Clearance Of Postprandial Triglyceride-rich Lipoproteins Research Articles

Abstract 1: Apolipoprotein E2 and Apolipoprotein E4 Promote Macrophage Inflammation and Dysfunction Through Distinct Mechanisms

ApoE3 is the most common apoE variant in humans. Carriers of the apoE4 variant have increased risk of coronary artery disease, while apoE2 carriers have lower plasma lipid levels, reduced risk of coronary artery disease, but an increased risk of peripheral vascular disease. The differing effects of apoE2 and apoE4 on vascular disease manifestation have not been completely elucidated. ApoE2, apoE3 and apoE4 gene replacement (knock-in) mice were used to examine the influence apoE variants have on macrophage function and response to dietary challenge. Peritoneal macrophages isolated from apoE4 mice displayed less efferocytosis capacity and were more apoptotic even under basal conditions when compared to apoE3 and apoE2 macrophages. Stimulation with LPS or oxLDL increased apoptosis more robustly in apoE4 and apoE2 macrophages compared with apoE3 macrophages. Immunoblot analysis showed increased endoplasmic reticulum (ER) stress in apoE4 macrophages, with IRE1α and JNK being up-regulated 2- and 4-fold, respectively, compared with apoE2 and apoE3 macrophage. Phosphorylated PERK was 2-fold higher in apoE4 macrophages when stressed with LPS or oxLDL, compared with apoE3 or apoE2 macrophages. The pro-apoptotic protein, CHOP, was also up-regulated in apoE4 macrophages compared with apoE3 macrophages when exposed to oxLDL. When knock-in mice were challenged with a lipid-rich meal via gastric lavage, postprandial increase in neutrophil cell count was similar between apoE3 and apoE4 mice but was much higher in apoE2 mice. The robust postprandial neutrophil response in apoE2 mice coincided with their delay in postprandial clearance of triglyceride-rich lipoproteins. Interestingly, when the animals were maintained on a high fat, high cholesterol Western-type diet for 4 weeks, tissue macrophages were found to be more inflamed in both apoE2 and apoE4 mice compared to apoE3 mice. These results document distinct pro-inflammatory mechanisms: apoE2 increases postprandial inflammation through defective receptor binding and delayed lipid clearance, while apoE4 promotes inflammation by reducing macrophage function and increasing ER stress. These findings may help to elucidate the disparate influences on peripheral versus coronary artery diseases.

Postprandial accumulation of chylomicrons and chylomicron remnants is determined by the clearance capacity

ObjectiveTo better understand the postprandial clearance of triglyceride-rich lipoproteins (TRLs) and its relation to the fasting kinetics of TRLs. MethodsTwo studies were performed on 30 male subjects: a fasting kinetic study to determine the fasting secretion and clearance rates of apolipoprotein B (apoB) 100 and triglycerides in the very low-density lipoprotein 1 and 2 (VLDL1 and VLDL2) fractions; and a postprandial study to determine the postprandial accumulation of apoB48, apoB100 and triglycerides in the chylomicron, VLDL1 and VLDL2 fractions. Results from these two studies were combined to characterize the postprandial clearance of TRLs in a physiologically relevant setting. ResultsOur results show that postprandial accumulation of the apoB48-carrying chylomicrons can be predicted from the clearance capacity of the lipolytic pathway, determined in the fasting state. Furthermore, we show that chylomicrons and VLDL1 particles are not cleared equally by the lipoprotein lipase pathway, and that chylomicrons seem to be the preferred substrate. Subjects with a rapid fasting lipid metabolism accumulate lower levels of postprandial triglycerides with less accumulation of apoB100 in the VLDL1 fraction and a faster transfer of apoB100 into the VLDL2 fraction. In contrast, fasting VLDL1 secretion does not predict postprandial triglyceride accumulation. ConclusionsNon-fasting triglyceride levels have recently been identified as a major predictor of future cardiovascular events. Here we show that the capacity of the lipolytic pathway is a common determinant of both the fasting and non-fasting triglyceride levels and may thus play an important role in the development of dyslipemia and atherosclerosis.

Apolipoproteins AI and B as therapeutic targets

Currently the apolipoprotein B:AI ratio integrates information about the potential for cardiovascular disease (CVD) risk reduction better than any other lipid or lipoprotein index. Certainly it could, with benefit, replace serum cholesterol and HDL cholesterol in the estimation of CVD risk. Defining the therapeutic target of statin therapy in terms of serum apolipoprotein B (apo B) rather than LDL cholesterol could also help to optimize statin treatment. Deciding whether a therapeutic response is adequate also requires knowledge of whether there is persisting hypertriglyceridaemia, because this gives an indication of whether small dense LDL is likely to have been satisfactorily reduced. Raising low levels of HDL, probably best measured as apo AI, may also prove to be an important aim of treatment. This is, however, a more complex issue and also depends on the mechanism by which a particular therapy alters HDL levels and on whether the capacity of HDL to perform its anti-inflammatory and antioxidative functions is restored. A meta-analysis of randomized clinical trials of statins in which apo B and apo AI have been reported could provide valuable information.

W16-P-039 Peripheral arterial disease: Is secondary prevention of cardiac disease comparable to patients with coronary artery disease?

To better understand the postprandial clearance of triglyceride-rich lipoproteins (TRLs) and its relation to the fasting kinetics of TRLs.Two studies were performed on 30 male subjects: a fasting kinetic study to determine the fasting secretion and clearance rates of apolipoprotein B (apoB) 100 and triglycerides in the very low-density lipoprotein 1 and 2 (VLDL1 and VLDL2) fractions; and a postprandial study to determine the postprandial accumulation of apoB48, apoB100 and triglycerides in the chylomicron, VLDL1 and VLDL2 fractions. Results from these two studies were combined to characterize the postprandial clearance of TRLs in a physiologically relevant setting.Our results show that postprandial accumulation of the apoB48-carrying chylomicrons can be predicted from the clearance capacity of the lipolytic pathway, determined in the fasting state. Furthermore, we show that chylomicrons and VLDL1 particles are not cleared equally by the lipoprotein lipase pathway, and that chylomicrons seem to be the preferred substrate. Subjects with a rapid fasting lipid metabolism accumulate lower levels of postprandial triglycerides with less accumulation of apoB100 in the VLDL1 fraction and a faster transfer of apoB100 into the VLDL2 fraction. In contrast, fasting VLDL1 secretion does not predict postprandial triglyceride accumulation.Non-fasting triglyceride levels have recently been identified as a major predictor of future cardiovascular events. Here we show that the capacity of the lipolytic pathway is a common determinant of both the fasting and non-fasting triglyceride levels and may thus play an important role in the development of dyslipemia and atherosclerosis.

Effect of the apolipoprotein A-IV Q360H polymorphism on postprandial plasma triglyceride clearance

Apolipoprotein (apo)A-IV is synthesized in the small intestine during fat absorption and is incorporated onto the surface of nascent chylomicrons. In circulation, apoA-IV is displaced from the chylomicron surface by high density lipoprotein-associated C and E apolipoproteins; this exchange is critical for activation of lipoprotein lipase and chylomicron remnant clearance. The variant allele A-IV-2 encodes a Q360H polymorphism that increases the lipid affinity of the apoA-IV-2 isoprotein. We hypothesized that this would impede the transfer of C and E apolipoproteins to chylomicrons, and thereby delay the clearance of postprandial triglyceride-rich lipoproteins. We therefore measured triglycerides in plasma, Sf > 400 chylomicrons, and very low density lipoproteins (VLDL) in 14 subjects heterozygous for the A-IV-2 allele (1/2) and 14 subjects homozygous for the common allele (1/1) who were fed a standard meal containing 50 gm fat per m2 body surface area. All subjects had the apoE-3/3 genotype. Postprandial triglyceride concentrations in the 1/2 subjects were significantly higher between 2–5 h in plasma, chylomicrons, and VLDL, and peaked at 3 h versus 2 h for the 1/1 subjects. The area under the triglyceride time curves was greater in the 1/2 subjects (plasma, P = 0.045; chylomicrons, P = 0.027; VLDL, P = 0.063). A posthoc analysis of the frequency of the apoA-IV T347S polymorphism suggested that it had an effect on triglyceride clearance antagonistic to that of the A-IV-2 allele. We conclude that individuals heterozygous for the A-IV-2 allele display delayed postprandial clearance of triglyceride-rich lipoproteins. —Hockey, K. J., R. A. Anderson, V. R. Cook, R. R. Hantgan, and R. B. Weinberg. Effect of the apolipoprotein A-IV Q360H polymorphism on postprandial plasma triglyceride clearance.

Postprandial Retinyl Palmitate and Squalene Metabolism Is Age Dependent

Most of our awake time is spent in a postprandial state. It has not been investigated in detail whether the post-prandial clearance of triglyceride-rich lipoproteins is age dependent. In addition, postabsorptive squalene metabolism has not been studied in relation to age. Accordingly, we investigated postprandial lipid metabolism in six young (22-25 years of age) and eight old (78-79 years of age) healthy men by use of an oral fat load containing 345,000 IU of vitamin A and 0.5 g of squalene as postprandial markers. Postprandial samples were drawn after 3, 4, 6, 9, 12 and 24 hours after the fat load. The retinyl palmitate area under the incremental curve of the old subjects was higher in plasma than that of the young subjects (p < .01). The pattern of postprandial very low density lipoprotein squalene responses differed significantly in the old compared with the young subjects (p < .01), but the areas under the incremental curve did not differ. Postprandial retinyl palmitate and squalene concentrations correlated significantly at 3-12 hours (p < .01). These data suggest that postprandial lipoprotein metabolism measured by retinyl palmitate and squalene is retarded with increasing age.

Lipoprotein lipase steady-state mRNA levels are lower in human omental versus subcutaneous abdominal adipose tissue

Adipose tissue synthesizes lipoprotein lipase (LPL), which helps in the postprandial clearance of triglyceride-rich lipoproteins. Because visceral adipose tissue is generally accepted as the most important metabolic tissue, we sought to verify whether there are regional differences in the expression of LPL. Samples of adipose tissue from subcutaneous and omental fat deposits were obtained from 20 adults undergoing surgery. Total adipose tissue LPL activity was measured using a conventional radioactive substrate assay. Steady-state levels of LPL mRNA were assessed using the very sensitive RNase protection assay technique with 18S ribosomal RNA as an internal control. A correlation was demonstrated between LPL activity levels in subcutaneous and omental tissue (r = .72; P < .01) and between mRNA levels at both sites (r = .47, P = .04). LPL mRNA levels were significantly lower in omental compared with subcutaneous depots (omental v subcutaneous, 1.7 ± 0.7 v 2.1 ± 0.7 arbitrary units [AU] over 18S, P < .05). In paired comparisons, LPL mRNA levels in omental adipose tissue were, on average, 20% ± 7% (range, −57% to +9.0%) lower than the levels measured in subcutaneous adipose tissue (P < .05). In conclusion, these data suggest that subcutaneous adipose tissue is a reliable surrogate of the expression (activity and mRNA) of LPL in omental adipose tissue, even though omental depots express proportionally less LPL than subcutaneous depots.

Effects of postprandial hyperlipemia on the vitamin E content of lipoproteins

Delayed postprandial clearance of triglyceride-rich lipoproteins (TGRL) could induce a decrease of low-density lipoprotein (LDL) and high-density lipoprotein (HDL) vitamin E content through its transfer into TGRL. We thus studied lipoprotein vitamin E content during postprandial hypertriglyceridemia induced by a high fat meal without vitamin E supplement. Venous blood was drawn from five healthy male volunteers following a 12 h fast at ( t 0) then 2 h ( t 2), 4 h ( t 4), 6 h ( t 6) and 8 h ( t 8) after a 80 g fat meal. In plasma, only TG significantly varied during the postprandial period with a large interindividual variability. Mean composition of lipoproteins in terms of mass was not significantly modified. The amount of vitamin E significantly increased in TGRL and decreased in LDL plus HDL at t 4 and t 6 relative to t 0. Vitamin E content of TGRL and LDL but not of HDL decreased significantly at t 4. The mean decrease was 20% (range 5%–54%) for the LDL. LDL– and HDL vitamin E content correlated inversely with plasma TGRL levels. Our data suggest that LDL from subjects with delayed chylomicron clearance could be less protected against oxidation.

Increased J774 macrophage cytotoxicity of late postprandial triglyceride-rich lipoproteins from normolipidemic young men expressing an apolipoprotein ε4 allele

It has been demonstrated that normolipidemic young men with apolipoprotein E4/3 phenotype have a prolonged postprandial clearance of triglyceride-rich lipoproteins following a high-fat diet. In the present study, we isolated fasting and postprandial (3 and 8 h) lipoprotein fractions from normolipidemic young men with E3/3 and E4/3 phenotypes and examined the in vitro cytotoxicity of these lipoproteins towards J774 macrophages. 8 h E4/3 very low density lipoprotein (VLDL) were significantly more cytotoxic than either 8 h E3/3 VLDL or fasting and 3 h E4/3 VLDL (lactate dehydrogenase (LDH) released: 161±21, 107±9, 88±16 and 101±12 I.U./l, respectively). Fasting E4/3 intermediate density lipoprotein (IDL) were also significantly more cytotoxic than either fasting E3/3 IDL or 3 h and 8 h E4/3 IDL (LDH released: 105±23, 60±9, 37±5 and 53±16 I.U./l, respectively), whereas either fasting or postprandial low density lipoprotein (LDL) and high density lipoprotein (HDL) samples did not show any difference in cytotoxicity between the two groups studied. 8 h E4/3 VLDL samples incubated with J774 macrophages had a lower esterified cholesterol (40±3 versus 52±3 μg), and higher triglyceride (783±133 versus 418±64 μg) and free fatty acid (FFA) (2.0±0.4 versus 0.9±0.1 μg) content than fasting E4/3 VLDL. The increased macrophage cytotoxicity of late postprandial triglyceride-rich lipoproteins seems to be related to the FFA content of E4/3 VLDL.

Endogenous triglyceride-rich lipoproteins accumulate in rat plasma when competing with a chylomicron-like triglyceride emulsion for a common lipolytic pathway

The rat liver secretes very low density lipoproteins (VLDL) containing either apoB-100 or apoB-48. After oral fat intake, chylomicrons containing apoB-48 and endogenously synthesized VLDL are mixed in the blood and the triglyceride clearance from these triglyceride-rich lipoprotein species compete for the same lipolytic pathway, i.e., lipoprotein lipase. A situation mimicking alimentary lipemia was induced by a short-term intravenous primed infusion of a chylomicron-like triglyceride emulsion to fed and fasted rats. The plasma concentration of apoB-100 and apoB-48 was monitored in triglyceride-rich lipoprotein subfractions after separation with density gradient ultracentrifugation by analytical SDS-PAGE. The net liver secretory output of VLDL was quantified by lipolytic blockade induced by Triton WR 1339. The chylomicron-like triglyceride emulsion induced a linear increase of large VLDL (Sf 60-400 subfraction containing both apoB-100 and apoB-48), almost to the same extent as that induced by Triton. The clearance of postprandial triglyceride-rich lipoproteins and both lipolysis and clearance of intravenously injected labeled rat chylomicrons was efficiently inhibited by the emulsion but not so complete as for fasting VLDL. The linearity of the VLDL increase and the very early response in the Intralipid-treated rats suggest that enhanced synthesis of VLDL is not a major cause for the accumulation. Rather, the present data indicate that a high plasma concentration of a chylomicron-like triglyceride emulsion competes efficiently with liver-derived VLDL for the same lipolytic pathway, which leads to accumulation in plasma of endogenous VLDL in the postprandial state.

Impaired clearance of postprandial triglyceride-rich lipoproteins in women with angiographically proven coronary artery disease

Impaired clearance of postprandial triglyceride-rich lipoproteins in women with angiographically proven coronary artery disease