Effect of chylomicron remnants on cholesterol metabolism in cultured rabbit hepatocytes: very low density lipoprotein and bile acid production

Glycemic and insulinemic meal responses modulate postprandial hepatic and intestinal lipoprotein accumulation in obese, insulin-resistant subjects

BACKGROUND

Exacerbated postprandial lipemia is a risk factor for cardiovascular disease and is linked to insulin status. Limited data on the effect of dietary carbohydrate on postprandial lipoprotein accumulation are available.

OBJECTIVE

We tested the hypothesis that dietary carbohydrates with different glucose availability alter postprandial lipoprotein metabolism differently in obese, insulin-resistant subjects.

DESIGN

After an overnight fast, 9 subjects with central obesity and insulin resistance but normal triacylglycerolemia randomly ingested 2 test meals with comparable amounts of fat (28-29 g) and digestible carbohydrate (91-94 g) but with different quantities of slowly available glucose (SAG) in cereal products (17 or 2 g SAG/100 g for biscuits and wheat flakes, respectively). Blood samples were collected before and for 6 h after meal intakes.

RESULTS

The postmeal 0-2-h areas under the curve (AUCs) for glycemia and insulinemia were significantly lower (P < 0.05) after the biscuit meal than after the flakes meal. Plasma triacylglycerol concentrations increased significantly after the flakes meal but not after the biscuit meal (1.5-fold higher 0-6-h AUC for the flakes meal). Apolipoprotein B-100 concentrations in the triacylglycerol-rich lipoprotein fraction increased significantly 2 h after the flakes meal but not after the biscuit meal (3-fold higher 0-6-h AUC for the flakes meal). Apolipoprotein B-48 concentrations increased (P < 0.05) 4 h after the flakes meal but not after the biscuit meal (2.3-fold higher 0-6-h AUC for the flakes meal).

CONCLUSION

Mixed meals containing slowly digestible carbohydrate that induces low glycemic and insulinemic responses reduce the postprandial accumulation of both hepatically and intestinally derived triacylglycerol-rich lipoproteins in obese subjects with insulin resistance.

The plasma and lipoprotein triglyceride postprandial response to a carbohydrate tolerance test differs in lean and massively obese normolipidemic women

The goal of the present study was to compare the plasma lipid responses of massively obese and lean women to a fat load and a carbohydrate load. For this purpose, 11 lean [body mass index (BMI), 21.6 +/- 2 kg/m(2)] and 8 obese (BMI, 50.8 +/- 7 kg/m(2)) normolipidemic women were given, in random order, either a dietary carbohydrate load (3.43 MJ, 166 g carbohydrates, 38 g proteins) or a dietary fat load (3.35 MJ, 70 g fat, 36 g proteins). Blood samples were collected hourly for 9 h after the test meal for measurements of triglyceride-rich lipoprotein (TRL)-lipid, apolipoprotein (apo)B-48 and apoB-100. Triglycerides (P < 0.0001), TRL triglycerides (P < 0.0001), TRL cholesterol (P < 0.04) and apoB-48 (P < 0.0001) peaked 3 h after the fat meal and returned progressively to baseline values in both obese women and lean controls. These lipid and apolipoprotein changes did not differ between the two groups. In contrast, after the carbohydrate load, the plasma triglyceride (P < 0.0001) and TRL triglyceride (P < 0.0001) increments were significantly greater in obese women than in lean controls. This carbohydrate-induced TRL triglyceride increment was half of that following the isocaloric fat load. The carbohydrate load did not affect apoB-100 and apoB-48 levels. These findings suggest that postprandial triglyceride metabolism is impaired after a carbohydrate load in normolipidemic massively obese women.

Dietary cholesterol is secreted in intestinally derived chylomicrons during several subsequent postprandial phases in healthy humans

BACKGROUND

The process of intestinal absorption and chylomicron resecretion of dietary cholesterol in humans is poorly understood.

OBJECTIVE

The present study aimed to test the hypothesis that dietary cholesterol ingested during a given meal is resecreted into chylomicrons (and plasma) during several subsequent postprandial periods.

DESIGN

Seven healthy subjects ingested 3 comparable mixed test meals (at 0, 8, and 24 h) containing a given amount of fat (49 g) and cholesterol (157 mg); blood samples were taken 3 and 6 h after each test meal and 48 and 72 h after the beginning of the experiment. Heptadeuterated dietary cholesterol was present in the first test meal only, enabling its specific determination with use of gas chromatography-mass spectrometry. Chylomicrons, LDL, and HDL were isolated and lipids were quantified.

RESULTS

In apolipoprotein B-48-containing chylomicrons, deuterated cholesterol concentrations were moderate after the first meal (1.3 x 10(-4) mmol/L), reached a maximum after the second meal (2.4 x 10(-4) mmol/L), and were still elevated after the third meal (1.7 x 10(-4) mmol/L). In plasma, LDL and HDL cholesterol enrichment in deuterated cholesterol was lower than in chylomicrons and plateaued after 24--48 h. Estimates of newly secreted exogenous deuterated cholesterol in chylomicrons indicate that 30.7%, 55.2%, and 14.1% of the total was secreted after the first, second, and third meals, respectively.

CONCLUSION

Ingested dietary cholesterol is secreted by the small intestine in chylomicrons into the circulation during > or =3 subsequent postprandial periods in healthy humans. This likely results from a complex multistep intestinal processing of cholesterol with dietary fat as a driving force.

Oxidation affects the regulation of hepatic lipid synthesis by chylomicron remnants

The effects of native and oxidized chylomicron remnants on lipid synthesis in normal and oxidatively stressed liver cells were investigated using MET murine hepatocytes (MMH cells), a nontransformed mouse hepatocyte cell line that maintains a highly differentiated hepatic phenotype in culture. Lipid synthesis was determined by measuring the incorporation of [(3)H]oleate into cholesteryl ester, triacylglycerol, and phospholipid by the cells. The formation of cholesteryl ester and phospholipid was decreased by chylomicron remnants in a dose-dependent manner, while triacylglycerol synthesis was increased. Exposure of MMH cells to mild oxidative stress by incubation with CuSO(4) (2.5 microM) for 24 h led to significantly increased incorporation of [(3)H]oleate into triacylglycerol and phospholipid, but not cholesteryl ester, in the absence of chylomicron remnants. In the presence of the lipoproteins, however, similar effects to those found in untreated cells were observed. Oxidatively modified chylomicron remnants prepared by incubation with CuSO(4) (10 microM, 18 h, 37 degrees C) did not influence cholesteryl ester or phospholipid synthesis in MMH cells, but had a similar effect to that found with native remnants on triacylglycerol synthesis. These findings show that hepatic lipid metabolism is altered by exposure to mild oxidative stress and by lipids from the diet delivered to the liver in chylomicron remnants, and these effects may play a role in the development of atherosclerosis.

Acute hyperinsulinism modulates plasma apolipoprotein B-48 triglyceride-rich lipoproteins in healthy subjects during the postprandial period

The role of postprandial insulin in the regulation of postprandial lipid metabolism is still poorly understood. The roles of hyperinsulinemia and insulin resistance in the alteration of postprandial lipid metabolism are not clear either. To improve knowledge in this area, we submitted healthy men to acute hyperinsulinemia in two different ways. In the first study, we compared in 10 men the effects of four isolipidic test meals that induce different degrees of hyperinsulinemia on postprandial lipid metabolism. Three different carbohydrate sources were compared according to their glycemic indexes (GIs; 35, 75, and 100 for white kidney bean, spaghetti, and white bread test meals, respectively); the fourth test meal did not contain any carbohydrates. Postprandial plasma insulin levels were proportional to the GIs (maximal plasma insulin concentrations: 113 +/- 16 to 266 +/- 36 pmol/l). We found a strong positive correlation during the 6-h postprandial period between apolipoprotein (apo) B-48 plasma concentration and insulin plasma concentration (r2 = 0.70; P = 0.0001). In a second study, 5 of the 10 subjects again ingested the carbohydrate-free meal, but during a 3-h hyperinsulinemic- (550 +/- 145 pmol/l plasma insulin) euglycemic (5.5 +/- 0.8 mmol/l plasma glucose) clamp. A biphasic response was observed with markedly reduced levels of plasma apoB-48 during insulin infusion, followed by a late accumulation of plasma apoB-48 and triglycerides. Overall, the data obtained showed that portal and peripheral hyperinsulinism delays and exacerbates postprandial accumulation of intestinally derived chylomicrons in plasma and thus is involved in the regulation of apoB-48-triglyceride-rich lipoprotein metabolism, in the absence of insulin-resistance syndrome.

Escherichia coli sepsis increases hepatic apolipoprotein B secretion by inhibiting degradation

Sepsis leads to hypertriglyceridemia in both humans and animals. Previously, we reported that plasma very low density lipoprotein apolipoprotein (apo) B and hepatic production of apoB increased during Escherichia coli sepsis. The present experiments were undertaken to determine whether the altered hepatic secretion of apoB was associated with an increase in synthesis or a decrease in degradation rate. Sepsis was induced in male, Lewis rats (225-275 g) by intravenous injection of 3.8 x 10(8) live E. coli colonies/100 g body. Twenty-four hours later rats were sacrificed, and primary hepatocytes were prepared and incubated overnight with 35S-methionine. Hepatocytes from E. coli-treated rats secreted twice as much apoB-48 and total apoB than the hepatocytes from control rats. Escherichia coil sepsis increased cellular triglyceride mass by 86%, which was due to a stimulation in triglyceride synthesis from newly synthesized fatty acids, measured by 3H2O incorporation into triglycerides. The apoB synthesis rate, apoB mRNA levels, and apoB mRNA editing were not altered during E. coil sepsis. The pulse-chase experiments showed that the rate of apoB degradation decreased in E. coli-treated rats. These findings demonstrate that the secretion of apoB is regulated posttranslationally during E. coli sepsis by decreasing the degradation of newly synthesized apoB, which contributes to the development of hypertriglyceridemia.

Dietary fat clearance in type V hyperlipoproteinaemia secondary to a rare variant of human apolipoprotein E: the apolipoprotein E3 (Arg 136-->Ser)

This present case report describes two siblings with severe type V hyperlipoproteinaemia, diagnosed very early in life and due to the combination of the common apolipoprotein (Apo) E2 allele and rare mutant variant of ApoE, ApoE3 (Arg 136-->Ser). Phenotyping of ApoE falsely identified E2/E2 phenotype. The presence of mutated ApoE was suspected on an unusual restriction polymorphism of a Hha 1 restriction site and confirmed by sequence analysis of the cloned polymerase chain reaction fragment of exon 4 and familial segregation study. The severity of the hypertriacylglycerolaemia was modulated by the lipid content of the diet. A low-fat diet enriched in medium-chain triacylglycerol (TAG) decreased but did not normalize plasma TAG levels in both affected patients of the pedigree. A standardized lipid-enriched test meal showed a marked impairment of TAG-rich lipoprotein (TRL) clearance, especially the exogeneous TRL bearing ApoB-48 which still represented 79% of total TRL 7 h after the fat load. Finally, differences between the male and female siblings with the existence of a consanguine relationship in their parents suggested the involvement of other genetic factors in modulating the severity of phenotypic expression. This observation reinforces the usefulness of genotyping of ApoE for the characterization of genetic hypertriacylglycerolaemia and selection of the appropriate diet and treatment.

Differential effects of chylomicron remnants derived from corn oil or palm oil on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes

The effects of chylomicron remnants derived from corn oil (rich in n-6 polyunsaturated fatty acids) and palm oil (rich in long chain saturated fatty acids) on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes were studied. Incubation of the cells with corn oil remnants led to increased bile acid production, while the secretion of lipid in very low density lipoprotein remained unchanged. In contrast, addition of palm oil remnants to the medium did not affect bile acid synthesis, but resulted in the secretion of cholesterol-rich very low density lipoprotein. These findings show that chylomicron remnants of different fatty acid composition have differential effects on cholesterol metabolism in liver cells, and provide part of the explanation for the hyper- and hypocholesterolaemic effects of saturated and polyunsaturated fatty acids.

Bile acid synthesis in hamster hepatocytes in primary culture: sources of cholesterol and comparison with other species

The synthesis of bile acids by primary hamster hepatocytes in culture has been studied. Measurable rates of bile acid synthesis were obtained from cells prepared from livers of animals fed 2% w/w cholestyramine to induce the synthesis of bile acids through the rate-limiting enzyme cholesterol 7 alpha-hydroxylase. The effects of various sources of substrate for bile acid synthesis in these cultured cells were examined over a period of 24 h and the results compared with published or parallel studies in primary rat hepatocytes or in the human hepatoma cell line, HepG2. In all the cells, bile acid synthesis was stimulated by the addition of 7 alpha-hydroxycholesterol, indicating the rate-limiting role of the cholesterol 7 alpha-hydroxylase. Bile acid synthesis in the hamster hepatocytes was also stimulated by a variety of sources of cholesterol as substrate, mevalonic acid (increasing the production of newly-synthesised cholesterol in the cell), and as an exogenous source, hamster LDL. Similarly, if cholesterol was diverted from intracellular esterification using the ACAT inhibitor Dup128, a further increase in bile acid synthesis could be demonstrated. These results show that hepatocytes obtained from cholestyramine-treated hamsters are deficient in substrate cholesterol for bile acid synthesis. A similar conclusion can be drawn from the published work with rat hepatocytes and is further supported by experiments on the regulation of cholesterol 7 alpha-hydroxylase activity at the mRNA and the protein level, although some in vivo studies in animals and studies in man have led authors to suggest that cholesterol 7 alpha-hydroxylase is saturated with substrate.