Effects of atorvastatin versus fenofibrate on apoB-100 and apoA-I kinetics in mixed hyperlipidemia
2004; Elsevier BV; Volume: 45; Issue: 1 Linguagem: Inglês
10.1194/jlr.m300309-jlr200
ISSN1539-7262
AutoresStefan Bilz, Stephan N. Wagner, Michaela Schmitz, Andrea Bedynek, Ulrich Keller, Thomas Demant,
Tópico(s)Computational Drug Discovery Methods
ResumoKinetics of apo B and apo AI were assessed in 8 patients with mixed hyperlipidemia at baseline and after 8 weeks of atorvastatin 80 mg q.d. and micronised fenofibrate 200 mg q.d. in a cross-over study. Both increased hepatic production and decreased catabolism of VLDL accounted for elevated cholesterol and triglyceride concentrations at baseline. Atorvastatin significantly decreased triglyceride, total, VLDL and LDL cholesterol and apo B concentrations (−65%, −36%, −57%, −40% and −33%, respectively, P < 0.05). Kinetic analysis revealed that atorvastatin stimulated the catabolism of apo B containing lipoproteins, enhanced the delipidation of VLDL1 and decreased VLDL1 production. Fenofibrate lowered triglycerides and VLDL cholesterol (−57% and −64%, respectively, P < 0.05) due to enhanced delipidation of VLDL1 and VLDL2 and increased VLDL1 catabolism. Changes of HDL particle composition accounted for the increase of HDL cholesterol during atorvastatin and fenofibrate (18% and 23%, P < 0.01). Only fenofibrate increased apo AI concentrations through enhanced apo AI synthesis (45%, P < 0.05).We conclude that atorvastatin exerts additional beneficial effects on the metabolism of apo B containing lipoproteins unrelated to an increase in LDL receptor activity. Fenofibrate but not atorvastatin increases apo AI production and plasma turnover. Kinetics of apo B and apo AI were assessed in 8 patients with mixed hyperlipidemia at baseline and after 8 weeks of atorvastatin 80 mg q.d. and micronised fenofibrate 200 mg q.d. in a cross-over study. Both increased hepatic production and decreased catabolism of VLDL accounted for elevated cholesterol and triglyceride concentrations at baseline. Atorvastatin significantly decreased triglyceride, total, VLDL and LDL cholesterol and apo B concentrations (−65%, −36%, −57%, −40% and −33%, respectively, P < 0.05). Kinetic analysis revealed that atorvastatin stimulated the catabolism of apo B containing lipoproteins, enhanced the delipidation of VLDL1 and decreased VLDL1 production. Fenofibrate lowered triglycerides and VLDL cholesterol (−57% and −64%, respectively, P < 0.05) due to enhanced delipidation of VLDL1 and VLDL2 and increased VLDL1 catabolism. Changes of HDL particle composition accounted for the increase of HDL cholesterol during atorvastatin and fenofibrate (18% and 23%, P < 0.01). Only fenofibrate increased apo AI concentrations through enhanced apo AI synthesis (45%, P < 0.05). We conclude that atorvastatin exerts additional beneficial effects on the metabolism of apo B containing lipoproteins unrelated to an increase in LDL receptor activity. Fenofibrate but not atorvastatin increases apo AI production and plasma turnover. Mixed hyperlipidemia, i.e., the increase of both plasma total cholesterol and triglycerides, refers to an etiologically heterogeneous lipoprotein phenotype that occurs in both primary and secondary dyslipidemias. The usually associated decrease of HDL cholesterol and the shift in the LDL size profile toward small, dense particles further contribute to the considerable cardiovascular risk in affected patients (1Grundy S.M. Atherogenic dyslipidemia: lipoprotein abnormalities and implications for therapy.Am. J. Cardiol. 1995; 75: 45B-52BAbstract Full Text PDF PubMed Scopus (53) Google Scholar).Among the currently available lipid-lowering compounds, HMG-CoA reductase inhibitors, also referred to as statins, and fibrates have been proven to effectively decrease cardiovascular morbidity and mortality in several large-scale primary and secondary prevention trials during the past two decades (2Frick M.H. Elo O. Haapa K. Heinonen O.P. Heinsalmi P. Helo P. Huttunen J.K. Kaitaniemi P. Koskinen P. Manninen V. Helsinki Heart Study: primary-prevention trial with gemfibrozil in middle-aged men with dyslipidemia. Safety of treatment, changes in risk factors, and incidence of coronary heart disease.N. Engl. J. Med. 1987; 317: 1237-1245Crossref PubMed Scopus (3407) Google Scholar, 3The Scandinavian Simvastatin Survival Study Group Randomised trial of cholesterol lowering in 4444 patients with coronary heart disease: the Scandinavian Simvastatin Survival Study (4S).Lancet. 1994; 344: 1383-1389PubMed Google Scholar, 4Shepherd J. Cobbe S.M. Ford I. Isles C.G. Lorimer A.R. MacFarlane P.W. McKillop J.H. Packard C.J. Prevention of coronary heart disease with pravastatin in men with hypercholesterolemia. West of Scotland Coronary Prevention Study Group.N. Engl. J. Med. 1995; 333: 1301-1307Crossref PubMed Scopus (7448) Google Scholar, 5Sacks F.M. Pfeffer M.A. Moye L.A. Rouleau J.L. Rutherford J.D. Cole T.G. 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In patients with mixed hyperlipidemia and moderately to severely increased plasma triglycerides, therapy with either compound may result in favorable changes of the lipoprotein pattern. Classic studies demonstrated that the dose-dependent decreases in plasma total and LDL cholesterol levels during competitive inhibition of HMG-CoA reductase by statin therapy are secondary to enhanced LDL receptor-mediated lipoprotein catabolism (10Ma P.T. Gil G. Sudhof T.C. Bilheimer D.W. Goldstein J.L. Brown M.S. Mevinolin, an inhibitor of cholesterol synthesis, induces mRNA for low density lipoprotein receptor in livers of hamsters and rabbits.Proc. Natl. Acad. Sci. USA. 1986; 83: 8370-8374Crossref PubMed Scopus (286) Google Scholar). More recently, the triglyceride-lowering capacity of statins has gained additional attention, and atorvastatin has been found to be particularly efficacious, most likely secondary to its longer duration of action and its enhanced lipid-lowering potency (11Bakker-Arkema R.G. Davidson M.H. Goldstein R.J. Davignon J. Isaacsohn J.L. Weiss S.R. Keilson L.M. Brown W.V. Miller V.T. Shurzinske L.J. Black D.M. Efficacy and safety of a new HMG-CoA reductase inhibitor, atorvastatin, in patients with hypertriglyceridemia.J. Am. Med. Assoc. 1996; 275: 128-133Crossref PubMed Google Scholar, 12Naoumova R.P. Dunn S. Rallidis L. Abu-Muhana O. Neuwirth C. Rendell N.B. Taylor G.W. Thompson G.R. Prolonged inhibition of cholesterol synthesis explains the efficacy of atorvastatin.J. Lipid Res. 1997; 38: 1496-1500Abstract Full Text PDF PubMed Google Scholar). Enhanced catabolism of apolipoprotein B (apoB)-containing triglyceride-rich lipoproteins via the LDL receptor pathway may in part explain the decrease in plasma triglycerides elicited by statins. However, data obtained from patients with LDL receptor-negative homozygous familial hypercholesterolemia and animal models clearly suggest that statins may also interfere with hepatic lipoprotein production (13Burnett J.R. Wilcox L.J. Telford D.E. Kleinstiver S.J. Barrett P.H. Newton R.S. Huff M.W. Barrett P. Inhibition of HMG-CoA reductase by atorvastatin decreases both VLDL and LDL apolipoprotein B production in miniature pigs.Arterioscler. Thromb. Vasc. Biol. 1997; 17: 2589-2600Crossref PubMed Scopus (94) Google Scholar, 14Marais A.D. Naoumova R.P. Firth J.C. Penny C. Neuwirth C.K. Thompson G.R. Decreased production of low density lipoprotein by atorvastatin after apheresis in homozygous familial hypercholesterolemia.J. Lipid Res. 1997; 38: 2071-2078Abstract Full Text PDF PubMed Google Scholar). The effects of fibrates, among them fenofibrate, on plasma lipids have been found to result from the activation of the peroxisome proliferator-activated receptor α (PPARα) (15Schoonjans K. Staels B. Auwerx J. Role of the peroxisome proliferator-activated receptor (PPAR) in mediating the effects of fibrates and fatty acids on gene expression.J. Lipid Res. 1996; 37: 907-925Abstract Full Text PDF PubMed Google Scholar). Specifically, fibrates have been demonstrated to reduce hepatic triglyceride production, to enhance delipidation and clearance of triglyceride-rich lipoproteins, and to activate reverse cholesterol transport (16Staels B. Dallongeville J. Auwerx J. Schoonjans K. Leitersdorf E. Fruchart J.C. Mechanism of action of fibrates on lipid and lipoprotein metabolism.Circulation. 1998; 98: 2088-2093Crossref PubMed Scopus (1393) Google Scholar). These alterations in lipid metabolism are associated with a profound decrease of plasma triglycerides, increased HDL cholesterol levels, and a favorable shift in the density distribution of LDL toward buoyant, less-atherogenic particles, whereas the effects on LDL cholesterol levels may vary. However, to our knowledge, data regarding the comparative effects of statins and fibrates on lipoprotein metabolism in the subset of patients with mixed hyperlipidemia and moderately to severely increased plasma triglyceride levels are not available from the literature. Therefore, we performed apoB and apolipoprotein A-I (apoA-I) kinetic studies using endogenous labeling of apoB and apoA-I with deuterated leucine (d3-leucine) and multicompartmental modeling to derive lipoprotein production and catabolic rates in the basal state and after 8 weeks of therapy with either atorvastatin or fenofibrate.METHODSSubjectsEight male patients (age 51 ± 4 years, mean body weight 86 ± 4 kg) with increased fasting triglyceride (3.7–26 mmol/l) and normal to increased LDL cholesterol (2.4–5.9 mmol/l) levels were selected from the lipid outpatient clinic at Basel University Hospital. Eight male subjects with normal fasting plasma triglyceride levels matched for age (48 ± 3 years) and weight (86 ± 6 kg) served as controls. The data for the control subjects were taken from a previous publication (17Packard C.J. Demant T. Stewart J.P. Bedford D. Caslake M.J. Schwertfeger G. Bedynek A. Shepherd J. Seidel D. Apolipoprotein B metabolism and the distribution of VLDL and LDL subfractions.J. Lipid Res. 2000; 41: 305-318Abstract Full Text Full Text PDF PubMed Google Scholar). ApoA-I kinetic data for hyperlipidemic patients at baseline were compared with data from a group of healthy controls (age 25 ± 0.4 years, mean body weight 73 ± 2 kg). Plasma lipids and lipoproteins were analyzed according to standard procedures (18Lipid Research Clinics Program Manual of Laboratory Operations, Volume 1: Lipid and Lipoprotein Analysis. DHES Publications No. (NIH), Bethesda, MD1974: 75-268Google Scholar). None of the study participants had diabetes mellitus, cholestasis, nephrotic syndrome, pancreatitis, primary hypothyroidism, renal or hepatic dysfunction, or chronic alcoholism. One patient (patient 7) suffering from panhypopituitarism was on stable replacement doses of levothyroxine, cortisol, recombinant human growth hormone, and testosterone throughout the study. Immunosuppressive agents, imidazole antimycotics, macrolide antibiotics, β blocking agents, diuretics, and isotretinoin were not allowed during the study period, nor were lipid-lowering agents other than the study medication.Study designThe study used an open, randomized, crossover design that consisted of two treatment periods of 8 weeks each. After discontinuing all preexisting lipid-lowering drugs, patients entered a 6 week run-in period and underwent the baseline apoB turnover study during the last 2 weeks of this period. Thereafter, they were randomized to receive either atorvastatin (80 mg/d) or micronized fenofibrate (200 mg/d) for 8 weeks together with their evening meal. After a washout period of 4 weeks, patients were switched to the other study drug. ApoB turnover studies were performed during the last 2 weeks of both treatment phases.All patients received dietary counseling by a dietitian immediately after inclusion into the study protocol and two times thereafter, once during each treatment period. They were asked to follow an isoenergetic diet containing 50% to 55% carbohydrates, 15% to 20% proteins, and 30% fats, with less than 300 mg of cholesterol and less than 10% of total energy as saturated fatty acids. Alcohol consumption of one drink or less per day was allowed, and patients were asked to refrain from vigorous physical exercise during the study period. Compliance with the dietary regimen was assessed by detailed food protocols of 3 days, including 1 weekend day, during each study period.Study medicationThe Clinical Pharmaceutical Operations Department of Warner Lambert provided 40 mg atorvastatin tablets. Micronized fenofibrate capsules (200 mg) were purchased from Fournier Pharmaceutical Company (Schwarz Pharma AG, Switzerland). Patients were instructed to take two atorvastatin 40 mg tablets or one micronized fenofibrate 200 mg capsule together with their evening meal during the treatment periods. Compliance with the study medication was assessed by pill counting. The lipid-lowering drugs were well tolerated in all but one patient, who developed a macular rash during fenofibrate therapy and therefore discontinued the drug.Turnover protocolThe detailed procedures for conducting the turnover study have been reported previously (19Demant T. Packard C.J. Demmelmair H. Stewart P. Bedynek A. Bedford D. Seidel D. Shepherd J. Sensitive methods to study human apolipoprotein B metabolism using stable isotope-labeled amino acids.Am. J. Physiol. 1996; 270: E1022-E1036Crossref PubMed Google Scholar). In brief, subjects fasted for 11 h overnight before starting the study at 7 AM in a metabolic ward. After insertion of intravenous lines for tracer administration and sample collection, controls received either an intravenous bolus injection of d3-leucine (6.0 mg/kg body weight) or a primed constant infusion (0.6 mg/kg, followed by 0.6 mg/kg per h for 10 h). Patients received a modified primed constant infusion (1.2 mg/kg, followed by 1.2 mg/kg per h for 5 h), which allowed for a better definition of VLDL1 tracer enrichment. The total dosage of tracer was nearly identical in all studies, and the three dosage schemes had been compared in previous studies, including the use of dual tracer applications (19Demant T. Packard C.J. Demmelmair H. Stewart P. Bedynek A. Bedford D. Seidel D. Shepherd J. Sensitive methods to study human apolipoprotein B metabolism using stable isotope-labeled amino acids.Am. J. Physiol. 1996; 270: E1022-E1036Crossref PubMed Google Scholar, 20Schmitz M. Michl G.M. Walli R. Bogner J. Bedynek A. Seidel D. Goebel F.D. Demant T. Alterations of apolipoprotein B metabolism in HIV-infected patients with antiretroviral combination therapy.J. Acquired Immune Defic. Syndr. 2001; 26: 225-235Crossref PubMed Scopus (53) Google Scholar). In the first 10 h period, patients and controls continued to fast but were allowed noncaloric drinks and remained ambulatory in the metabolic ward. At 6 PM, all study participants were allowed to leave the hospital. Blood samples were collected in EDTA-containing tubes before administration of tracer and thereafter at 0.16, 0.33, 0.5, 0.75, 1, 2, 4, 6, 8, 10, 12, 15, and 24 h in controls and at 0.25, 0.5, 1, 2, 3, 4, 5, 5.33, 5.66, 6, 7, 10, and 24 h in patients. The additional samples taken in patients at 5, 5.33, and 5.66 h were necessary because of the rapid decline of plasma d3-leucine concentrations after stopping the tracer infusion. Further fasting samples were obtained at approximately 8 AM daily for the next 10–14 days.Lipoprotein isolation and preparation of apoBThe preparation of VLDL subfractions VLDL1 [Svedberg flotation unit (Sf) 60–400] and VLDL2 (Sf 20–60), intermediate density lipoprotein (IDL; Sf 12–20), LDL (Sf 0–12), LDL1 (Sf 6–12), and LDL2 (Sf 0–6) has been described previously and is based on the procedure of Lindgren, Jensen, and Hatch (19Demant T. Packard C.J. Demmelmair H. Stewart P. Bedynek A. Bedford D. Seidel D. Shepherd J. Sensitive methods to study human apolipoprotein B metabolism using stable isotope-labeled amino acids.Am. J. Physiol. 1996; 270: E1022-E1036Crossref PubMed Google Scholar, 21Lindgren F.T. Jensen L.C. Hatch F.T. The Isolation and Quantitation Analysis of Serum Lipoproteins. Wiley-Interscience, New York1972: 181-274Google Scholar). Briefly, 2 ml of plasma was adjusted to a density (d) of 1.118 g/ml by the addition of 0.03410 g of solid NaCl and layered over a 0.5 ml cushion of d = 1.182 g/ml NaBr solution in a Beckman SW 40 rotor tube. A discontinuous six-step salt gradient was constructed above this, and VLDL1, VLDL2, and IDL were harvested as described (19Demant T. Packard C.J. Demmelmair H. Stewart P. Bedynek A. Bedford D. Seidel D. Shepherd J. Sensitive methods to study human apolipoprotein B metabolism using stable isotope-labeled amino acids.Am. J. Physiol. 1996; 270: E1022-E1036Crossref PubMed Google Scholar). Finally, LDL1 and LDL2 were isolated after further centrifugation at 36,000 rpm for 5 h, 6 min and at 32,000 rpm for 12 h, 9 min, respectively. Total LDL was reconstituted by mixing equal volumes of LDL1 and LDL2. From the infranatant, the HDL fraction was prepared after adjusting density to 1.21 g/ml by centrifugation at 40,000 rpm for 16 h in a Beckman 50.4 Ti rotor. From each apoB-containing lipoprotein fraction, apoB was precipitated by the addition of an equal volume of isopropanol at room temperature (22Egusa G. Brady D.W. Grundy S.M. Howard B.V. Isopropanol precipitation method for the determination of apolipoprotein B specific activity and plasma concentrations during metabolic studies of very low density lipoprotein and low density lipoprotein apolipoprotein B.J. Lipid Res. 1983; 24: 1261-1267Abstract Full Text PDF PubMed Google Scholar). The pellet was delipidated with ethanol-ether (3:1) and dried with ether until apoB remained as a fine white protein pellet. HDL apoA-I was prepared by SDS-PAGE using an apoA-I standard, and apoA-I-containing bands were cut from the gel.Preparation and analysis of leucine in apoB, apoA-I, and plasmaApoB and apoA-I were hydrolyzed in glass tubes (Schott, Mainz, Germany) in the presence of 0.5–1.0 ml of 6 N HCl at 110°C for 20–24 h. The amino acid hydrolysate was concentrated in a vacuum concentrator centrifuge (Univapo 150 H; Uniequip, Martinsried/Munich, Germany) and aliquoted into microvials (Chromacol, Herts, UK). After complete removal of HCl, samples were ready for derivatization and mass spectrometric analysis.Proteins were precipitated from 1 ml of plasma by adding 1 ml of TCA (10%), and amino acids were prepared from the supernatant by cation-exchange chromatography using 2 ml columns filled with Dowex AG-50W-X8 resin (H+-form, 50–100 mesh; Bio-Rad, Richmond, CA). The amino acids that bound to the resin were desorbed by 4 M NH4OH, which was subsequently removed by evaporation in a vacuum pump (Univapo 150 H), transferred into microvials, and dried again for derivatization.The method used for the analysis of d3-leucine enrichment in protein hydrolysates and plasma amino acids is presented in detail elsewhere (19Demant T. Packard C.J. Demmelmair H. Stewart P. Bedynek A. Bedford D. Seidel D. Shepherd J. Sensitive methods to study human apolipoprotein B metabolism using stable isotope-labeled amino acids.Am. J. Physiol. 1996; 270: E1022-E1036Crossref PubMed Google Scholar). Ion mass fragments at m/z 277, 276, and 274 were monitored by selective ion recording. The m/z 277:276 ratio showed a linear relationship with isotopic enrichment over the range 0.0–10.0% atom percent excess. The ratio of m/z 277:276 was multiplied by an average value for the constant ratio of m/z 276:274 (determined repeatedly throughout the analytical run, it shows no change over the range 0.0–10% atom percent excess), and the resulting m/z 277:274 values were used to calculate specific isotopic enrichments and tracer-tracee ratios (19Demant T. Packard C.J. Demmelmair H. Stewart P. Bedynek A. Bedford D. Seidel D. Shepherd J. Sensitive methods to study human apolipoprotein B metabolism using stable isotope-labeled amino acids.Am. J. Physiol. 1996; 270: E1022-E1036Crossref PubMed Google Scholar).Measurement of apoB and apoA-I masses in lipoprotein fractionsThe apoB content of VLDL1, VLDL2, IDL, and LDL was determined as the difference between total and isopropanol-soluble protein measured by the procedure of Lowry et al. (23Lowry O. Rosebrough N. Farr A. Randall R. Protein measurement with the Folin phenol reagent.J. Biol. Chem. 1951; 193: 265-275Abstract Full Text PDF PubMed Google Scholar). Because more than 90% of plasma apoA-I is associated with HDL, the plasma apoA-I concentration was considered to equal the HDL apoA-I concentration. From apoB and apoA-I concentrations (in milligrams per milliliter of plasma) and an estimate of the plasma volume (4% of the body weight), apoB and apoA-I plasma pools were determined for the lipoprotein fractions of interest. The leucine content of the apoB and apoA-I pools was then calculated assuming leucine contents of 12.12% and 15.68%, respectively.Kinetic analyses and multicompartmental modelingThe change in tracer-tracee ratios with time in plasma and in apoB in the four apoB-containing lipoprotein fractions, together with the measured apoB pool size for VLDL1, VLDL2, IDL, and LDL, was used as the data set for the derivation of apoB kinetic parameters. Figure 1Ashows the multicompartmental model of apoB metabolism, which was constructed using the SAAM II modeling program (SAAM Institute, Seattle, WA), the development of which has been described (19Demant T. Packard C.J. Demmelmair H. Stewart P. Bedynek A. Bedford D. Seidel D. Shepherd J. Sensitive methods to study human apolipoprotein B metabolism using stable isotope-labeled amino acids.Am. J. Physiol. 1996; 270: E1022-E1036Crossref PubMed Google Scholar). Briefly, its basic features include a four-compartment representation of free leucine kinetics (compartments 1–4), a sequence of lipoprotein compartments accounting for the stepwise delipidation of VLDL1 through VLDL2 and IDL to LDL (compartments 6, 7, 9, 10, 12, and 14), plus the three remnant compartments 8, 11, and 13 for VLDL1, VLDL2, and IDL particles that are removed directly from plasma. Free leucine and the apoB-containing compartments 6, 9, 12, and 14 in the VLDL1, VLDL2, IDL, and LDL density range are linked via delay compartment 5, which is set at 0.5 h, the time required for apoB biosynthesis. Compartment 15 allows for some intravascular/extravascular exchange of LDL, which is not observed for less-dense lipoproteins. Typical examples of the time courses of leucine tracer-tracee ratios measured in the four apoB-containing lipoproteins VLDL1, VLDL2, IDL, and LDL at baseline and during therapy with each of the study drugs are shown in Fig. 2A. Similarly, apoA-I kinetic data were derived from changes in the tracer-tracee ratios in HDL apoA-I over time and in the apoA-I pool size in this lipoprotein fraction. Figure 2B shows a typical example of the time course of the leucine tracer-tracee ratio in HDL-associated apoA-I. The model to describe HDL apoA-I metabolism is shown in Fig. 1B and has been published previously (24Gylling H. Vega G.L. Grundy S.M. Physiologic mechanisms for reduced apolipoprotein A-I concentrations associated with low levels of high density lipoprotein cholesterol in patients with normal plasma lipids.J. Lipid Res. 1992; 33: 1527-1539Abstract Full Text PDF PubMed Google Scholar). As for the apoB model, compartments 1–4 describe plasma leucine metabolism. An intrahepatic delay compartment accounts for hepatic synthesis and the secretion of apoA-I and is connected to a single intravascular compartment (compartment 6), from which apoA-I is cleared. The exchange of apoA-I with an extracellular pool is accounted for by the addition of compartment 7 to the model.Fig. 1A: Multicompartmental model of apolipoprotein B (apoB) metabolism. Because plasma-free tracer kinetics are fully accounted for, both bolus and primed constant infusion data could be analyzed using the same model. Plasma leucine is represented by a compartment (compartment 1) that received the tracer and distributed it to body protein pools (compartments 3 and 4) and an intracellular compartment (compartment 2) that was the precursor to apoB synthesis. Compartment 5 represents an intrahepatic pool accounting for the delay (0.5 h) associated with the synthesis of apoB, lipoprotein assembly, and secretion. Compartments 6–9, 12, and 14 form a delipidation chain, and tracer appeared throughout this chain, i.e., in VLDL1 (compartment 6), VLDL2 (compartment 9), intermediate density lipoprotein (IDL; compartment 12), and LDL (compartment 14). Compartments 8, 11, and 13 represent lipoprotein remnants that are directly removed from the circulation. Compartment 15 accounts for an extravascular exchange of LDL, which does not occur for the other apoB-containing lipoproteins. A detailed description of the model is given in ref. (19Demant T. Packard C.J. Demmelmair H. Stewart P. Bedynek A. Bedford D. Seidel D. Shepherd J. Sensitive methods to study human apolipoprotein B metabolism using stable isotope-labeled amino acids.Am. J. Physiol. 1996; 270: E1022-E1036Crossref PubMed Google Scholar). B: Multicompartmental model of apolipoprotein A-I (apoA-I) metabolism. Plasma leucine is represented by a compartment (compartment 1) that received the tracer and distributed it to body protein pools (compartments 3 and 4) and an intracellular pool (compartment 2). Compartment 5 represents an intrahepatic pool accounting for the delay (0.15 h) associated with the synthesis of apoA-I, lipoprotein assembly, and secretion. All tracer input into HDL apoA-I occurs in a single compartment (compartment 6) from which apoA-I is cleared. Compartment 7 accounts for an extravascular HDL pool.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Fig. 2Tracer-tracee ratios recorded after a primed constant infusion of deuterated leucine as a metabolic tracer. ApoB from VLDL1, VLDL2, IDL, and LDL (A) and apoA-I from HDL (B) at baseline and during atorvastatin and fenofibrate treatment are shown from patient 7.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Fig. 2Tracer-tracee ratios recorded after a primed constant infusion of deuterated leucine as a metabolic tracer. ApoB from VLDL1, VLDL2, IDL, and LDL (A) and apoA-I from HDL (B) at baseline and during atorvastatin and fenofibrate treatment are shown from patient 7.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Ethical considerationsAll subjects participating in this study gave written informed consent. The study was approved by the Ethics Committee of Basel University Hospital.Statistical analysesAll statistical analyses were performed using Statistica 6 software (StatSoft, Inc., Tulsa, OK). Values are given as means ± SEM. Plasma lipids and lipoproteins during different treatments were analyzed using Student's t-test. The effects of treatments on lipoprotein kinetics data were assessed using the Wilcoxon test. Pearson correlation coefficients were used to express the relation between kinetic variables. Comparisons between patients at baseline and controls were performed using the Kruskal-Wallis test.RESULTSLipids and apoB and apoA-I kinetic data of patients and controls at baselineTable 1 demonstrates that both plasma total cholesterol and triglycerides were increased markedly in patients (7.23 ± 1.39 and 8.84 ± 7.38 mmol/l, respectively). Whereas the increase in total cholesterol was secondary to the increased VLDL cholesterol levels in most patients, some presented also with increased LDL cholesterol. Compared with patients, age- and sex-matched controls showed significantly lower plasma triglyceride concentrations and VLDL cholesterol levels. Total and LDL cholesterol levels, however, did not differ significantly. HDL cholesterol levels were decreased compared with those of both ag
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