Stable Isotope Kinetic Studies Research Articles

Abstract P182: Recombinant Human Lecithin-cholesterol Acyltransferase Treatment In Patients With ASCVD Increases HDL Through Increases In Cholesterol Ester Content.

Lecithin-cholesterol acyltransferase (LCAT) mediates the esterification of free cholesterol (FC) to cholesteryl ester (CE) in HDL and, therefore, plays a critical role in reverse cholesterol transport. MEDI6012, a recombinant human LCAT (rhLCAT) increases HDL-C. Our goal was to interrogate the pathways regulating the increase in HDL-C and effects of rhLCAT on apoB metabolism. Methods: We enrolled five subjects (4 Man, mean age 67) with stable ASCVD into a Phase II, placebo controlled, double-blind, randomized cross-over study to determine the effects of two IV doses of MEDI6012, administered 48-hrs apart, versus placebo, on plasma lipids and lipoproteins. Stable isotope kinetic studies with D2-Leu, 13C-Phe, D2-Glycerol were performed to examine the metabolism of apoB100, ApoA1, ApoA2 and triglyceride. Results: As expected two doses of IV MEDI6012 increased total cholesterol and HDL-C levels significantly (Table1). We did not observe significant changes in other measured lipids or lipoproteins. The significant increase in HDL-C was 34.9±10.3 (p=0.002) and due to an increase in the amount of CE (33±8.9 p=0.001); there was no change in free cholesterol (1.9±1.9). We found no changes in levels of sterols (i.e. Lathosterol) between the two periods. Preliminary in vivo kinetic studies of HDL metabolism in 3 subjects, showed no changes in the mean fractional clearance rate (FCR) or production rate (PR) of apoA1 between placebo (0.3±0.2pool/day, 1.3±0.5 mg/kg/day) and rhLCAT treatment (0.28±0.1pools/day, 1.3±0.3). There were no changes in ApoA2 FCR and PR after rhLCAT administration. Complete data for ApoA1, ApoA2 and ApoB100 and TG kinetics will be presented. Conclusions: In subjects with ASCVD, treatment with rhLCAT increased HDL-C mainly by increasing HDL CE; there were no changes in the FCR or PR of HDL ApoA1 or ApoA2. Together, these results suggest that rhLCAT treatment is associated with increased steady-state transport of CE by HDL.

Effects of mipomersen, an apolipoprotein B100 antisense, on lipoprotein (a) metabolism in healthy subjects

Elevated lipoprotein (a) [Lp(a)] levels increase the risk for CVD. Novel treatments that decrease LDL cholesterol (LDL-C) have also shown promise for reducing Lp(a) levels. Mipomersen, an antisense oligonucleotide that inhibits apoB synthesis, is approved for the treatment of homozygous familial hypercholesterolemia. It decreases plasma levels of LDL-C by 25% to 39% and lowers levels of Lp(a) by 21% to 39%. We examined the mechanisms for Lp(a) lowering during mipomersen treatment. We enrolled 14 healthy volunteers who received weekly placebo injections for 3 weeks followed by weekly injections of mipomersen for 7 weeks. Stable isotope kinetic studies were performed using deuterated leucine at the end of the placebo and mipomersen treatment periods. The fractional catabolic rate (FCR) of Lp(a) was determined from the enrichment of a leucine-containing peptide specific to apo(a) by LC/MS. The production rate (PR) of Lp(a) was calculated from the product of Lp(a) FCR and Lp(a) concentration (converted to pool size). In a diverse population, mipomersen reduced plasma Lp(a) levels by 21%. In the overall study group, mipomersen treatment resulted in a 27% increase in the FCR of Lp(a) with no significant change in PR. However, there was heterogeneity in the response to mipomersen therapy, and changes in both FCRs and PRs affected the degree of change in Lp(a) concentrations. Mipomersen treatment decreases Lp(a) plasma levels mainly by increasing the FCR of Lp(a), although changes in Lp(a) PR were significant predictors of reductions in Lp(a) levels in some subjects.

Systemic inflammation is associated with exaggerated skeletal muscle protein catabolism in maintenance hemodialysis patients.

Systemic inflammation and muscle wasting are highly prevalent and coexist in patients on maintenance hemodialysis (MHD). We aimed to determine the effects of systemic inflammation on skeletal muscle protein metabolism in MHD patients. Whole body and skeletal muscle protein turnover were assessed by stable isotope kinetic studies. We incorporated expressions of E1, E214K, E3αI, E3αII, MuRF-1, and atrogin-1 in skeletal muscle tissue from integrin β1 gene KO CKD mice models. Among 129 patients with mean (± SD) age 47 ± 12 years, 74% were African American, 73% were male, and 22% had diabetes mellitus. Median high-sensitivity C-reactive protein (hs-CRP) concentration was 13 (interquartile range 0.8, 33) mg/l. There were statistically significant associations between hs-CRP and forearm skeletal muscle protein synthesis, degradation, and net forearm skeletal muscle protein balance (P < 0.001 for all). The associations remained statistically significant after adjustment for clinical and demographic confounders, as well as in sensitivity analysis, excluding patients with diabetes mellitus. In attempting to identify potential mechanisms involved in this correlation, we show increased expressions of E1, E214K, E3αI, E3αII, MuRF-1, and atrogin-1 in skeletal muscle tissue obtained from an animal model of chronic kidney disease. These data suggest that systemic inflammation is a strong and independent determinant of skeletal muscle protein homeostasis in MHD patients, providing rationale for further studies using anticytokine therapies in patients with underlying systemic inflammation. This study was in part supported by NIH grants R01 DK45604 and 1K24 DK62849, the Clinical Translational Science Award UL1-TR000445 from the National Center for Advancing Translational Sciences, the Veterans Administration Merit Award I01 CX000414, the SatelliteHealth Normon Coplon Extramural Grant Program, and the FDA grant 000943.

Abstract 21310: ApoCIII RX19 Mutation Lowers the Production of ApoCIII and Increases the Clearance of VLDL-TG

The APOCIII R19X mutation (rs76353203), has been found in ~5% of the Lancaster Old Order Amish. It is a C -&gt;T substitution at the 55th nucleotide in human aolipoprotein C-III mRNA that prevents translation of mature apoC-III protein. The mutation decreases apoC-III levels by ~50% and confers favorable lipid profiles and less coronary artery calcification, providing evidence that ApoC-III deficiency is cardioprotective. This conclusion is supported by recent data in Danish and U.S. cohorts showing a 40% reduction in myocardial infarction rates in people with this mutation. The mechanisms whereby apoc-III deficiency exerts its effects are not fully understood. The aim of this study was to describe the fractional clearance rates (FCR) and production rates (PR) of apoB containing lipoproteins, apoCIII, and very low density lipoprotein (VLDL) tryglyceride (TG) in five Amish sibling pairs (CT vs. CC) discordant for the APOC3 R19X mutation and matched for age and sex. Methods: Sib pairs completed a 24-hr stable isotope kinetic study using leucine and glycerol. Blood samples were collected over 24-hr period. We measured plasma lipids and apoCIII by Elisa. We determined the FCR and PR of VLDL apoB, apoCIII, and VLDL-TG. Results: The CT subjects had a 38% decrease in baseline TG levels compared to CC. ApoCIII levels were reduced by 42% in the CT vs. CC. We observed reductions in VLDL-Chol (49%), IDL-Chol (52%), VLDL-TG(44%), and IDL-TG(39%) in the CT when compared to CC. The changes in plasma levels were due to an increase in the VLDL apoB FCR (p = 0.001) without significant changes in production. VLDL-TG FCR increased by 76% in the CT when compared to CC (p=0.03). We observed no differences in post-heparin plasma levels of hepatic and lipoprotein lipase activity in the CT vs. CC. The plasma changes in apoCIII were due to and increased FCR in the CT compared to CC. Conclusions: Reductions in plasma TG levels in heterozygous apoC-III deficient individuals is due to enhanced FCRs of VLDL-apoB and VLDL-TG.

CETP (Cholesteryl Ester Transfer Protein) Inhibition With Anacetrapib Decreases Production of Lipoprotein(a) in Mildly Hypercholesterolemic Subjects.

Lp(a) [lipoprotein (a)] is composed of apoB (apolipoprotein B) and apo(a) [apolipoprotein (a)] and is an independent risk factor for cardiovascular disease and aortic stenosis. In clinical trials, anacetrapib, a CETP (cholesteryl ester transfer protein) inhibitor, causes significant reductions in plasma Lp(a) levels. We conducted an exploratory study to examine the mechanism for Lp(a) lowering by anacetrapib. We enrolled 39 participants in a fixed-sequence, double-blind study of the effects of anacetrapib on the metabolism of apoB and high-density lipoproteins. Twenty-nine patients were randomized to atorvastatin 20 mg/d, plus placebo for 4 weeks, and then atorvastatin plus anacetrapib (100 mg/d) for 8 weeks. The other 10 subjects were randomized to double placebo for 4 weeks followed by placebo plus anacetrapib for 8 weeks. We examined the mechanisms of Lp(a) lowering in a subset of 12 subjects having both Lp(a) levels >20 nmol/L and more than a 15% reduction in Lp(a) by the end of anacetrapib treatment. We performed stable isotope kinetic studies using 2H3-leucine at the end of each treatment to measure apo(a) fractional catabolic rate and production rate. Median baseline Lp(a) levels were 21.5 nmol/L (interquartile range, 9.9-108.1 nmol/L) in the complete cohort (39 subjects) and 52.9 nmol/L (interquartile range, 38.4-121.3 nmol/L) in the subset selected for kinetic studies. Anacetrapib treatment lowered Lp(a) by 34.1% (P≤0.001) and 39.6% in the complete and subset cohort, respectively. The decreases in Lp(a) levels were because of a 41% reduction in the apo(a) production rate, with no effects on apo(a) fractional catabolic rate. Anacetrapib reduces Lp(a) levels by decreasing its production. URL: http://www.clinicaltrials.gov. Unique identifier: NCT00990808.

Stable isotope kinetic study of apolipoprotein M in healthy subjects

Stable isotope kinetic study of apolipoprotein M in healthy subjects

Optimization of N‐methyl‐N‐[tert‐butyldimethylsilyl]trifluoroacetamide as a derivatization agent for determining isotopic enrichment of glycerol in very‐low density lipoproteins

Stable isotope kinetic studies play an important role in the study of very-low density lipoprotein (VLDL) metabolism, including basic and clinical research. Today, [1,1,2,3,3-(2)H(5)]glycerol is the most cost-effective alternative to measure glycerol and triglyceride kinetics. Recycling of glycerol from glycolysis and gluconeogenesis may lead to incompletely labelled tracer molecules. Many existing methods for the measurement of glycerol isotopic enrichment involve the production of glycerol derivatives that result in fragmentation of the glycerol molecule after ionization. It would be favourable to measure the intact tracer molecule since incompletely labelled tracer molecules may be measured as fully labelled. The number of methods available to measure the intact tracer in biological samples is limited. The aim of this project was to develop a gas chromatography/mass spectrometry (GC/MS) method for glycerol enrichment that measures the intact glycerol backbone and is suitable for electron ionization (EI), which is widely available. A previously published method for N-methyl-N-[tert-butyldimethylsilyl]trifluoroacetamide (MTBSTFA) derivatization was significantly improved; we produced a stable derivative and increased recovery 27-fold in standards. We used the optimized MTBSTFA method in VLDL-triglyceride and found that further modification was required to take matrix effects into account. We now have a robust method to measure glycerol isotopic enrichment by GC/EI-MS that can be used to rule out the known problem of tracer recycling in studies of VLDL kinetics.

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.

Effects of cholesteryl ester transfer protein inhibition on apolipoprotein A-II-containing HDL subspecies and apolipoprotein A-II metabolism

This study was designed to establish the mechanism responsible for the increased apolipoprotein (apo) A-II levels caused by the cholesteryl ester transfer protein inhibitor torcetrapib. Nineteen subjects with low HDL cholesterol (<40 mg/dl), nine of whom were also treated with 20 mg of atorvastatin daily, received placebo for 4 weeks, followed by 120 mg of torcetrapib daily for the next 4 weeks. Six subjects in the nonatorvastatin cohort participated in a third phase, in which they received 120 mg of torcetrapib twice daily for 4 weeks. At the end of each phase, subjects underwent a primed-constant infusion of [5,5,5-(2)H(3)]L-leucine to determine the kinetics of HDL apoA-II. Relative to placebo, torcetrapib significantly increased apoA-II concentrations by reducing HDL apoA-II catabolism in the atorvastatin (-9.4%, P < 0.003) and nonatorvastatin once- (-9.9%, P = 0.02) and twice- (-13.2%, P = 0.02) daily cohorts. Torcetrapib significantly increased the amount of apoA-II in the alpha-2-migrating subpopulation of HDL when given as monotherapy (27%, P < 0.02; 57%, P < 0.003) or on a background of atorvastatin (28%, P < 0.01). In contrast, torcetrapib reduced concentrations of apoA-II in alpha-3-migrating HDL, with mean reductions of -14% (P = 0.23), -18% (P < 0.02), and -18% (P < 0.01) noted during the atorvastatin and nonatorvastatin 120 mg once- and twice-daily phases, respectively. Our findings indicate that CETP inhibition increases plasma concentrations of apoA-II by delaying HDL apoA-II catabolism and significantly alters the remodeling of apoA-II-containing HDL subpopulations.

Effects of different doses of atorvastatin on human apolipoprotein B-100, B-48, and A-I metabolism

Nine hypercholesterolemic and hypertriglyceridemic subjects were enrolled in a randomized, placebo-controlled, double-blind, crossover study to test the effect of atorvastatin 20 mg/day and 80 mg/day on the kinetics of apolipoprotein B-100 (apoB-100) in triglyceride-rich lipoprotein (TRL), intermediate density lipoprotein (IDL), and LDL, of apoB-48 in TRL, and of apoA-I in HDL. Compared with placebo, atorvastatin 20 mg/day was associated with significant reductions in TRL, IDL, and LDL apoB-100 pool size as a result of significant increases in fractional catabolic rate (FCR) without changes in production rate (PR). Compared with the 20 mg/day dose, atorvastatin 80 mg/day caused a further significant reduction in the LDL apoB-100 pool size as a result of a further increase in FCR. ApoB-48 pool size was reduced significantly by both atorvastatin doses, and this reduction was associated with nonsignificant increases in FCR. The lathosterol-campesterol ratio was decreased by atorvastatin treatment, and changes in this ratio were inversely correlated with changes in TRL apoB-100 and apoB-48 PR. No significant effect on apoA-I kinetics was observed at either dose of atorvastatin. Our data indicate that atorvastatin reduces apoB-100- and apoB-48-containing lipoproteins by increasing their catabolism and has a dose-dependent effect on LDL apoB-100 kinetics. Atorvastatin-mediated changes in cholesterol homeostasis may contribute to apoB PR regulation.

No change in apolipoprotein AI metabolism when subcutaneous insulin infusion is replaced by intraperitoneal insulin infusion in type 1 diabetic patients

In type 1 diabetic patients, the replacement of subcutaneous insulin infusion by intraperitoneal insulin infusion restores the normal physiological gradient between the portal vein and the peripheral circulation, which is likely to modify HDL metabolism. This stable isotope kinetic study was designed to compare HDL apolipoprotein (apo) AI metabolism in seven type 1 diabetic patients first treated by continuous subcutaneous insulin infusion by an external pump and then 3 months after the beginning of intraperitoneal insulin infusion by an implantable pump. Glycaemic control was comparable under subcutaneous and intraperitoneal insulin infusion (HbA1c = 7.34 ± 0.94% versus 7.24 ± 1.00%, NS). HDL composition was similar under both insulin regimens (esterified cholesterol = 20.1 ± 2.5% versus 24.0 ± 3.0% (NS), free cholesterol = 3.4 ± 1.1% versus 3.3 ± 0.9% (NS), triglycerides = 2.4 ± 0.9% versus 2.1 ± 0.9% (NS), phospholipids = 22.7 ± 5.3% versus 25.2 ± 6.5% (NS) and proteins = 51.2 ± 6.3% versus 45.5 ± 4.7% (NS)). The replacement of subcutaneous insulin infusion by intraperitoneal insulin infusion induced significant changes neither in apoAI fractional catabolic rate, nor in apoAI production rate, nor in apoAI pool size (respectively, 0.199 ± 0.051 pool d −1 versus 0.211 ± 0.017 pool d −1, 12.0 ± 3.2 mg kg −1 d −1 versus 12.1 ± 1.8 mg kg −1 d −1, 60.4 ± 5.0 mg kg −1 versus 57.5 ± 7.5 mg kg −1). In conclusion, HDL metabolism is not modified by the replacement of subcutaneous insulin infusion by intraperitoneal insulin infusion when glycaemia is well controlled under both insulin regimens. As far as HDL metabolism is concerned there is no advantage in favour of one way of insulin administration or another.

Apolipoprotein C-III isoforms: kinetics and relative implication in lipid metabolism

Apolipoprotein C-III (apoC-III) production rate (PR) is strongly correlated with plasma triglyceride (TG) levels. ApoC-III exists in three different isoforms, according to the sialylation degree of the protein. We investigated the kinetics and respective role of each apoC-III isoform in modulating intravascular lipid/lipoprotein metabolism. ApoC-III kinetics were measured in a sample of 18 healthy men [mean age (+/-SD) 42.1 +/- 9.5 years, body mass index 29.8 +/- 4.6 kg/m2] using a primed-constant infusion of l-(5,5,5-D3) leucine for 12 h. Mono-sialylated and di-sialylated apoC-III (apo-CIII1 and apoC-III2) exhibited similar PRs (means +/- SD, 1.22 +/- 0.49 mg/kg/day vs. 1.15 +/- 0.59 mg/kg/day, respectively) and similar fractional catabolic rates (FCRs) (0.51 +/- 0.13 pool/day vs. 0.61 +/- 0.24 pool/day, respectively). Nonsialylated apoC-III (apoC-III0) had an 80% lower PR (0.25 +/- 0.12 mg/kg/day) and a 60% lower FCR (0.21 +/- 0.07 pool/day) (P < 0.0001 for comparison with CIII1 and CIII2 isoforms). The PRs of apoC-III1 and apoC-III2 were more strongly correlated with plasma TG levels (r > 0.8, P < 0.0001) than was apoC-III0 PR (r = 0.54, P < 0.05). Finally, the PR of apoC-III2 was strongly correlated with the proportion of LDL <255 A (r = 0.72, P = 0.002). These results suggest that all apoC-III isoforms, especially the predominant CIII1 and CIII2 isoforms, contribute to hypertriglyceridemia and that apoC-III2 may play a significant role in the expression of the small, dense LDL phenotype.

Use of Intralipid for kinetic analysis of HDL apoC-III: evidence for a homogeneous kinetic pool of apoC-III in plasma

Apolipoprotein C-III (apoC-III) is an important regulator of lipoprotein metabolism. Radioisotope and stable isotope kinetic studies show differing results in relation to the kinetics of apoC-III in HDL. Kinetic analysis of HDL apoC-III may be difficult because of its low concentration, as well as the presence of other apoproteins at higher concentration, in the HDL fraction. We used Intralipid(R) (IL), known to preferentially extract apoC proteins from plasma, as a means of extracting apoC-III from HDL before apoprotein separation by isoelectric focusing gel electrophoresis for the measurement of tracer enrichment. Protein purity was assessed by an isoleucine-to-leucine (Ile/Leu) ratio, as apoC-III contains no isoleucine. We compared apoC-III kinetics in 14 men using a bolus infusion of deuterated leucine. The Ile/Leu ratio for IL-extracted HDL (IL-HDL) apoC-III (3.0 +/- 0.7%) was not different from that of VLDL apoC-III (2.6 +/- 0.6%) but was significantly lower than that of untreated HDL apoC-III (9.0 +/- 2.9%) (P < 0.001). The isotopic enrichment curves and fractional catabolic rates (FCRs) for IL-HDL apoC-III were not different from those of VLDL apoC-III. In contrast, HDL apoC-III had significantly lower isotopic enrichments and FCRs than IL-HDL apoC-III (P < 0.001). In conclusion, this simple IL method can be used to isolate apoC-III from HDL with minimal interference from other HDL apoproteins, and it demonstrates that the kinetics of apoC-III in VLDL and HDL are similar, supporting the concept of a single kinetically homogeneous pool of apoC-III in plasma.

Delayed In Vivo Catabolism of Intermediate-Density Lipoprotein and Low-Density Lipoprotein in Hemodialysis Patients as Potential Cause of Premature Atherosclerosis

Premature cardiovascular disease is the leading cause of death in patients with end-stage renal disease treated by hemodialysis (HD). Low-density lipoprotein (LDL) levels are not generally increased in HD patients, but their LDL metabolism is still poorly understood. We therefore investigated the in vivo metabolism of apoB-containing lipoproteins in two different ethnic populations of HD patients and controls. We performed stable isotope kinetic studies using a primed constant infusion of deuterated leucine in 12 HD patients and 13 healthy controls. Tracer/tracee ratio of apoB was determined by means of gas chromatography/mass spectrometry, and the modeling program SAAMII was used to estimate the fractional catabolic rate (FCR) of apoB. Mean LDL-apoB plasma concentrations were almost identical in both groups (HD: 95+/-30 mg/dL, controls: 91+/-40 mg/dL), whereas LDL-apoB FCR was 50% lower in HD patients as compared with controls (0.22+/-0.12 days(-1) versus 0.46+/-0.20 days(-1), P=0.001) with concomitantly decreased production rates of LDL. Compared with controls, intermediate-density lipoprotein (IDL)-apoB FCR was 65% lower (2.87+/-1.02 days(-1) versus 8.89+/-4.94 days(-1), P=0.014), accompanied by 1.5-fold higher IDL-apoB levels in HD. Very low-density lipoprotein metabolism was similar in both study groups. In vivo catabolism of LDL and IDL is severely impaired in HD patients but misleadingly masked by normal plasma cholesterol levels. The resulting markedly prolonged residence times of both IDL and LDL particles might thus significantly contribute to the well-documented high risk for premature cardiovascular disease in HD patients.

Lipoprotein transport in the metabolic syndrome: pathophysiological and interventional studies employing stable isotopy and modelling methods.

The accompanying review in this issue of Clinical Science [Chan, Barrett and Watts (2004) Clin. Sci. 107, 221-232] presented an overview of lipoprotein physiology and the methodologies for stable isotope kinetic studies. The present review focuses on our understanding of the dysregulation and therapeutic regulation of lipoprotein transport in the metabolic syndrome based on the application of stable isotope and modelling methods. Dysregulation of lipoprotein metabolism in metabolic syndrome may be due to a combination of overproduction of VLDL [very-LDL (low-density lipoprotein)]-apo (apolipoprotein) B-100, decreased catabolism of apoB-containing particles and increased catabolism of HDL (high-density lipoprotein)-apoA-I particles. These abnormalities may be consequent on a global metabolic effect of insulin resistance, partly mediated by depressed plasma adiponectin levels, that collectively increases the flux of fatty acids from adipose tissue to the liver, the accumulation of fat in the liver and skeletal muscle, the hepatic secretion of VLDL-triacylglycerols and the remodelling of both LDL (low-density lipoprotein) and HDL particles in the circulation. These lipoprotein defects are also related to perturbations in both lipolytic enzymes and lipid transfer proteins. Our knowledge of the pathophysiology of lipoprotein metabolism in the metabolic syndrome is well complemented by extensive cell biological data. Nutritional modifications may favourably alter lipoprotein transport in the metabolic syndrome by collectively decreasing the hepatic secretion of VLDL-apoB and the catabolism of HDL-apoA-I, as well as by potentially increasing the clearance of LDL-apoB. Several pharmacological treatments, such as statins, fibrates or fish oils, can also correct the dyslipidaemia by diverse kinetic mechanisms of action, including decreased secretion and increased catabolism of apoB, as well as increased secretion and decreased catabolism of apoA-I. The complementary mechanisms of action of lifestyle and drug therapies support the use of combination regimens in treating dyslipoproteinaemia in subjects with the metabolic syndrome.

Lipoprotein transport in the metabolic syndrome: methodological aspects of stable isotope kinetic studies.

The metabolic syndrome encapsulates visceral obesity, insulin resistance, diabetes, hypertension and dyslipidaemia. Dyslipidaemia is a cardinal feature of the metabolic syndrome that accelerates the risk of cardiovascular disease. It is usually characterized by high plasma concentrations of triacylglycerol (triglyceride)-rich and apoB (apolipoprotein B)-containing lipoproteins, with depressed concentrations of HDL (high-density lipoprotein). However, lipoprotein metabolism is complex and abnormal plasma concentrations can result from alterations in the rates of production and/or catabolism of these lipoprotein particles. Our in vivo understanding of kinetic defects in lipoprotein metabolism in the metabolic syndrome has been achieved chiefly by ongoing developments in the use of stable isotope tracers and mathematical modelling. This review deals with the methodological aspects of stable isotope kinetic studies. The design of in vivo turnover studies requires considerations related to stable isotope tracer administration, duration of sampling protocol and interpretation of tracer data, all of which are critically dependent on the kinetic properties of the lipoproteins under investigation. Such models provide novel insight that further understanding of metabolic disorders and effects of treatments. Future investigations of the pathophysiology and therapy of the dyslipoproteinaemia of the metabolic syndrome will require the development of novel kinetic methodologies. Specifically, new stable isotope techniques are required for investigating in vivo the turnover of the HDL subpopulation of particles, as well as the cellular efflux of cholesterol into the extracellular space and its subsequent transport in plasma and metabolic fate in the liver.

Lipoprotein Metabolism in Obesity and Diabetes: Insights from Stable Isotope Kinetic Studies in Humans

Lipoprotein Metabolism in Obesity and Diabetes: Insights from Stable Isotope Kinetic Studies in Humans

Lipoprotein metabolism in obesity and diabetes: insights from stable isotope kinetic studies in humans.

The paradigm of obesity leading to insulin resistance and type 2 diabetes is examined in relation to dyslipidemia, which typically consists of high levels of triglycerides (TG) and low levels of high-density lipoprotein (HDL). Kinetic studies with stable isotope-labeled amino acid precursors have shown that the development of visceral obesity, as well as type 2 diabetes, leads to overproduction of the apolipoprotein B-100 and TG in very low-density lipoproteins. Elevated plasma levels or increased flux of albumin-bound free fatty acids to the liver appear to be underlying metabolic events in this process. Low levels of HDL are due to increased catabolism, which can be related to TG enrichment.

Lipoprotein Metabolism in Obesity and Diabetes: Insights from Stable Isotope Kinetic Studies in Humans

Lipoprotein Metabolism in Obesity and Diabetes: Insights from Stable Isotope Kinetic Studies in Humans

Increased VLDL-apoB and IDL-apoB production rates in nonlipodystrophic HIV-infected patients on a protease inhibitor-containing regimen: a stable isotope kinetic study

The aim of this study was to identify the first abnormalities of apolipoprotein B (apoB) metabolism in HIV-infected patients treated by antiretroviral therapy (ART) with protease inhibitors (PIs). The influence of ART on the metabolism of apoB in VLDL, IDL, and LDL was investigated in six patients receiving dual nucleoside reverse transcriptase inhibitors (NRTIs) and PI, and in five patients receiving NRTI and nevirapine. None of the patients had lipodystrophy. The study was performed in the fed state. Each subject received an intravenous injection of a 0.7 mg.kg-1 bolus of l-[1-13C]leucine, immediately followed by a 16 h constant infusion at 0.7 mg.kg-1.h-1. The VLDL- and IDL-apoB concentrations were significantly higher in PI-treated patients compared to non-PI-treated patients. The VLDL-apoB and IDL-apoB production rates were markedly higher in PI-treated patients compared to non-PI-treated patients (54.5 +/- 30.1 vs. 30.9 +/- 8.4 mg.kg-1.d-1, P = 0.04; and 43.5 +/- 20.0 vs. 18.7 +/- 7.8 mg.kg-1.d-1, P = 0.04, respectively). In conclusion, our study shows that patients receiving ART with PI present altered metabolism of the VLDL-IDL-LDL chain compared with patients treated without PI. These data confirm that PI therapy is associated with a physiopathological mechanism for dyslipidemia in addition to the effect of lipodystrophy on lipid metabolism.