Postprandial plasma lipoprotein changes in human subjects of different ages

Plasma lipoprotein changes were monitored for 12 hr after a fat-rich meal (1 g of fat/kg body weight) in 22 subjects (9 males, 13 females, 22-79 yr old). Plasma triglyceride, measured hourly, peaked once in some subjects, but twice or three times in others. The magnitude of postprandial triglyceridemia varied considerably between subjects (range: 650-4082 mg.hr/dl). Males tended to have greater postprandial triglyceridemia than females, and elderly subjects had significantly (P less than 0.05) greater postprandial triglyceridemia than younger subjects. Total plasma cholesterol, measured every three hr, increased significantly (6.0 +/- 2.1%) in 7 subjects, decreased significantly (7.1 +/- 1.2%) in 10 subjects, and remained unchanged in the remainder. Single spin ultracentrifugation and dextran sulfate precipitation procedures were used to quantitate triglyceride and cholesterol in triglyceride-rich lipoproteins (TRL, d less than 1.006 g/ml), low density lipoproteins (LDL), and high density lipoproteins (HDL). Plasma TRL and HDL triglyceride increased after the fat meal, while LDL triglyceride decreased at 3 hr but increased at 9 and 12 hr. TRL cholesterol increased postprandially, while LDL and HDL cholesterol decreased. Phospholipid (PL), free (FC) and esterified (EC) cholesterol measurements were carried out on the plasma and lipoprotein fractions of 8 subjects. Plasma PL increased significantly at 3, 6, and 9 hr after the fat-rich meal, due to increases in TRL and HDL PL. TRL CE increased postprandially, but a greater decrease in LDL and HDL CE caused plasma CE to be decreased. Plasma FC increased, predominantly due to an increase in TRL FC. Plasma concentrations of apolipoprotein A-I and apolipoprotein B both decreased after the fat-rich meal. The magnitude of postprandial triglyceridemia was inversely correlated with HDL cholesterol levels (r = -0.502, P less than 0.05) and positively correlated with age (r = -0.449, P less than 0.05), fasting levels of plasma triglyceride (r = 0.636, P less than 0.01), plasma apoB (r = 0.510, P less than 0.05), TRL triglyceride (r = 0.564, P less than 0.01), TRL cholesterol (r = 0.480, P less than 0.05) and LDL triglyceride (r = 0.566, P less than 0.01). Change in postprandial cholesterolemia was inversely correlated with fasting levels of HDL cholesterol (r = -0.451, P less than 0.05) and plasma apoA-I (r = -0.436, P less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)     

Sources and properties of triglyceride-rich lipoproteins containing apoB-48 and apoB-100 in postprandial blood plasma of patients with primary combined hyperlipidemia

Although editing of apolipoprotein (apo)B in the small intestine, yielding apoB-48, is thought to be nearly complete in adult humans, small amounts of intestinal apoB-100 may also be produced. We have evaluated the fraction of unedited apoB secreted from the intestine postprandially in subjects with primary combined hyperlipidemia, a disorder in which secretion of apoB-100 into the blood is increased. Three hours after these subjects and healthy controls were fed a fat-rich meal containing retinol, the distribution of retinyl esters (RE) between plasma triglyceride-rich lipoprotein (TRL) fractions containing apoB-100 and apoB-48 was measured under conditions minimizing transfer of RE between lipoprotein particles. The estimated maximal percentage of unedited intestinal apoB-100 (approximately 3%) was not increased in subjects with primary combined hyperlipidemia, suggesting that reduced editing of intestinal mRNA does not contribute to the pathogenesis of this disorder. Postprandially, the triglyceride content of TRL containing apoB-48 more than doubled, leading to a 20% increase in mean diameter, yet the surface concentration of phospholipids and soluble apolipoproteins (apoE and total apoC) was unchanged. Furthermore, the surface concentrations of these components did not differ among TRL containing apoB-48 and two smaller fractions of apoB-100 TRL with distinct immunoreactivities. These findings suggest that available surface area is a major determinant of the particle content of each of these surface components of TRL species of differing size and origin.     

Evidence of increased secretion of apolipoprotein B-48-containing lipoproteins in subjects with type 2 diabetes

Patients with type 2 diabetes have high levels of triglyceride-rich lipoproteins (TRLs), including apolipoprotein B-48 (apoB-48)-containing TRLs of intestinal origin, but the mechanism leading to overaccumulation of these lipoproteins remains to be fully elucidated. Therefore, the objective of this study was to examine the in vivo kinetics of TRL apoB-48 and VLDL, intermediate density lipoprotein (IDL), and LDL apoB-100 in type 2 diabetic subjects (n = 11) and nondiabetic controls (n = 13) using a primed-constant infusion of l-[5,5,5-D(3)]leucine for 12 h in the fed state. Diabetic subjects had significantly higher fasting glycemia, higher fasting insulinemia, higher plasma triglyceride, and lower HDL-cholesterol levels than controls. Compared with controls, diabetic subjects had increased TRL apoB-48, VLDL apoB-100, and IDL apoB-100 pool sizes as a result of increased production rates (PRs) and reduced fractional catabolic rates of these lipoprotein subfractions. Furthermore, multiple linear regression analyses revealed that the diabetic/control status was an independent predictor of TRL apoB-48 PR and represented nearly 35% of its variance. These results suggest that the overaccumulation of TRLs seen in patients with type 2 diabetes is attributable to increased PRs of both intestinally derived apoB-48-containing lipoproteins and TRL apoB-100 of hepatic origin and to decreased catabolism of these subfractions.     

Minor Contribution of Endogenous GLP-1 and GLP-2 to Postprandial Lipemia in Obese Men

Context

Glucose and lipids stimulate the gut-hormones glucagon-like peptide (GLP)-1, GLP-2 and glucose-dependent insulinotropic polypeptide (GIP) but the effect of these on human postprandial lipid metabolism is not fully clarified.

Objective

To explore the responses of GLP-1, GLP-2 and GIP after a fat-rich meal compared to the same responses after an oral glucose tolerance test (OGTT) and to investigate possible relationships between incretin response and triglyceride-rich lipoprotein (TRL) response to a fat-rich meal.

Design

Glucose, insulin, GLP-1, GLP-2 and GIP were measured after an OGTT and after a fat-rich meal in 65 healthy obese (BMI 26.5–40.2 kg/m²) male subjects. Triglycerides (TG), apoB48 and apoB100 in TG-rich lipoproteins (chylomicrons, VLDL₁ and VLDL₂) were measured after the fat-rich meal.

Main Outcome Measures

Postprandial responses (area under the curve, AUC) for glucose, insulin, GLP-1, GLP-2, GIP in plasma, and TG, apoB48 and apoB100 in plasma and TG-rich lipoproteins.

Results

The GLP-1, GLP-2 and GIP responses after the fat-rich meal and after the OGTT correlated strongly (r = 0.73, p<0.0001; r = 0.46, p<0.001 and r = 0.69, p<0.001, respectively). Glucose and insulin AUCs were lower, but the AUCs for GLP-1, GLP-2 and GIP were significantly higher after the fat-rich meal than after the OGTT. The peak value for all hormones appeared at 120 minutes after the fat-rich meal, compared to 30 minutes after the OGTT. After the fat-rich meal, the AUCs for GLP-1, GLP-2 and GIP correlated significantly with plasma TG- and apoB48 AUCs but the contribution was very modest.

Conclusions

In obese males, GLP-1, GLP-2 and GIP responses to a fat-rich meal are greater than following an OGTT. However, the most important explanatory variable for postprandial TG excursion was fasting triglycerides. The contribution of endogenous GLP-1, GLP-2 and GIP to explaining the variance in postprandial TG excursion was minor.     

Remnant lipoproteins are related to intima-media thickness of the carotid artery independently of LDL cholesterol and plasma triglycerides

Remnants of triglyceride-rich lipoproteins (TRL) have been implicated in the early development of atherosclerosis. We tested this hypothesis by quantifying the plasma concentration of remnant-like particle cholesterol (RLP-C) in a cohort of healthy 50-year-old men in whom the common carotid artery intima-media thickness (CCA-IMT) was assessed by B-mode ultrasound as a surrogate marker for atherosclerosis. The subjects were given a fat-rich meal to study the generation of RLP-C during postprandial lipemia. Fasting plasma RLP-C and other major fasting plasma lipids and lipoproteins were determined twice, and the mean RLP-C concentration was strongly correlated with CCA-IMT (r = 0.32, P = 0.002). In addition, low density lipoprotein (LDL) cholesterol (r = 0.25, P = 0.01) and plasma triglycerides (r = 0.20, P = 0.05) were significantly related to CCA-IMT. Multivariate analyses showed a triglyceride-independent contribution of RLP-C to CCA-IMT. After fat intake, the median plasma RLP-C concentration was doubled after 3 h. The increase was strongly related to the postprandial generation of TRL apolipoprotein (apo)B-48, and large (S(f) 60;-400) TRL apoB-100. The association with CCA-IMT was somewhat stronger for the 3-h RLP-C level than for the fasting RLP-C concentration [r = 0.27, P < 0.01 (3 h) compared with r = 0.22, P < 0.05 (0 h)].We conclude that the plasma concentration of RLP-C is related to CCA-IMT, independent of plasma triglycerides and LDL cholesterol, in a healthy middle-aged male population. - Karpe, F., S. Boquist, R. Tang, G. M. Bond, U. de Faire, and A. Hamsten. Remnant lipoproteins are related to intima-media thickness of the carotid artery independently of LDL cholesterol and plasma triglycerides. J. Lipid Res. 2001. 42: 17;-21.     

Postprandial lipemia in the elderly involves increased incorporation of ingested fat in plasma free fatty acids and small (Sf 20-400) triglyceride-rich lipoproteins

In the elderly, the rise in postprandial plasma triglyceride (TG) concentrations is increased, contributing to their increased risk of cardiovascular disease. We sought to determine the incorporation of ingested fat (whipping cream enriched with [1,1,1-(13)C]triolein) into plasma lipids during the postprandial period in six healthy elderly (67 ± 1 yr old) and six healthy young (23 ± 2 yr old) subjects. Blood and expired air samples were taken before and at 2-h intervals during the 8-h postprandial period. As expected, the area under the curve of postprandial plasma TG concentrations was larger in the elderly compared with the young subjects (152 ± 38 vs. 66 ± 27 mg·dl(-1)·h, P < 0.05). The incorporation of [(13)C]oleate in plasma free fatty acids (FFAs) and TG of the small (S(f) = 20-400) triglyceride-rich lipoprotein (TRL) fraction was significantly higher in the elderly compared with the young subjects, resulting in increased postprandial contributions of the ingested lipid to plasma FFAs (41 ± 3 vs. 26 ± 6%, P < 0.05) and the small TRL fraction (36 ± 5 vs. 21 ± 3%, P < 0.05) in elderly. Plasma apoB-100 concentration was higher, whereas the rate of oxidation of the ingested lipid was lower (P < 0.05) in the elderly. We conclude that increased postprandial lipemia in the elderly involves increased contribution of ingested lipid to the plasma small TRLs. This appears to be driven at least in part by increased appearance of the ingested fat as plasma FFA and increased availability of apo B-100 lipoproteins in the elderly.     

Postprandial changes of apoB-100 and apoB-48 in TG rich lipoproteins in familial combined hyperlipidemia

Impaired chylomicron (chylo) remnant clearance and small VLDL overproduction are major metabolic abnormalities in familial combined hyperlipidemia (FCHL). Quantitative data on postprandial apolipoprotein B-48 (apoB-48) and apolipoprotein B-100 (apoB-100) in TG rich lipoproteins (TRL) in FCHL have not been reported before. Eight untreated FCHL patients and 10 matched controls underwent a 24 h oral fat load. Fasting apoB-48 and apoB-100 were significantly higher in all TRL in FCHL. Maximal concentrations of chylo-[Svedberg's flotation rate (Sf) >400] apoB-48 and apoB-100 were reached later in FCHL (at t = 6 h), in contrast to controls (t = 4 h). Maximal VLDL1-(Sf60-400)-apoB-48 occurred at the same time point (t = 2 h) in both, whereas VLDL1-apoB-100 was maximal at t = 4 h in both, most likely representing delayed VLDL clearance by preferential clearance of chylo and their remnants by competition for the same clearance mechanisms. VLDL2-(Sf20-60)-apoB-48 and VLDL2- apoB-100 decreased in FCHL, in contrast to an increase of apoB-48, and no change of apoB-100 in controls, suggesting impaired conversion of VLDL1-apoB-48 into VLDL2-apoB-48 in FCHL, and partly also of VLDL1-apoB-100. In conclusion, in FCHL clearance of large postprandial Sf >400 apoB-48 and apoB-100 TRL is delayed. ApoB-100 accumulates in the VLDL1 range postprandially in both FCHL and controls, reaching higher levels in FCHL and thereby conferring a higher atherogenic burden in the postprandial situation in FCHL.     

Relationship between apo[a] isoforms and Lp[a] density in subjects with different apo[a] phenotype: a study before and after a fatty meal

Plasma Lp[a] levels and apo[a] isoform distribution among lipoproteins isolated by density gradient ultracentrifugation were studied in subjects with one-band or two-band apo[a] phenotypes as assessed by gradient gel electrophoresis before and after an oral fat load. There were no significant differences in the ultracentrifugal profile between fasting plasma and postprandial plasma that was freed of triglyceride-rich particles (TRP). One-band phenotypes exhibited a single symmetrical peak in the density gradient, whereas two-band phenotypes exhibited a multi-modal distribution. Low molecular weight apo[a] isoforms were preferentially associated with low density Lp[a] whereas high molecular weight apo[a] isoforms were found with high density Lp[a] particles. Feeding a high fat meal caused no significant increase in the total plasma level of Lp[a]. However, the isolated TRP contained the apoB-100-apo[a] complex in a quantity that represented only about 1% of its total amount in the fasting plasma. In all cases the apo[a] isoforms present in TRP were also present in the fasting plasma; however, in the two-band apo[a] phenotypes the ratio of the slow over the fast migrating band was in all cases about eightfold higher in TRP than in the fasting plasma. These observations indicate that postprandially a small percentage of apoB-100-apo[a] associates with TRP and suggest that this complex may derive from de novo synthesis rather than from a pre-existing Lp[a] plasma pool. The liver would be the source of the complex due to the presence in the latter of apoB-100.     

Effect of atorvastatin on postprandial lipoprotein metabolism in hypertriglyceridemic patients

Postprandial lipoprotein metabolism is impaired in hypertriglyceridemia. It is unknown how and to what extent atorvastatin affects postprandial lipoprotein metabolism in hypertriglyceridemic patients. We evaluated the effect of 4 weeks of atorvastatin therapy (10 mg/day) on postprandial lipoprotein metabolism in 10 hypertriglyceridemic patients (age, 40 +/- 3 years; body mass index, 27 +/- 1 kg/m2; cholesterol, 5.74 +/- 0.34 mmol/l; triglycerides, 3.90 +/- 0.66 mmol/l; HDL-cholesterol, 0.85 +/- 0.05 mmol/l; and LDL-cholesterol, 3.18 +/- 0.23 mmol/l). Patients were randomized to be studied with or without atorvastatin therapy. Postprandial lipoprotein metabolism was evaluated with a standardized oral fat load. Plasma was obtained every 2 h for 14 h. Large triglyceride-rich lipoproteins (TRLs) (containing chylomicrons) and small TRLs (containing chylomicron remnants) were isolated by ultracentrifugation, and cholesterol, triglyceride, apolipoprotein B-100 (apoB-100), apoB-48, apoC-III, and retinyl-palmitate concentrations were determined. Atorvastatin significantly (P < 0.01) decreased fasting cholesterol (-27%), triglycerides (-43%), LDL-cholesterol (-28%), and apoB-100 (-31%), and increased HDL-cholesterol (+19%). Incremental area under the curve (AUC) significantly (P < 0.05) decreased for large TRL-cholesterol, -triglycerides, and -retinyl-palmitate, while none of the small TRL parameters changed. These findings contrast with the results in normolipidemic subjects, in which atorvastatin decreased the AUC for chylomicron remnants (small TRLs) but not for chylomicrons (large TRLs). We conclude that atorvastatin improves postprandial lipoprotein metabolism in addition to decreasing fasting lipid levels in hypertriglyceridemia. Such changes would be expected to improve the atherogenic profile.     

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.     

The peroxisome proliferator-activated receptor alpha (PPARalpha) agonist ciprofibrate inhibits apolipoprotein B mRNA editing in low density lipoprotein receptor-deficient mice: effects on plasma lipoproteins and the development of atherosclerotic lesions

Low density lipoprotein receptor (LDLR)-deficient mice fed a chow diet have a mild hypercholesterolemia caused by the abnormal accumulation in the plasma of apolipoprotein B (apoB)-100- and apoB-48-carrying intermediate density lipoproteins (IDL) and low density lipoproteins (LDL). Treatment of LDLR-deficient mice with ciprofibrate caused a marked decrease in plasma apoB-48-carrying IDL and LDL but at the same time caused a large accumulation of triglyceride-depleted apoB-100-carrying IDL and LDL, resulting in a significant increase in plasma cholesterol levels. These plasma lipoprotein changes were associated with an increase in the hepatic secretion of apoB-100-carrying very low density lipoproteins (VLDL) and a decrease in the secretion of apoB-48-carrying VLDL, accompanied by a significant decrease in hepatic apoB mRNA editing. Hepatic apobec-1 complementation factor mRNA and protein abundance were significantly decreased, whereas apobec-1 mRNA and protein abundance remained unchanged. No changes in apoB mRNA editing occurred in the intestine of the treated animals. After 150 days of treatment with ciprofibrate, consistent with the increased plasma accumulation of apoB-100-carrying IDL and LDL, the LDLR-deficient mice displayed severe atherosclerotic lesions in the aorta. These findings demonstrate that ciprofibrate treatment decreases hepatic apoB mRNA editing and alters the pattern of hepatic lipoprotein secretion toward apoB-100-associated VLDL, changes that in turn lead to increased atherosclerosis.     

Direct effects of growth hormone on production and secretion of apolipoprotein B from rat hepatocytes

The aim of this study was to investigate the direct effects of growth hormone (GH) on production and secretion of apolipoprotein B (apoB)-containing lipoproteins from hepatocytes. Bovine GH (5-500 ng/ml) was given for 1 or 3 days to rat hepatocytes cultured on laminin-rich matrigel in serum-free medium. The effects of GH were compared with those of 3 nM insulin and 500 microM oleic acid. GH increased the editing of apoB mRNA, and the proportion of newly synthesized apoB-48 (of total apoB) in the cells and secreted into the medium changed in parallel. GH increased total secretion of apoB-48 (+30%) and apoB-48 in very low density lipoproteins (VLDL) more than twofold. Total apoB-100 secretion decreased 63%, but apoB-100-VLDL secretion was unaffected by GH. Pulse-chase studies indicated that GH increased intracellular early degradation of apoB-100 but not apoB-48. GH had no effect on apoB mRNA or LDL receptor mRNA levels. The triglyceride synthesis, the mass of triglycerides in the cells, and the VLDL fraction of the medium increased after GH incubation. Three days of insulin incubation had effects similar to those of GH. Combined incubation with oleic acid and GH had additive effects on apoB mRNA editing and apoB-48-VLDL secretion. In summary, GH has direct effects on production and secretion of apoB-containing lipoproteins, which may add to the effects of hyperinsulinemia and increased flux of fatty acids to the liver during GH treatment in vivo.     

Delayed clearance of postprandial large TG-rich particles in normolipidemic carriers of LPL Asn291Ser gene variant.

The carrier frequency of Asn291Ser polymorphism of the lipoprotein lipase (LPL) gene is 4;-6% in the Western population. Heterozygotes are prone to fasting hypertriglyceridemia and low high density lipoprotein (HDL) cholesterol concentrations especially when secondary factors are superimposed on the genetic defect. We studied the LPL Asn291Ser gene variant as a modulator of postprandial lipemia in heterozygote carriers. Ten normolipidemic carriers were compared to ten control subjects, who were selected to have similar age, sex, BMI, and apolipoprotein (apo)E-phenotype. The subjects were given a lipid-rich mixed meal and their insulin sensitivity was determined by euglycemic hyperinsulinemic clamp technique. The two groups had comparable fasting triglycerides and glucose utilization rate during insulin infusion, but fasting HDL cholesterol was lower in carriers (1.25 +/- 0.05 mmol/L) than in the control subjects (1. 53 +/- 0.06 mmol/L, P = 0.005). In the postprandial state the most pronounced differences were found in the very low density lipoprotein 1 (VLDL1) fraction, where the carriers displayed higher responses of apoB-48 area under the curve (AUC), apoB-100 AUC, triglyceride AUC, and retinyl ester AUC than the control subjects. The most marked differences in apoB-48 and apoB-100 concentrations were observed late in the postprandial period (9 and 12 h), demonstrating delayed clearance of triglyceride-rich particles of both hepatic and intestinal origin. Postprandially, the carriers exhibited enrichment of triglycerides in HDL fraction. Thus, in normolipidemic carriers the LPL Asn291Ser gene variant delays postprandial triglyceride, apoB-48, apoB-100, and retinyl ester metabolism in VLDL1 fraction and alters postprandial HDL composition compared to matched non-carriers.     

The ACAT inhibitor avasimibe increases the fractional clearance rate of postprandial triglyceride-rich lipoproteins in miniature pigs

Previously, we have shown, in vivo, that the acyl coenzyme A: cholesterol acyltransferase (ACAT) inhibitor avasimibe decreases hepatic apolipoprotein (apo) B secretion into plasma. To test the hypothesis that avasimibe modulates postprandial triglyceride-rich lipoprotein (TRL) metabolism in vivo, an oral fat load (2 g fat/kg) containing retinol was given to 9 control miniature pigs and to 9 animals after 28 days treatment with avasimibe (10 mg/kg/day, n=5; 25 mg/kg/day, n=4). The kinetic parameters for plasma retinyl palmitate (RP) metabolism were determined by multi-compartmental modeling using SAAM II. Avasimibe decreased the 2-h TRL (d<1.006 g/mL; S(f)>20) triglyceride concentrations by 34%. The TRL triglyceride 0-12 h area under the curve (AUC) was decreased by 21%. In contrast, avasimibe had no effect on peak TRL RP concentrations, time to peak, or its rate of appearance into plasma, however, the TRL RP 0-12 h AUC was decreased by 17%. Analysis of the RP kinetic parameters revealed that the TRL fractional clearance rate (FCR) was increased 1.4-fold with avasimibe. The TRL RP FCR was negatively correlated with very low density lipoprotein (VLDL) apoB production rate measured in the fasting state (r=-0.504). No significant changes in total intestinal lipid concentrations were observed. Thus, although avasimibe had no effect on intestinal TRL secretion, plasma TRL clearance was significantly increased; an effect that may relate to a decreased competition with hepatic VLDL for removal processes.     

Effects of ezetimibe and simvastatin on apolipoprotein B metabolism in males with mixed hyperlipidemia

Sixteen hyperlipidemic men were enrolled in a randomized, placebo-controlled, double-blind, cross-over study to evaluate the effect of ezetimibe 10 mg and simvastatin 40 mg, coadministered and alone, on the in vivo kinetics of apolipoprotein (apo) B-48 and B-100 in humans. Subjects underwent a primed-constant infusion of a stable isotope in the fed state. The coadministration of simvastatin and ezetimibe significantly reduced plasma concentrations of cholesterol (−43.0%), LDL-C (−53.6%), and triglycerides (−44.0%). Triglyceride-rich lipoproteins (TRL) apoB-48 pool size (PS) was significantly decreased (−48.9%) following combination therapy mainly through a significant reduction in TRL apoB-48 production rate (PR) (−38.0%). The fractional catabolic rate (FCR) of VLDL and LDL apoB-100 were significantly increased with all treatment modalities compared with placebo, leading to a significant reduction in the PS of these fractions. We also observed a positive correlation between changes in TRL apoB-48 PS and changes in TRL apoB-48 PR (r = 0.85; P < 0.0001) with combination therapy. Our results indicate that treatment with simvastatin plus ezetimibe is effective in reducing plasma TRL apoB-48 levels and that this effect is most likely mediated by a reduction in the intestinal secretion of TRL apoB-48. Our study also indicated that the reduction in LDL-C concentration following combination therapy is mainly driven by an increase in FCR of apoB-100 containing lipoproteins.     

Ezetimibe ameliorates intestinal chylomicron overproduction and improves glucose tolerance in a diet-induced hamster model of insulin resistance

Ezetimibe is a cholesterol uptake inhibitor that targets the Niemann-Pick C1-like 1 cholesterol transporter. Ezetimibe treatment has been shown to cause significant decreases in plasma cholesterol levels in patients with hypercholesterolemia and familial hypercholesterolemia. A recent study in humans has shown that ezetimibe can decrease the release of atherogenic postprandial intestinal lipoproteins. In the present study, we evaluated the mechanisms by which ezetimibe treatment can lower postprandial apoB48-containing chylomicron particles, using a hyperlipidemic and insulin-resistant hamster model fed a diet rich in fructose and fat (the FF diet) and fructose, fat, and cholesterol (the FFC diet). Male Syrian Golden hamsters were fed either chow or the FF or FFC diet ± ezetimibe for 2 wk. After 2 wk, chylomicron production was assessed following intravenous triton infusion. Tissues were then collected and analyzed for protein and mRNA content. FFC-fed hamsters treated with ezetimibe showed improved glucose tolerance, decreased fasting insulin levels, and markedly reduced circulating levels of TG and cholesterol in both the LDL and VLDL fractions. Examination of triglyceride (TG)-rich lipoprotein (TRL) fractions showed that ezetimibe treatment reduced postprandial cholesterol content in TRL lipoproteins as well as reducing apoB48 content. Although ezetimibe did not decrease TRL-TG levels in FFC hamsters, ezetimibe treatment in FF hamsters resulted in decreases in TRL-TG. Jejunal apoB48 protein expression was lower in ezetimibe-treated hamsters. Reductions in jejunal protein levels of scavenger receptor type B-1 (SRB-1) and fatty acid transport protein 4 were also observed. In addition, ezetimibe-treated hamsters showed significantly lower jejunal mRNA expression of a number of genes involved in lipid synthesis and transport, including srebp-1c, sr-b1, ppar-γ, and abcg1. These data suggest that treatment with ezetimibe not only inhibits cholesterol uptake, but may also alter intestinal function to promote improved handling of dietary lipids and reduced chylomicron production. These, in turn, promote decreases in fasting and postprandial lipid levels and improvements in glucose homeostasis.     

Influence of peroxisome proliferator-activated receptor alpha agonists on the intracellular turnover and secretion of apolipoprotein (Apo) B-100 and ApoB-48

The peroxisome proliferator-activated receptor (PPAR) alpha agonist WY 14,643 increased the secretion of apolipoprotein (apo) B-100, but not that of apoB-48, and decreased triglyceride biosynthesis and secretion from primary rat hepatocytes. These effects resulted in decreased secretion of apoB-100-very low density lipoprotein (VLDL) and an increased secretion of apoB-100 on low density lipoproteins/intermediate density lipoproteins. ApoB-48-VLDL was also replaced by more dense particles. The proteasomal inhibitor lactacystin did not influence the recovery of apoB-100 or apoB-48 in primary rat hepatocytes, indicating that co-translational (proteasomal) degradation is of less importance in these cells. Treatment with WY 14,643 made the recovery of apoB-100 sensitive to lactacystin, most likely reflecting the decreased biosynthesis of triglycerides. The PPAR alpha agonist induced a significant increase in the accumulation of pulse-labeled apoB-100 even after a short pulse (2-5 min). There was also an increase in apoB-100 nascent polypeptides, indicating that the co-translational degradation of apoB-100 was inhibited. However, a minor influence on an early posttranslation degradation cannot be excluded. This decreased co-translational degradation of apoB-100 explained the increased secretion of the protein. The levels of apoB-48 remained unchanged during these pulse-chase experiments, and albumin production was not affected, indicating a specific effect of PPAR alpha agonists on the co-translational degradation of apoB-100. These findings explain the difference in the rate of secretion of the two apoB proteins seen after PPAR alpha activation. PPAR alpha agonists increased the expression and biosynthesis of liver fatty acid-binding protein (LFABP). Increased expression of LFABP by transfection of McA-RH7777 cells increased the secretion of apoB-100, decreased triglyceride biosynthesis and secretion, and increased PPAR alpha mRNA levels. These findings suggest that PPAR alpha and LFABP could interact to amplify the effect of endogenous PPAR alpha agonists on the assembly of VLDL.     

Cholesteryl ester flux from HDL to VLDL-1 is preferentially enhanced in type IIB hyperlipidemia in the postprandial state

Postprandial triglyceride-rich lipoproteins (TRL) exert proatherogenic effects at the arterial wall, including lipid deposition. Following consumption of a mixed meal (1200 kcal), plasma-mediated cellular free cholesterol (FC) efflux, lecithin:cholesterol acyltransferase (LCAT), and cholesteryl ester transfer protein (CETP) activities were determined in subjects (n = 12) displaying type IIB hyperlipidemia and compared with those in a normolipidemic control group (n = 14). The relative capacity of plasma to induce FC efflux from Fu5AH cells via the SR-BI receptor was significantly increased 4 h postprandially (+23%; P < 0.005) in the type IIB group, whereas it remained unchanged for postprandial plasma from normolipidemic subjects. LCAT activity was significantly elevated 2 h postprandially in both the IIB and control groups, (+46% and +36%, respectively; P < 0.005 vs. respective baseline value). In type IIB subjects, total cholesteryl ester (CE) mass transfer from HDL to total TRL [chylomicrons (CMs) + VLDL-1 + VLDL-2 + IDL] increased progressively from 15 +/- 2 micro g CE/h/ml at baseline to 28 +/- 2 micro g CE transferred/h/ml (+87%; P = 0.0004) at 4 h postprandially. CE transfer to CMs and VLDL-1 was preferentially stimulated (2.6-fold and 2.3-fold respectively) at 4 h in IIB subjects and occurred concomitantly with elevation in mass and particle number of both CMs (2.3-fold) and VLDL-1 (1.3-fold). Furthermore, in type IIB subjects, CETP-mediated total CE flux over the 8 h postprandial period from HDL to potentially atherogenic TRL was significantly enhanced, and notably to VLDL-1 (32-fold elevation; P < 0.005), relative to control subjects. Such CE transfer flux was reflected in a significant postprandial increase in CE-TG ratio in both CMs and VLDL-1 in type IIB plasmas. In conclusion, HDL-CE is preferentially targeted to VLDL-1 via the action of CETP during alimentary lipemia, thereby favoring formation and accumulation of atherogenic CE-rich remnant particles.