Chylomicron apoB48 Research Articles

Investigation of human apoB48 metabolism using a new, integrated non-steady-state model of apoB48 and apoB100 kinetics.

BackgroundTriglyceride‐rich lipoproteins and their remnants have emerged as major risk factors for cardiovascular disease. New experimental approaches are required that permit simultaneous investigation of the dynamics of chylomicrons (CM) and apoB48 metabolism and of apoB100 in very low‐density lipoproteins (VLDL).MethodsMass spectrometric techniques were used to determine the masses and tracer enrichments of apoB48 in the CM, VLDL 1 and VLDL 2 density intervals. An integrated non‐steady‐state multicompartmental model was constructed to describe the metabolism of apoB48‐ and apoB100‐containing lipoproteins following a fat‐rich meal, as well as during prolonged fasting.ResultsThe kinetic model described the metabolism of apoB48 in CM, VLDL 1 and VLDL 2. It predicted a low level of basal apoB48 secretion and, during fat absorption, an increment in apoB48 release into not only CM but also directly into VLDL 1 and VLDL 2. ApoB48 particles with a long residence time were present in VLDL, and in subjects with high plasma triglycerides, these lipoproteins contributed to apoB48 measured during fasting conditions. Basal apoB48 secretion was about 50 mg day−1, and the increment during absorption was about 230 mg day−1. The fractional catabolic rates for apoB48 in VLDL 1 and VLDL 2 were substantially lower than for apoB48 in CM.DiscussionThis novel non‐steady‐state model integrates the metabolic properties of both apoB100 and apoB48 and the kinetics of triglyceride. The model is physiologically relevant and provides insight not only into apoB48 release in the basal and postabsorptive states but also into the contribution of the intestine to VLDL pool size and kinetics.

Dietary n−3 PUFA Improves Post-Prandial Metabolism by Reducing Chylomicron apoB48 Production and Lymphatic Cholesterol in the Insulin Resistant JCR:LA-cp rat

Dietary n−3 PUFA Improves Post-Prandial Metabolism by Reducing Chylomicron apoB48 Production and Lymphatic Cholesterol in the Insulin Resistant JCR:LA-cp rat

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.

Chylomicron and apoB48 metabolism in the JCR:LA corpulent rat, a model for the metabolic syndrome

Postprandial (PP) lipaemia is a significant contributor to the development of dyslipidaemia and cardiovascular disease (CVD). It is also evident that PP lipaemia is prevalent during conditions of obesity and insulin resistance (IR) and may contribute to increased progression of CVD. Our group has assessed the potential of the obese JCR:LA-cp rat as a model of PP lipaemia in order to explore CM (chylomicron) metabolism during the onset and development of IR in the metabolic syndrome. Studies confirm that both fasting plasma and PP apoB48 (apolipoprotein B48) area under the curve are significantly elevated in the obese JCR:LA-cp phenotype as compared with lean controls. Mechanistic studies have also shown that the concentration of lymphatic CM apoB48 and CM size are significantly increased in this model. Furthermore, PP dyslipidaemia in the obese rat can be improved acutely with supplementation of n-3 polyunsaturated fatty acids. Using a different approach, we have subsequently hypothesized that the vascular remodelling that accompanies IR may explain accelerated entrapment of apoB48-containing particles. Small leucine-rich proteoglycans (including biglycan and decorin) have been observed to co-localize with apoB in human tissue. However, the potential impact of IR on vascular remodelling, particularly in the presence of obesity, remains unclear. Preliminary observations from the JCR:LA-cp model indicate that biglycan protein core content increases with age and is exacerbated by IR, suggestive of pro-atherogenic remodelling. The focus of this review is to contribute to the perspective of PP lipaemia in CVD risk associated with the metabolic syndrome through the use of animal models.

Th-P15:83 Chylomicron apoB48 is increased in diabetic patients

Th-P15:83 Chylomicron apoB48 is increased in diabetic patients

Microsomal triglyceride transfer protein: does insulin resistance play a role in the regulation of chylomicron assembly?

We have previously demonstrated that diabetes is associated with an increase in intestinal microsomal triglyceride transfer protein (MTP) mRNA in both the rat and rabbit models. The present study was designed to investigate the relationship between MTP expression and chylomicron assembly in an insulin resistant non-diabetic animal model. Ten insulin resistant Zucker obese fa/fa rats and ten lean fa/− rats were examined at 8–10 weeks of age. The lymph duct was cannulated and lymph collected for 4 h. Lymph chylomicrons were isolated by ultracentrifugation and their composition determined. RNA was extracted from intestinal mucosa and from the liver. MTP mRNA was measured using the RNase protection assay. Blood sugar in the fatty rats was significantly higher (6.3±1.2 vs. 5.4±0.4 P<0.05) and plasma insulin was almost six times that of the lean rats ( P<0.001). Plasma cholesterol and phospholipid but not triglyceride were significantly increased in the obese animals ( P<0.01). Obese animals secreted significantly more lymph chylomicron apo B48 (0.05±0.02 vs. 0.02±0.01mg/h P<0.005), triglyceride (9.7±5.3 vs. 3.8±1.9 mg/h P<0.005) and phospholipid (1.5±0.7 vs. 0.4±0.3 mg/h P<0.001). The only difference in the chylomicron particle composition between the two groups was a significant increase in phospholipid ( P<0.01). Intestinal MTP mRNA expression was significantly higher in the fatty compared to the lean rats (22.1±9.5 vs. 7.8±5.6 amol MTP mRNA/μg total RNA P<0.001) as was hepatic MTP mRNA expression (6.9±3.5 vs. 3.4±1.5 amol MTP mRNA/μg total RNA, P<0.01). Thus in this animal model of insulin resistance, increased MTP, which was associated with increased chylomicron particle number, may play a crucial role in the development of atherosclerosis.

The relationship between cholesterol absorption and intestinal cholesterol synthesis in the diabetic rat model.

The chylomicron remnant particle is thought to be particularly atherogenic and we have previously shown alterations in post-prandial lipoproteins which could contribute to their atherogenicity. Cholesterol metabolism is disturbed in diabetes, yet the effect of diabetes on intestinal cholesterol synthesis and absorption has rarely been investigated. The aim of this study was to examine cholesterol absorption and intestinal synthesis of cholesterol in the streptozotocin diabetic rat. Twelve diabetic rats were paired with 12 control rats. [14C]-Cholesterol emulsion was administered and the lymph duct was canulated. Lymph was collected for 4 h. At sacrifice blood was taken for plasma lipoprotein measurements. Chylomicrons were prepared from the lymph by ultracentrifugation and [14C]-cholesterol content was determined by liquid scintillation counting. Lymph apolipoprotein B48 was isolated by gradient gel electrophoresis, and quantified by densitometric scanning. Serum triglyceride and cholesterol were greatly elevated in diabetic compared to control animals (260 ± 90 and 9.8 ± 8.0 mg/ml vs. 1.0 ± 0.4 and 0.6 ± 0.3 mg/ml, p < 0.0001 respectively). Lymph chylomicron apo B48 was similar in the two groups. Cholesterol absorption was not significantly different in diabetic compared to control rats but cholesterol synthesis was significantly, higher in the diabetic animals (550 ± 352 vs. 322 ± 113 μg/h p < 0.03). There was a positive correlation between apo B48 and cholesterol absorption (r = 0.70, p < 0.01) in the diabetic rats and control rats (r = 0.71, p < 0.01) but no correlation between apo B48 and cholesterol synthesis in either group. This study demonstrates that cholesterol synthesis was increased in diabetes whereas cholesterol absorption was unaffected suggesting that intestinal cholesterol synthesis made an important contribution to the hypercholesterolaemia seen in the diabetic animals.

The role of microsomal triglyceride transfer protein and dietary cholesterol in chylomicron production in diabetes.

The aim of this study was to examine factors involved in chylomicron production in the streptozotocin diabetic rat, our hypothesis being that the synthesis of the chylomicron is abnormal in diabetes. Diabetic rats (n = 20) were paired with control rats (n = 20). Cholesterol emulsion was given by gavage and the lymph duct was cannulated. Lymph was collected for 4 h. Chylomicrons were prepared from the lymph by ultracentrifugation. Lymph apolipoprotein B48 was isolated by gradient gel electrophoresis and quantified by densitometric scanning. Intestinal microsomal triglycerol transfer protein mRNA was measured by solution hybridisation nuclease protection, using a rat specific [32P]-labelled cRNA probe. Serum triglyceride and cholesterol were greatly increased in diabetic compared with control animals (258 +/- 77 and 8.9 +/- 6.4 mg/ml vs 1.04 +/- 0.37 and 0.54 +/- 0.03 mg/ml, p < 0.0001). Lymph chylomicron triglyceride and cholesterol were also higher in diabetic rats (29.4 +/- 27.3 and 0.28 +/- 0.3 mg/h vs 16.8 +/- 10.6 and 0.18 +/- 0.09 mg/h, p < 0.05). Lymph chylomicron apo B48 was similar in the two groups. Intestinal microsomal triglycerol transfer protein mRNA was higher in the diabetic rats (12.6 +/- 3.2 vs 3.8 +/- 3.0 amol/microgram RNA, p < 0.0001) and there was a positive correlation between lymph triglyceride and microsomal triglycerol transfer protein mRNA in the whole group (r = 0.65, p < 0.01). The study shows that microsomal triglycerol transfer protein mRNA is raised in diabetes without an increase in apolipoprotein B48 in the lymph suggesting that microsomal triglycerol transfer protein regulates chylomicron triglyceride content but not particle number.

Postprandial elevation of ApoB-48-containing triglyceride-rich particles and retinyl esters in normolipemic males who smoke.

Smokers have an increased risk for coronary artery disease (CAD), which can only partly be explained by fasting lipoprotein changes. Recent studies have indicated that smokers express metabolic abnormalities characteristic of insulin resistance syndrome. A preliminary study reported an increased postprandial triglyceride (TG) response in smokers compared with nonsmokers. To investigate the effect of smoking on postprandial lipemia, a fat-rich mixed meal (837 kcal, 63 g of fat) was served to 12 healthy smokers and 12 controls with similar fasting lipoprotein profiles, body composition, and lifestyles. Blood was drawn before and 3, 4, 6, and 8 hours postprandially, and triglyceride-rich lipoprotein (TRL) fractions (chylomicrons, VLDL1, VLDL2, and IDL) were separated with density gradient ultracentrifugation. Pre- and postprandial TG, retinyl esters (RE), apolipoprotein B-48 (apoB-48) and B-100 (apoB-100) were measured in each fraction. Smokers showed a significantly increased postprandial TG response in chylomicrons, VLDL1, and VLDL2. The areas under the incremental curve (AUIC) of apoB-48 in chylomicrons (2.83 +/- 0.84 versus 0.56 +/- 0.17; P < .05) and VLDL1 (10.17 +/- 1.96 versus 2.95 +/- 2.44; P = < .01) were markedly higher in smokers than in controls. Changes of RE responses of all TRL fractions were consistent with those of apoB-48. Postprandial apoB-100 concentrations and lipolytic enzymes were similar between the two groups. In conclusion, smokers have the syndrome of impaired TG tolerance because of defective clearance of chylomicrons and their remnants. Prolonged residence time of atherogenic remnant particles may constitute a significant risk factor for CAD in smokers.

Studies on the Expression of Genes Encoding Apolipoproteins B100 and B48 and the Low Density Lipoprotein Receptor in Nonhuman Primates: Comparison of Dietary Fat and Cholesterol

African green monkeys were fed diets containing low and moderate cholesterol concentrations with either polyunsaturated or unsaturated fat as 40% of calories. Plasma total cholesterol, low density lipoprotein (LDL) cholesterol, and apoB concentrations generally were higher in animals fed (a) the higher dietary cholesterol concentration and (b) saturated fat. At necropsy, liver and intestine were removed, and measurement of mRNAs for LDL receptors (liver) and for apolipoprotein B (liver and intestine) was done. Monkey small intestine mucosa made exclusively apoB48 while the liver made only apoB100, although apoB mRNA in both tissues was the same size (14 kilobases). No dietary cholesterol or fat effects were found for apoB mRNA abundance in the liver, while the animals fed the higher dietary cholesterol level had 50% lower levels of hepatic LDL receptor mRNA. In a separate group of animals, livers were perfused and the rate of apoB secretion was measured. No dietary fat effect on apoB secretion rate was found, and no relationship between plasma LDL cholesterol concentration and the rate of hepatic apoB production existed. These findings support the idea that the dietary factors that increase LDL concentrations act by reducing clearance of apoB-containing particles rather than by increasing production of these lipoproteins. Hepatic LDL receptor mRNA was similar in abundance in polyunsaturated fat and saturated fat-fed animals, suggesting that the difference in plasma cholesterol concentration between these groups is not mediated via effects on LDL receptor mRNA abundance. The level of intestinal apoB mRNA was about 30% higher in animals fed the moderate dietary cholesterol concentration. Earlier studies have shown that more cholesterol is transported in chylomicrons from the intestine when dietary cholesterol levels are higher, and the increased intestinal apoB mRNA abundance may reflect increased intestinal cholesterol transport and chylomicron apoB48 production.