VLDL-apolipoprotein B-100 Research Articles

Estimates of the Effect of Feeding on Whole-Body Protein Degradation in Women Vary with the Amino Acid Used as Tracer

We measured how feeding status affects the kinetics of multiple indispensable amino acids in four adult female subjects studied both in fed and fasted state. The subjects ingested one dose of uniformly 13C-labeled algae (Spirulina platensis). The isotopic enrichments (measured with negative chemical ionization gas chromatography-mass spectrometry) of the branched chain amino acids, phenylalanine, lysine and threonine were followed for 24 h in both the plasma and in VLDL-apolipoprotein B-100 (apoB-100). Fasting lowered body protein degradation when measured with the branched chain amino acids, increased it when measured with phenylalanine and had no statistically significant effect when determined from the kinetics of lysine and threonine. These apparent differences challenge the adequacy of current models of whole-body protein turnover. The ratio of the peak labeling of amino acids in plasma and apoB-100 was used as an estimate of the isotopic dilution in the hepatic pool. In contrast to our earlier observations during intravenous tracer amino acid administration, in the present study fasting lowered the ratio of the peak isotopic enrichments of apoB-100 and plasma amino acids. This supports our contention that feeding increases the use of hepatic portal amino acids for hepatic secretory protein synthesis.

The Effects of Mipomersen on Inhibiting Hepatic VLDL Apolipoprotein B100 Synthesis and Propensity for Hepatic Steatosis

Mipomersen is a 20-nucleotide, second-generation, antisense oligonucleotide that inhibits human apolipoprotein B (apo B)–1003 production by binding to and preventing translation of apo B messenger RNA. As apo B–100 is an essential structural component of VLDL, intermediate-density lipoprotein (IDL), LDL, and lipoprotein(a), decreased hepatic production of apo B by mipomersen should lead to reduced circulating concentrations of all of these atherogenic lipoprotein particles (1). However, Reyes-Soffer et al., in a stable isotope study of 17 healthy volunteers without hyperlipidemia and presumably on a restricted fat diet, reported that mipomersen 200 mg weekly for 7 weeks, although reducing VLDL and its apo B and triglyceride content, did not reduce the production rates of VLDL apo B but surprisingly increased its clearance (2). The study did, however, find modest reductions in IDL- and LDL apo B–associated production and postulated that these lipoproteins were decreased independently and directly rather than being a consequence of reduced VLDL production. The authors postulated that, due to the partial inhibition of apo B synthesis by mipomersen, compensation occurred by increasing the proportion of newly synthesized apo B directed to VLDL production, which in turn helped maintain hepatic lipid homeostasis (2). In addition, there was increased loading of triglyceride onto VLDL …

Changes in Lipoprotein Kinetics Associated With Type 2 Diabetes Affect the Distribution of Lipopolysaccharides Among Lipoproteins

Lipopolysaccharides (LPSs) are inflammatory components of the outer membrane of Gram-negative bacteria and, in plasma, are mostly associated with lipoproteins. This association is thought to promote their catabolism while reducing their proinflammatory effects. Our aim was to determine the impact of lipoprotein kinetics on plasma LPS distribution and how it may affect patients with type 2 diabetes mellitus (T2DM). We performed a kinetic study in 30 individuals (16 T2DM patients, 14 controls) and analyzed the impact of changes in lipoprotein kinetics on LPS distribution among lipoproteins. Plasma LPS levels in T2DM patients were not different from those in controls, but LPS distribution in the two groups was different. Patients with T2DM had higher LPS-very low-density lipoprotein (VLDL; 31% ± 7% vs 22% ± 11%, P = .002), LPS-high-density lipoprotein (HDL; 29% ± 9% vs 19% ± 10%, P = .015), free (nonlipoprotein bound) LPS (10% ± 4% vs 7% ± 4%, P = .043) and lower LPS-low-density lipoprotein (LDL; 30% ± 13% vs 52% ± 16%, P = .001). In multivariable analysis, VLDL-LPS was associated with HDL-LPS (P < .0001); LDL-LPS was associated with VLDL-LPS (P = .004), and VLDL apolipoprotein (apo) B100 catabolism (P = .002); HDL-LPS was associated with free LPS (P < .0001) and VLDL-LPS (P = .033); free LPS was associated with HDL-LPS (P < .0001). In a patient featuring a dramatic decrease in VLDL catabolism due to apoA-V mutation, LDL-LPS was severely decreased (0.044 EU/mL vs 0.788 EU/mL in controls). The difference between T2DM patients and controls for LDL-LPS fraction was no longer significant after controlling for VLDL apoB100 total fractional catabolic rate. Our data suggest that in humans, free LPS transfers first to HDL and then to VLDL, whereas the LPS-bound LDL fraction is mainly derived from VLDL catabolism; the latter may hence represent a LPS catabolic pathway. T2DM patients show lower LDL-LPS secondary to reduced VLDL catabolism, which may represent an impaired catabolic pathway.

One Day of Mixed Meal Overfeeding Reduces Hepatic Insulin Sensitivity and Increases VLDL Particle But Not VLDL-Triglyceride Secretion in Overweight and Obese Men

The exact mechanisms responsible for increased plasma triglyceride (TG) concentration in obese people are unclear, and it is not known whether excess energy intake per se is involved in the pathophysiology of this abnormality. The purpose of our study was to examine how excess energy intake from a balanced diet for 1 day affects very-low-density lipoprotein (VLDL)-TG kinetics and its putative regulators hepatic insulin sensitivity and plasma free fatty acid availability. We used stable isotope-labeled tracer methods to evaluate glucose and lipid kinetics in 8 overweight and obese men (age, 38 ± 3 years; body mass index, 33.7 ± 1.7 kg/m(2); means ± SEM) on 2 occasions (randomized crossover design): once, the day after they consumed a balanced diet that provided an amount of energy that matched their energy expenditure, and another time, the day after they consumed a balanced diet that provided 30% excess calories. Eight healthy, lean men (34 ± 1 years; 22.5 ± 0.6 kg/m(2)) were studied under isocaloric conditions only to provide a reference for normal lipid kinetics. VLDL-TG and VLDL-apolipoprotein B-100 (apoB-100) concentrations and secretion rates were significantly greater (P < .01) in overweight/obese compared with lean men. Hypercaloric, compared with isocaloric, feeding in overweight/obese men increased glucose rate of appearance in plasma (904 ± 21 vs 873 ± 26 μmol/min), the hepatic insulin resistance index (10.9 ± 2.2 vs 8.3 ± 1.8), and VLDL-apoB-100 concentration and secretion rate (1.91 ± 0.24 vs. 1.53 ± 0.13 nmol/min), whereas VLDL-apoB-100 plasma clearance rate, VLDL-TG secretion and plasma clearance rates, and free fatty acid rate of appearance in plasma were not affected by overfeeding. One day of moderate overfeeding (30% excess energy intake) stimulates hepatic glucose and VLDL-apo B-100 secretion rates but has no effect on hepatic and adipose tissue fatty acid metabolism in overweight/obese men.

Tracking fatty acid kinetics in distinct lipoprotein fractions in vivo: a novel high-throughput approach for studying dyslipidemia in rodent models

Isotopic tracers have been used to examine lipid trafficking for many years, and data from those studies have typically yielded novel insight regarding the pathophysiology of dyslipidemia. Previous experimental designs were suitable for studies in humans because relatively large volumes of plasma could be regularly sampled. We have expanded on the earlier logic by applying high-throughput analytical methods that require reduced sample volumes. Specifically, we have examined the possibility of coupling gel-based separations of lipoproteins (e.g., lipoprint) with LC-MS/MS analyses of complex lipid mixtures as a way to routinely measure the labeling profiles of distinct lipids in discrete lipoprotein subfractions. We demonstrate the ability to measure the incorporation of [U-(13)C]oleate into triglycerides (TG), PLs (PL), and cholesterol esters (CE) in VLDL, LDL, and HDL particles in mice. Although rodent models of dyslipidemia are inherently different from humans because of alterations in enzyme activities and underlying metabolism, rodent models can be used to screen novel compounds for efficacy in altering a given biochemical pathway and therein enable studies of target engagement in vivo. We expect that it is possible to translate our approach for application in other systems, including studies in humans.

Nutritional Repletion of Children with Severe Acute Malnutrition Does Not Affect VLDL Apolipoprotein B-100 Synthesis Rate ,

VLDL apo B-100 is essential for the secretion of liver fat. It is thought that synthesis of this lipoprotein is impaired in childhood severe acute malnutrition (SAM), especially in the edematous syndromes, and that this contributes to the common occurrence of hepatic steatosis in this condition. However, to our knowledge, it has not been confirmed that VLDL apo B-100 synthesis is slower in edematous compared with nonedematous SAM and that it is impaired in the malnourished compared with the well-nourished state. Therefore, VLDL apo B-100 kinetics were measured in 2 groups of children with SAM (15 edematous and 7 nonedematous), aged 4–20 mo, at 3 stages during treatment. Measurements were done at 4 ± 1 d postadmission, mid- catch-up growth in weight, and at recovery (normal weight-for-length). VLDL apo B-100 synthesis was determined using stable isotope methodology to measure the rate of incorporation of 2H3-leucine into its apoprotein moiety. The fractional and absolute synthesis of VLDL apo B-100 did not differ between the groups or from the initial malnourished stage to the recovery stage. Concentrations of VLDL apo B-100 were greater in the edematous than in the nonedematous group (P < 0.04) and did not differ from the initial stage to recovery. The data indicate that VLDL apo B-100 synthesis is not reduced when children develop either edematous or nonedematous SAM.

Measurement of Fractional Synthetic Rates of Multiple Protein Analytes by Triple Quadrupole Mass Spectrometry

Current approaches to measure protein turnover that use stable isotope-labeled tracers via GC-MS are limited to a small number of relatively abundant proteins. We developed a multiplexed liquid chromatography-selected reaction monitoring mass spectrometry (LC-SRM) assay to measure protein turnover and compared the fractional synthetic rates (FSRs) for 2 proteins, VLDL apolipoprotein B100 (VLDL apoB100) and HDL apoA-I, measured by both methods. We applied this technique to other proteins for which kinetics are not readily measured with GC-MS. Subjects were given a primed-constant infusion of [5,5,5-D(3)]-leucine (D(3)-leucine) for 15 h with blood samples collected at selected time points. Apolipoproteins isolated by SDS-PAGE from lipoprotein fractions were analyzed by GC-MS or an LC-SRM assay designed to measure the M+3/M+0 ratio at >1% D(3)-leucine incorporation. We calculated the FSR for each apolipoprotein by curve fitting the tracer incorporation data from each subject. The LC-SRM method was linear over the range of tracer enrichment values tested and highly correlated with GC-MS (R(2) > 0.9). The FSRs determined from both methods were similar for HDL apoA-I and VLDL apoB100. We were able to apply the LC-SRM approach to determine the tracer enrichment of multiple proteins from a single sample as well as proteins isolated from plasma after immunoprecipitation. The LC-SRM method provides a new technique for measuring the enrichment of proteins labeled with stable isotopes. LC-SRM is amenable to a multiplexed format to provide a relatively rapid and inexpensive means to measure turnover of multiple proteins simultaneously.

Testosterone increases the muscle protein synthesis rate but does not affect very-low-density lipoprotein metabolism in obese premenopausal women

Men and women with hyperandrogenemia have a more proatherogenic plasma lipid profile [e.g., greater triglyceride (TG) and total and low-density lipoprotein-cholesterol and lower high-density lipoprotein-cholesterol concentrations] than healthy premenopausal women. Furthermore, castration of male rats markedly reduces testosterone availability below normal and decreases plasma TG concentration, and testosterone replacement reverses this effect. Testosterone is, therefore, thought to be an important regulator of plasma lipid homeostasis. However, little is known about the effect of testosterone on plasma TG concentration and kinetics. Furthermore, testosterone is a potent skeletal muscle protein anabolic agent in men, but its effect on muscle protein turnover in women is unknown. We measured plasma lipid concentrations, hepatic very low density lipoprotein (VLDL)-TG and VLDL-apolipoprotein B-100 secretion rates, and the muscle protein fractional synthesis rate in 10 obese women before and after trandermal testosterone (1.25 g of 1% AndroGel daily) treatment for 3 wk. Serum total and free testosterone concentrations increased (P < 0.05) by approximately sevenfold in response to testosterone treatment, reaching concentrations that are comparable to those in women with hyperandrogenemia, but lower than the normal range for eugonadal men. Except for a small (∼10%) decrease in plasma high-density lipoprotein particle and cholesterol concentrations (P < 0.04), testosterone therapy had no effect on plasma lipid concentrations, lipoprotein particle sizes, and hepatic VLDL-TG and VLDL-apolipoprotein B-100 secretion rates (all P > 0.05); the muscle protein fractional synthesis rate, however, increased by ∼45% (P < 0.001). We conclude that testosterone is a potent skeletal muscle protein anabolic agent, but not an important regulator of plasma lipid homeostasis in obese women.

Subclinical Hypothyroidism and Hyperthyroidism Have Opposite Effects on Hepatic Very-Low-Density Lipoprotein-Triglyceride Kinetics

Clinically overt thyroid dysfunction is associated with alterations in triglyceride (TG) metabolism. The effect of subclinical thyroid disease on very-low-density lipoprotein (VLDL) kinetics is not known. Our objective was to investigate whether subclinical thyroid disease is associated with alterations in hepatic VLDL metabolism. DESIGN AND OUTCOMES: We measured VLDL-TG and VLDL-apolipoprotein B-100 (apoB-100) kinetics by infusing stable isotopically labeled tracers, in conjunction with mathematical modeling. Ten women with subclinical hypothyroidism, 10 women with subclinical hyperthyroidism, and 25 euthyroid women, matched on age, body mass index, and percent body fat, were studied in the Clinical Research Unit at Washington University School of Medicine. Plasma VLDL-TG concentrations were 0.75±0.13, 0.51±0.06, and 0.37±0.07 mmol/liter (P=0.029), and hepatic VLDL-TG secretion rates were 6.5±0.7, 5.0±0.4, and 4.1±0.6 μmol/liter·min (P=0.026) in hypothyroid, euthyroid, and hyperthyroid women, respectively. The differences in VLDL-TG secretion rates were due to differences in the incorporation of systemic plasma free fatty acids into VLDL-TG (4.3±0.3, 3.1±0.3, and 2.5±0.3 μmol/liter·min in hypothyroid, euthyroid, and hyperthyroid women, respectively; P=0.005). Plasma VLDL-apoB-100 concentration and hepatic secretion rate did not differ among groups (P>0.400), so the molar ratios of VLDL-TG to VLDL-apoB-100 secretion rates were 21,469±3,477, 16,025±1,273, and 11,889±1,319 in hypothyroid, euthyroid, and hyperthyroid women, respectively (P=0.019). Subclinical thyroid disease affects hepatic VLDL-TG but not VLDL-apoB-100 metabolism: subclinical hypothyroidism increases, whereas subclinical hyperthyroidism decreases, hepatic VLDL-TG secretion rate compared with the euthyroid state. Plasma VLDL-TG concentration is greater in subclinical hypothyroid than euthyroid and hyperthyroid subjects, due to greater secretion of large, TG-rich VLDL particles from the liver.

Very Low Density Lipoprotein Metabolism in Patients with Chronic Kidney Disease

Background: Hypertriglyceridemia is a common metabolic complication of chronic kidney disease (CKD) and an important risk factor for coronary heart disease in this patient population. The mechanisms responsible for the development of hypertriglyceridemia in subjects with CKD are not clear. Methods: We studied very low density lipoprotein triglyceride (VLDL-TG) and VLDL-apolipoprotein B-100 (VLDL-apoB-100) kinetics in vivo in 6 subjects with non-dialysis-dependent CKD (CKD-ND), 6 subjects with CKD treated with peritoneal dialysis (CKD-PD) and 24 sex-, age- and body mass index-matched control subjects with normal renal function (12 control subjects each matched with the CKD-ND and CKD-PD group, respectively). Results: The secretion rates of VLDL-TG and VLDL-apoB-100 into plasma were not different between CKD-ND or CKD-PD and their respective control groups. The mean residence times of VLDL-TG and VLDL-apoB-100 in plasma, which represents the time VLDL-TG and VLDL-apoB-100 spend in the circulation after secretion by the liver, tended to be greater in subjects with CKD-ND than in control subjects (222 ± 38 vs. 143 ± 21 min, p = 0.07, and 352 ± 102 vs. 200 ± 20 min, p = 0.06, respectively) and were about two-fold greater in subjects with CKD-PD compared with their control group (248 ± 51 vs. 143 ± 21 min and 526 ± 116 vs. 182 ± 16 min, respectively; both p ≤ 0.01). Conclusion: Impaired plasma clearance of VLDL-TG and VLDL-apoB-100 is the major abnormality associated with hypertriglyceridemia in patients with either CKD-ND or CKD-PD.

Portal vein and systemic adiponectin concentrations are closely linked with hepatic glucose and lipoprotein kinetics in extremely obese subjects

Low systemic plasma adiponectin concentrations are associated with abnormalities in hepatic glucose and lipoprotein metabolism in obese people. However, the relationship between the delivery of adiponectin to the liver via the portal vein and hepatic glucose and lipoprotein metabolism is not known. We examined the relationship between hepatic substrate metabolism (glucose rate of appearance into plasma and hepatic very low-density lipoprotein [VLDL]–triglyceride [TG] and VLDL–apolipoprotein B-100 [apoB-100] secretion rates, determined by using stable isotope-labeled tracer techniques) and portal vein adiponectin concentration, in 8 insulin-resistant, extremely obese subjects (body mass index, 65 ± 7 kg/m 2). Portal vein adiponectin concentration was inversely associated with basal glucose rate of appearance ( r = −0.820, P = .013) and VLDL-TG ( r = −0.823, P = .012) and VLDL–apoB-100 ( r = −0.787, P = .020) secretion rates. Very similar correlations were obtained for radial artery adiponectin as a result of a mirroring relationship between portal and arterial adiponectin concentrations ( r = 0.899, P = .002) and the absence of significant arteriovenous concentration differences ( P = .570). Insulin resistance, assessed with the homeostasis model assessment score, was also strongly associated with hepatic glucose and lipid metabolic parameters, as well as with adiponectin concentrations in the portal vein and radial artery. These results suggest that adiponectin delivery to the liver, whether via the portal or the systemic circulation, may be an important regulator of basal hepatic glucose, VLDL-TG, and VLDL–apoB-100 production rates in obese people, possibly through direct effects on the liver or changes in hepatic insulin sensitivity. However, portal vein adiponectin does not appear to be superior to arterial adiponectin as a marker of hepatic metabolic dysregulation. Additional studies are needed to elucidate the mechanism(s) responsible for the strong association we observed between adiponectin and hepatic substrate metabolism.

Increased Whole‐Body Adiposity Without a Concomitant Increase in Liver Fat is Not Associated With Augmented Metabolic Dysfunction

Aim of this study was to determine whether an increase in adiposity, without a concomitant increase in intrahepatic triglyceride (IHTG) content, is associated with a deterioration in metabolic function. To this end, multiorgan insulin sensitivity, assessed by using a two-stage hyperinsulinemic-euglycemic clamp procedure in conjunction with stable isotopically labeled tracer infusion, and very low-density lipoprotein (VLDL) kinetics, assessed by using stable isotopically labeled tracer infusion and mathematical modeling, were determined in 10 subjects with class I obesity (BMI: 31.6 +/- 0.3 kg/m(2); 37 +/- 2% body fat; visceral adipose tissue (VAT): 1,225 +/- 144 cm(3)) and 10 subjects with class III obesity (BMI: 41.5 +/- 0.5 kg/m(2); 43 +/- 2% body fat; VAT: 2,121 +/- 378 cm(3)), matched on age, sex, and IHTG content (14 +/- 4 and 14 +/- 3%, respectively). No differences between class I and class III obese groups were detected in insulin-mediated suppression of palmitate (67 +/- 3 and 65 +/- 3%, respectively; P = 0.635) and glucose (67 +/- 3 and 73 +/- 5%, respectively; P = 0.348) rates of appearance in plasma, and the insulin-mediated increase in glucose disposal (218 +/- 18 and 193 +/- 30%, respectively; P = 0.489). In addition, no differences between class I and class III obese groups were detected in secretion rates of VLDL-triglyceride (6.5 +/- 1.0 and 6.0 +/- 1.4 micromol/l x min, respectively; P = 0.787) and VLDL-apolipoprotein B-100 (0.40 +/- 0.05 and 0.41 +/- 0.04 nmol/l x min, respectively; P = 0.866), and plasma clearance rates of VLDL-triglyceride (31 (16-59) and 29 (18-46) ml/min, respectively; P = 0.888) and VLDL-apolipoprotein B-100 (15 (11-19) and 17 (11-25) ml/min, respectively; P = 0.608). We conclude that increased adiposity without a concomitant increase in IHTG content does not cause additional abnormalities in adipose tissue, skeletal muscle, and hepatic insulin sensitivity, or VLDL metabolism.

Chronic kidney disease delays VLDL-apoB-100 particle catabolism: potential role of apolipoprotein C-III

To determine the relative contribution of obesity and/or insulin resistance (IR) in the development of dyslipidemia in chronic kidney disease (CKD), we investigated the transport of apolipoprotein (apo) B-100 in nonobese, nondiabetic, nonnephrotic CKD subjects and healthy controls (HC). We determined total VLDL, VLDL(1), VLDL(2), intermediate density lipoprotein (IDL), and LDL-apoB-100 using intravenous D3-leucine, GC-MS, and multicompartmental modeling. Plasma apoC-III and apoB-48 were immunoassayed. In this case control study, we report higher plasma triglyceride, IDL-, VLDL-, VLDL(1)-, and VLDL(2)-apoB-100 concentrations in CKD compared with HC (P < 0.05). This was associated with decreased fractional catabolic rates [FCRs (pools/day)] [IDL:CKD 3.4 (1.6) vs. HC 5.0 (3.2), P < 0.0001; VLDL:CKD 4.8 (5.2) vs. HC 7.8 (4.8), P = 0.038; VLDL(1):CKD 10.1 (8.5) vs. HC 29.5 (45.1), P = 0.007; VLDL(2):CKD 5.4 (4.6) vs. HC 10.4 (3.4), P = 0.001] with no difference in production rates. Plasma apoC-III and apoB-48 were significantly higher in CKD (P < 0.001) and both correlated with impaired FCRs of VLDL, VLDL(1), and VLDL(2) apoB-100 (P < 0.05). In CKD, apoC-III concentration was the only independent predictor of clearance defects in VLDL and its subfractions. Moderate CKD in the absence of central adiposity and IR is associated with mild hypertriglyceridemia due to delayed catabolism of triglyceride rich lipoproteins, IDL, and VLDL, without changes in production rate. Altered apoC-III metabolism may contribute to dyslipidemia in CKD, and this requires further investigation.

Inhibition of apoB secretion from HepG2 cells by insulin is amplified by naringenin, independent of the insulin receptor

Hepatic overproduction of apolipoprotein B (apoB)-containing lipoproteins is characteristic of the dyslipidemia associated with insulin resistance. Recently, we demonstrated that the flavonoid naringenin, like insulin, decreased apoB secretion from HepG2 cells by activation of both the phosphoinositide-3-kinase (PI3-K) pathway and the mitogen-activated protein kinase/extracellular-regulated kinase (MAPK(erk)) pathway. In the present study, we determined whether naringenin-induced signaling required the insulin receptor (IR) and sensitized the cell to the effects of insulin, and whether the kinetics of apoB assembly and secretion in cells exposed to naringenin were similar to those of insulin. Immunoblot analysis revealed that insulin stimulated maximal phosphorylation of IR and IR substrate-1 after 10 min, whereas naringenin did not affect either at any time point up to 60 min. The combination of naringenin and submaximal concentrations of insulin potentiated extracellular-regulated kinase 1/2 activation and enhanced upregulation of the LDL receptor, downregulation of microsomal triglyceride transfer protein expression, and inhibition of apoB-100 secretion. Multicompartmental modeling of apoB pulse-chase studies revealed that attenuation of secreted radiolabeled apoB in naringenin- or insulin-treated cells was similar under lipoprotein-deficient or oleate-stimulated conditions. Naringenin and insulin both stimulated intracellular apoB degradation via a kinetically defined rapid pathway. Therefore, naringenin, like insulin, inhibits apoB secretion through activation of both PI3-K and MAPK(erk) signaling, resulting in similar kinetics of apoB secretion. However, the mechanism for naringenin-induced signaling is independent of the IR. Naringenin represents a possible strategy for reduction of hepatic apoB secretion, particularly in the setting of insulin resistance.

A single bout of brisk walking increases basal very low-density lipoprotein triacylglycerol clearance in young men

Very low-density lipoprotein triacylglycerol (VLDL-TG) turnover rate was evaluated in the morning, 12 hours after a single bout of brisk walking (90 minutes at approximately 60% of V̇ o 2max; EXE), compared to a resting control period (CON) in 10 recreationally active men. VLDL-TG fractional catabolic rate was calculated from the decay in isotopic enrichment after a bolus injection of [ 2H 5]glycerol. Plasma VLDL-TG concentration was 24% lower in the morning after the EXE trial compared to control (0.47 ± 0.04 and 0.36 ± 0.04 mmol L −1, for CON and EXE, respectively; P < .01). Serum insulin (7.4 ± 0.7 and 5.6 ± 0.4 mIU L −1, CON and EXE, respectively; P < .05) and plasma glucose (5.6 ± 0.1 and 5.4 ± 0.1 mmol/L, CON and EXE, respectively; P < .05) concentrations were also significantly lower in the EXE trial. Insulin sensitivity (homeostasis model assessment [HOMA] index) was improved by 27% in EXE compared with the CON trial ( P < .05).VLDL–apolipoprotein B-100 and plasma fatty acid concentrations were similar in the two trials. Hepatic VLDL-TG secretion rates were not significantly affected by exercise (13.1 ± 1.2 and 13.2 ± 1.6 μmol .min −1 for the CON and EXE trials, respectively), whereas VLDL-TG clearance rate increased by 36% (28.1 ± 1.3 and 38.1 ± 3.5 mL .min −1 for the CON and EXE trials, respectively; P < .05). It is concluded that the decrease in fasting plasma VLDL-TG concentration observed 12 hours after brisk walking is related mainly to increased VLDL-TG clearance from plasma.

Reproducibility of stable isotope-labeled tracer measures of VLDL-triglyceride and VLDL-apolipoprotein B-100 kinetics

To gain insight into the mechanisms regulating plasma lipid homeostasis, FFA, VLDL-triglyceride (TG), and VLDL-apolipoprotein B-100 (apoB-100) kinetics are commonly assessed using stable isotope-labeled tracer methods. The reproducibility of these measurements, which is critical for the experimental design, is unknown. Therefore, we investigated the repeatability of plasma FFA, VLDL-TG, and VLDL-apoB-100 kinetics in eight healthy men using stable isotope-labeled tracer techniques. There were no systematic differences in plasma FFA, VLDL-TG, and VLDL-apoB-100 concentrations and kinetics between the two studies. Intraindividual day-to-day variability for various outcome variables ranged from 15% to 25%, and almost all of this was of biological origin. The most robust outcome variables were FFA rate of appearance and hepatic VLDL-TG and VLDL-apoB-100 secretion rates; the least robust were VLDL-TG and VLDL-apoB-100 plasma clearance rates and mean residence times. Overall, physiologically meaningful differences in mean values (i.e., 25-30% in magnitude) can be obtained with a sample size of 6-10 subjects for paired studies and 12-20 subjects per group for cross-sectional studies, assuming a type I error rate of 0.05 and a type II error rate of 0.20 (i.e., 80% power). These findings will be useful for future studies investigating FFA, VLDL-TG, and VLDL-apoB-100 kinetics with the methods described.

Women Produce Fewer but Triglyceride-Richer Very Low-Density Lipoproteins than Men

Very low-density lipoproteins (VLDL) are a major risk factor for cardiovascular disease. The concentrations of VLDL particles and VLDL-triglyceride (TG) in plasma are lower in women than men, but the mechanisms responsible for these differences are unclear. The objective of the study was to investigate the effects of sex on VLDL-TG and VLDL-apolipoprotein B-100 (apoB-100) metabolism. EXPERIMENTAL DESIGN AND MAIN OUTCOME MEASURES: We measured basal VLDL-TG and VLDL-apoB-100 kinetics by using stable isotope labeled tracers. Twenty-six healthy, lean subjects (13 men, aged 29+/-5 yr; 13 women, aged 28+/-6 yr) were studied in the General Clinical Research Center at Washington University School of Medicine. VLDL-TG and VLDL-apoB-100 concentrations were less in women than men (P<0.05). The secretion rate of VLDL-TG was approximately 70% greater (P<0.05), whereas the secretion rate of VLDL-apoB-100 (i.e. VLDL particles) was approximately 20% less (P<0.05) in women than men. The molar ratio of VLDL-TG and VLDL-apoB-100 secretion rates was therefore more than double (P<0.05) in women than men. VLDL-TG plasma clearance rate was approximately 70% greater in women than men (P<0.05), whereas VLDL-apoB-100 plasma clearance rate was not different between sexes. However, VLDL-TG and VLDL-apoB-100 mean residence times in plasma were both shorter (by 45 and 25%, respectively; P<0.05) in women than men. Increased VLDL-TG plasma clearance is responsible for lower VLDL-TG concentration, whereas decreased VLDL-apoB-100 secretion rate, combined with shorter VLDL-apoB-100 residence time in plasma, is responsible for lower VLDL-apoB-100 concentration in women than men. The greater molar ratio of VLDL-TG and VLDL-apoB-100 secretion rates suggests that the liver in women secretes fewer but TG-richer VLDL particles than the liver in men.

A single 1-h bout of evening exercise increases basal FFA flux without affecting VLDL-triglyceride and VLDL-apolipoprotein B-100 kinetics in untrained lean men

Our group (Magkos F, Wright DC, Patterson BW, Mohammed BS, Mittendorfer B, Am J Physiol Endocrinol Metab 290: E355-E362, 2006) has recently demonstrated that a single, prolonged bout of moderate-intensity cycling (2 h at 60% of peak oxygen consumption) in the evening increases basal whole-body free fatty acid (FFA) flux and fat oxidation, decreases hepatic VLDL-apolipoprotein B-100 (apoB-100) secretion, and enhances removal efficiency of VLDL-triglyceride (TG) from the circulation the following day in untrained, healthy, lean men. In the present study, we investigated the effect of a single, shorter-duration bout of the same exercise (1 h cycling at 60% of peak oxygen consumption) on basal FFA, VLDL-TG, and VLDL-apoB-100 kinetics in seven untrained, healthy, lean men by using stable isotope-labeled tracer techniques. Basal FFA rate of appearance in plasma and plasma FFA concentration were approximately 55% greater (P < 0.05) the morning after exercise than rest, whereas resting metabolic rate and whole-body substrate oxidation rates were not different after rest and exercise. Exercise had no effect on plasma VLDL-TG and VLDL-apoB-100 concentrations, hepatic VLDL-TG and VLDL-apoB-100 secretion rates, and VLDL-TG and VLDL-apoB-100 plasma clearance rates (all P > 0.05). We conclude that in untrained, healthy, lean men 1) the exercise-induced changes in basal whole-body fat oxidation, VLDL-TG, and VLDL-apoB-100 metabolism during the late phase of recovery from exercise are related to the duration of the exercise bout; 2) single sessions of typical recreational activities appear to have little effect on basal, fasting plasma TG homeostasis; and 3) there is a dissociation between systemic FFA availability and VLDL-TG and VLDL-apoB-100 secretion by the liver.

No effect of menstrual cycle phase on basal very-low-density lipoprotein triglyceride and apolipoprotein B-100 kinetics

Dyslipidemia, manifested by increased plasma triglyceride (TG), increased total and LDL-cholesterol concentrations and decreased HDL-cholesterol concentration, is an important risk factor for cardiovascular disease. Premenopausal women have a less atherogenic plasma lipid profile and a lower risk of cardiovascular disease than men, but this female advantage disappears after menopause. This suggests that female sex steroids affect lipoprotein metabolism. The impact of variations in the availability of ovarian hormones during the menstrual cycle on lipoprotein metabolism is not known. We therefore investigated whether very-low-density lipoprotein (VLDL)-TG and VLDL-apolipoprotein B-100 (apoB-100) kinetics are different during the follicular (FP) and luteal phases (LP) of the menstrual cycle. We studied seven healthy, premenopausal women (age 27 +/- 2 yr, BMI 25 +/- 2 kg/m(2)) once during FP and once during LP. We measured VLDL-TG, VLDL-apoB-100, and plasma free fatty acid (FFA) kinetics by using stable isotope-labeled tracers, VLDL subclass profile by nuclear magnetic resonance spectroscopy, whole body fat oxidation by indirect calorimetry, and the plasma concentrations of lipoprotein lipase (LPL) and hepatic lipase (HL) by ELISA. VLDL-TG and VLDL-apoB-100 concentrations in plasma, VLDL-TG and VLDL-apoB-100 secretion rates and mean residence times, VLDL subclass distribution, FFA concentration and rate of appearance in plasma, whole body substrate oxidation, and LPL and HL concentrations in plasma were not different during the FP and the LP. We conclude that VLDL-TG and VLDL-apoB-100 metabolism is not affected by menstrual cycle phase.

Physiologic growth hormone replacement improves fasting lipid kinetics in patients with HIV lipodystrophy syndrome

HIV lipodystrophy syndrome (HLS) is characterized by accelerated lipolysis, inadequate fat oxidation, increased hepatic reesterification, and a high frequency of growth hormone deficiency (GHD). The effect of growth hormone (GH) replacement on these lipid kinetic abnormalities is unknown. We aimed to measure the effects of physiologic GH replacement on lipid kinetics in men with HLS and GHD. Seven men with HLS and GHD were studied with the use of infusions of [13C1]palmitate, [2H5]glycerol, and [2H3]leucine to quantify total and net lipolysis, palmitate and free fatty acid (FFA) oxidation, and VLDL apolipoprotein B-100 synthesis before and after 6 mo of GH replacement (maximum: 5 microg x kg(-1) x d(-1)). GH replacement decreased the rates of total lipolysis [FFA(total) rate of appearance (x +/- SE): from 4.80 +/- 1.24 to 3.32 +/- 0.76 mmol FFA x kg fat(-1) x h(-1); P < 0.05] and net lipolysis (FFA(net) rate of appearance: from 1.87 +/- 0.34 to 1.20 +/- 0.25 mmol FFA x kg fat(-1) x h(-1); P < 0.05). Fat oxidation decreased (from 0.28 +/- 0.02 to 0.20 +/- 0.02 mmol FFA x kg lean body mass(-1) x h(-1); P < 0.002), as did the rate of appearance of FFAs available for intrahepatic reesterification (from 0.50 +/- 0.13 to 0.29 +/- 0.09 mmol FFA x kg fat(-1) x h(-1); P < 0.03). Fractional and absolute synthetic rates of VLDL apolipoprotein B-100 were unaltered. These kinetic changes were associated with a decrease in the waist-to-hip ratio but no significant change in fasting plasma lipid concentrations. Fasting plasma glucose concentrations increased after treatment (from 5.2 +/- 0.2 to 5.8 +/- 0.3 mmol/L; P < 0.01). Physiologic GH replacement has salutary effects on abnormal lipid kinetics in HLS. The effects are mediated by diminished lipolysis and hepatic reesterification rather than by increased fat oxidation.