Intestinal lipoprotein synthesis. Comparison of nascent Golgi lipoproteins from chow-fed and hypercholesterolemic rats

A proposed model for the assembly of chylomicrons

The intestine synthesizes very low density lipoproteins (VLDL) and chylomicrons (CM) to transport fat and fat-soluble vitamins into the blood. VLDL assembly occurs constitutively whereas CM assembly is a characteristic property of the enterocytes during the postprandial state. The secretion of CM is specifically inhibited by Pluronic L81. CM are very heterogeneously-sized particles that consist of a core of triglycerides (TG) and cholesterol esters and a monolayer of phospholipids (PL), cholesterol and proteins. The fatty acid composition of TG, but not PL, in CM mirrors the fatty acid composition of fat in the diet. CM assembly is deficient in abetalipoproteinemia and CM retention disease. Abetalipoproteinemia results due to mutation in the mttp gene and is characterized by the virtual absence of apoB-containing lipoproteins in the plasma. Patients suffer from neurologic disorders, visual impairment, and exhibit acanthocytosis. CM retention disease, an inherited recessive disorder, is characterized by chronic diarrhea with steatorrhea in infancy, abdominal distention and failure to thrive. It is caused by a specific defect in the secretion of intestinal lipoproteins; secretion of lipoproteins by the liver is not affected. Besides human disorders, mice that do not assemble intestinal lipoproteins have been developed. These mice are normal at birth, but defective in fat and fat-soluble vitamin absorption, and fail to thrive. Thus, fat and fat-soluble vitamin transport by the intestinal lipoproteins is essential for proper growth and development of neonates. Recently, differentiated Caco-2 cells and rabbit primary enterocytes have been described that synthesize and secrete CM. These cells can be valuable in distinguishing between the two different models proposed for the assembly of CM. In the first model, the assembly of VLDL and CM is proposed to occur by two 'independent' pathways. Second, CM assembly is proposed to be a product of 'core expansion' that results in the synthesis of lipoproteins of different sizes. According to this model, intestinal lipoprotein assembly begins with the synthesis of 'primordial' lipoprotein particles and involves release of the nascent apoB with PL derived from the endoplasmic reticulum (ER) membrane. In addition, TG-rich 'lipid droplets' of different sizes are formed independent of apoB synthesis. The fusion of lipid droplets and primordial lipoproteins results in the formation of different size lipoproteins due to the 'core expansion' of the primordial lipoproteins.

Postprandial lipaemia is associated with increased levels of apolipoprotein A-IV in the triacylglycerol-rich fraction and decreased levels in the denser plasma fractions

Apolipoprotein (apo) A-IV is primarily associated with HDL or with the lipoprotein-free fraction of plasma, and in small amounts with chylomicrons and VLDL. The aim of the present study was to assess the effect of a fatty meal on the postprandial variation in plasma apo A-IV and on its distribution among lipoprotein fractions following absorption of fat. Twenty healthy male subjects participated in the study. After an overnight fast, subjects were given a fatty breakfast containing 1 g fat/kg body weight (% energy: fat 65, carbohydrate 20; protein 15). Blood samples were taken every hour during the next 10 h. Apo A-IV was measured by ELISA. Postprandial lipaemia was associated with a moderate, although significant, increase in the plasma levels of apo A-IV. Apo A-IV increased from the median baseline value of 0.15 g/l to 0.165 g/l (median +17%; P < 0.01) 5 h after fat ingestion. The postprandial peak of apo A-IV occurred 1 h after the triacylglycerol peak. There were no statistically significant correlations between baseline lipids, baseline apo A-IV and postprandial changes in apo A-IV levels, or between postprandial changes in lipids and apo A-IV at any time. To assess apo A-IV distribution among lipoproteins, plasma was fractionated by fast performance liquid chromatography at baseline and 3, 6 and 10 h postprandially. There was a substantial heterogeneity in the apo A-IV distribution among lipoproteins following the fatty meal. At 3 h after fat ingestion, apo A-IV levels increased in the triacyglycerol-rich lipoprotein (TRL) fraction and decreased in the denser plasma fraction. At 6 h after the fatty meal, apo A-IV was still present in the TRL but was decreased in the HDL fractions. The findings of the present study support the concept that apo A-IV particles transfer from the denser plasma fraction to TRL during postprandial lipaemia.

Effect of pluronic L-81 on intestinal lipoprotein secretion in the rat

The hydrophobic nonionic detergent Pluronic L-81 has been shown to lower plasma very-low- and low-density lipoprotein cholesterol, thus preventing diet-induced atherogenesis. The major effect of this agent is a pronounced interference with intestinal lipid metabolism. For studying mesenteric lymph lipoproteins during detergent exposure, a combined micromorphological and biochemical assessment of mucosa and lymph during steady-state lipid absorption was performed. Pluronic L-81 was infused intraduodenally at a constant rate in combination with mixed micellar solutions or saline in mesenteric lymph fistula rats. Pluronic L-81 impairs transepithelial lipid flux during fat absorption, trapping export lipids within the enterocytes and leading to a cytosolic and endoplasmic reticulum lipid accumulation sparing the Golgi region. Pluronic L-81 markedly (P < 0.001) reduces mesenteric triglyceride, phospholipid, and total cholesterol secretion almost exclusively by a reduction of chylomicron formation. Chylomicron and very-low-density lipoprotein lipid composition was only insignificantly altered, except for somewhat higher phospholipid/triglyceride ratios. The chylomicron apoprotein pattern was almost unaffected. Thus, chylomicron formation decreased dramatically without major compositional alterations. The reduction of lipid and apoprotein secretion without particle augmentation is not in favour of a selective interference of Pluronic L-81 with intestinal apoprotein B-48 secretion.

Chylomicron assembly and catabolism: role of apolipoproteins and receptors

Chylomicrons are lipoproteins synthesized exclusively by the intestine to transport dietary fat and fat-soluble vitamins. Synthesis of apoB48, a translational product of the apob gene, is required for the assembly of chylomicrons. The apob gene transcription in the intestine results in 14 and 7 kb mRNAs. These mRNAs are post-transcriptionally edited creating a stop codon. The edited mRNAs chylomicrons from the shorter apoB48 peptide remains to be elucidated. In addition, the roles of proteins involved in the assembly pathway, e.g. apobec-1, MTP and apoA-IV, needs to be studied. Cloning of enzymes involved in the intestinal biosynthesis of triglycerides will be crucial to fully appreciate the assembly of chylomicrons. There is a need for cell culture and transgenic animal models that can be used for intestinal lipoprotein assembly. The catabolism of chylomicrons is far more complex and efficient than the catabolism of VLDL. Even though the major steps involved in the catabolism of chylomicrons are now known, the determinants for apolipoprotein exchange, processing of remnants in the space of Disse, as well as the mechanism of uptake of these particles by extra-hepatic tissue needs further exploration.

Ontogenetic regulation of apolipoprotein B mRNA editing during human and rat development in vivo

The solubilization and delivery of lipids in plasma rely on both forms of apolipoprotein B (apo B): apo B-100 and apo B-48. Apo B-48 is the translational product of apo B-100 mRNA that undergoes peritranscriptional conversion of C----U, replacing codon CAA (glutamine 2,153) with the inframe stop codon (UAA). We examined mRNA editing activity in the human and the rat by reverse transcription-polymerase chain reaction primer-extension analysis of intestine and liver total RNA. In rat intestine the percentage of apo B transcripts that undergo editing increases dramatically the day before birth (from approximately 1% to 80%), whereas the rat liver acquires an adult level of editing activity during the third postnatal week (rising from approximately 8% to 30%), when weaning is completed, bile acid composition matures, and plasma thyroid hormone levels peak. In contrast to the rat, the human intestine acquires adult levels of apo B mRNA editing relatively early in fetal development, rising from 10% at 10 weeks to approximately 80% by the end of the second trimester. Our results establish that apo B mRNA editing is 1) developmentally regulated in a tissue- and species-specific manner; 2) fully developed prenatally in both human and rat intestine, suggesting a crucial role of apo B-48 in mammalian fetal adaptation to extrauterine life; and 3) acquired early in human fetal intestine, implying a potential role for apo B-48 in prenatal lipid metabolism.

The absorption and transport of lipids by the small intestine

Dietary lipid provides as much as 40% of the caloric intake in the Western diet. Triacylglycerol is the main dietary fat. The human small intestine is also presented daily with 11-12 g of phospholipid, predominantly phosphatidylcholine. The predominant sterol in the Western diet is cholesterol, which is derived from animal fat. Plant sterols account for up to 20-25% of total dietary sterol. This paper reviews our current understanding of the process and the factors that regulate the absorption and transport of different dietary lipids by the human small intestine.

Distribution of apolipoprotein E between free and A-II complexed forms in very-low- and high-density lipoproteins: functional implications

The stability of apolipoprotein E/lipoprotein associations has been examined as a function of apolipoprotein E phenotype. Visualisation by immunoblotting showed plasma apolipoprotein E to be present in two forms; the free form and, as previously described, an E-A-II complex. In very low density lipoproteins isolated by gel filtration from subjects with E3/3 and E4/3 phenotypes, apolipoprotein E was present essentially in the free form (ratio free: complex of 12.2 and 37.5, respectively). Exploiting ultracentrifugation as the disruptive agent, very-low-density lipoproteins thus isolated were shown to have substantially lower ratios (5.6 and 5.4, respectively) reflecting preferential loss of free apolipoprotein E. In high-density lipoproteins isolated by gel filtration from E3/3 phenotypes, apolipoprotein E was largely present as an E-A-II complex (80.3%). In contrast, the majority of apolipoprotein E in high-density lipoproteins from E4/3 phenotypes was present in the free form (58.7%). In both phenotypes, the content of free apolipoprotein E was markedly reduced by ultracentrifugation. The results confirm the notion that the formation of the E-A-II complex is a major determinant of the stability of apolipoprotein E-high-density lipoprotein associations. Moreover, that the predominant, ancestral isoform, apolipoprotein E3, exists largely as an E-A-II complex in higher density lipoproteins has important functional implications for this plasma source of apolipoprotein E.

The assembly of lipids into lipoproteins during secretion

The process of assembly and secretion of lipoproteins is discussed with particular reference to the role of lipids. The majority of circulating lipoproteins is produced by the liver (80%) with the remainder being supplied by the intestine. The liver secretes both very low density lipoproteins and high density lipoproteins, but the assembly and secretion of these two types of particles may follow different routes. The major lipid components of lipoproteins are triacylglycerols, cholesterol, cholesterol esters and phospholipids. The biosynthesis of these lipids occurs on membranes of the endoplasmic reticulum, with many of the enzymes also being present in the Golgi; the roles of these two subcellular organelles in the assembly of lipoproteins are discussed. There appears to be a compartmentalization of lipids in cells, such that defined pools, often those newly-synthesized, are preferred, or even required, for lipoprotein assembly. The process of hepatic very low density lipoprotein secretion appears to be regulated by the supply of lipids. Indeed, the synthesis of new lipid may be a major driving force in lipoprotein assembly and secretion.

Intestinal lipoprotein synthesis in control and hypercholesterolemic rats

Previous studies in our laboratory have shown that very-low-density lipoproteins (VLDL) synthesized by the intestine of the diet-induced hypercholesterolemic rat are enriched in cholesteryl esters and unesterified cholesterol compared with intestinal VLDL from control rats. In these studies, we isolated and characterized nascent intestinal Golgi intermediate-density lipoproteins (IDL, d 1.006-1.040 g/ml) and studied isotope incorporation into apoliproteins of Golgi VLDL from control and hypercholesterolemic rats. IDL were triacylglycerol-rich lipoproteins but contained more cholesteryl ester and protein than the corresponding Golgi VLDL fractions. IDL from hypercholesterolemic rats were enriched in cholesteryl esters to a greater extent than IDL from control rats. The apolipoprotein patterns of IDL fractions were the same as those of intestinal Golgi VLDL, consisting of apolipoproteins (apo) B-48, A-I and A-IV. Time-course isotope incorporation curves for apo A-I and A-IV in Golgi VLDL were similar, but they differed from curves for apo B-48. None of these curves was markedly altered in the hypercholesterolemic rat. We conclude that the major effect of increased dietary cholesterol on intestinal lipoprotein biosynthesis is to increase the percentage of cholesteryl esters in Golgi lipoproteins. Dietary cholesterol does not alter the apolipoprotein composition of Golgi lipoproteins, nor does it have a significant effect on the pattern of isotope incorporation into apolipoproteins of Golgi VLDL. The effect of cholesteryl ester enrichment on the subsequent metabolism of these particles in the circulation and the effect of these particles on hepatic lipoprotein production remain to be determined.

Effect of marginal zinc deficiency on the apolipoprotein-B content and size of mesenteric lymph chylomicrons in adult rats

To investigate the mechanisms underlining the impaired intestinal absorption of lipids in zinc deficiency, the apo-B content and chemical composition of chylomicrons from marginally zinc-deficient rats fed 2.8 ppm of dietary zinc (ZD) were compared with those from pair-fed (PF) and ad libitum control (CT) groups fed an adequate level (30.8 ppm) of zinc. Chylomicrons, obtained by cannulating the mesenteric lymph, were isolated by ultracentrifugation at 1.3 X 10(6) g/min at 12 C and purified by 2% agarose column chromatography. Apolipoprotein- (apo) B was separated by the method of isopropanol precipitation. The apo-B concentration of chylomicrons was lowered significantly in ZD group. The apo-B contents of chylomicrons in ZD, PF and CT rats, as expressed as % chylomicron protein, were 8.7 +/- 0.1, 11.5 +/- 0.5 and 10.7 +/- 0.7%, respectively. No significant differences were noted between ZD and PF groups in total protein (TP), phospholipid (PL), triglyceride (TG) and cholesterol (CH), although there was a slight decrease in TG and an increase in CH in CT rats compared with ZD and PF groups. The ratio of the core to surface constituents, as determined by TG/(TP + PL), was significantly higher in ZD group relative to the controls, suggesting that chylomicrons from ZD rats were larger. This finding was consistent with the appearance of larger chylomicron particles in the lacteal of the intestinal mucosa following lipid ingestion. These findings suggest that the intestinal synthesis of apo-B may be defective in zinc-deficient rats and may explain in part the impaired absorption of dietary lipids observed in zinc deficiency.

Post-secretory acquisition of apolipoprotein E by nascent rat hepatic very-low-density lipoproteins in the absence of cholesteryl ester transfer

Previous work has shown that nascent hepatic very-low-density lipoproteins (VLDL) in the rat are biosynthesized without the obligatory co-factor (apolipoprotein C-II) for lipoprotein lipase-mediated hydrolysis of their core triacylglycerols. Upon secretion, apolipoproteins C-II and C-III are rapidly transferred to the particles from high-density lipoprotein (HDL) within the space of Disse and upon the entry into the plasma. Here we extend those studies to include observations on the apolipoprotein E content and lipid composition of nascent hepatic VLDL before and after exposure to plasma components. We have elected to use hepatic secretory vesicle VLDL rather than liver perfusate VLDL as truly representative of the nascent lipoproteins. Nascent VLDL from fed rats has an apolipoprotein B/E ratio of 6.6 +/- 0.5, whereas that from fasted animals is 13.9 +/- 2.3. Incubation of nascent VLDL from fed and fasted rats with d greater than 1.063 g/ml rat serum, HDL or the d greater than 1.21 g/ml fraction resulted in a mass transfer of apolipoprotein E to the VLDL such that the apolipoprotein B/E ratio decreased to at least that of serum VLDL (3.4 +/- 0.3). The d greater than 1.21 g/ml fraction appeared to contain a species of apolipoprotein E which most actively transferred to VLDL. The acquisition of apolipoprotein E by nascent secretory vesicle VLDL was attended by a loss of phospholipids, particularly the C40 (stearoylarachidonyl) molecular species, and an increase in the cholesterol-to-phospholipid ratio from 0.11 +/- 0.01 to 0.18 +/- 0.03. No evidence was obtained to suggest a simultaneous acquisition of cholesteryl esters upon incubation of nascent VLDL with VLDL-free serum. We conclude that nascent hepatic VLDL is modified after secretion by acquisition of apolipoproteins C-II, C-III and E with a concomitant loss of phospholipids.

Effects of high cholesterol high fat diet on plasma lipoproteins in familial hypercholesterolemia

Heterozygous individuals with familial hypercholesterolemia possess about half of the normal numbers of functioning receptors on their cells. This is thought to be responsible for their hypercholesterolemia. In normals, dietary cholesterol increases LDL production and decreases LDL receptor-related LDL clearance, resulting in elevations in LDL cholesterol levels of approximately 30 mg/dL. To assess the effects of high fat and high cholesterol diets on the lipoproteins of individuals with diminished LDL receptors, three kinds of diets, including ones high in cholesterol, were fed to four patients with familial hypercholesterolemia, in the expectation that diet effects on apoB- or apoE-containing lipoproteins would be exaggerated. The basal diet consisted of 15% protein, 30% fat, 55% carbohydrate, 300 mg/d cholesterol, P/S ratio 0.4; the high fat diet was identical except that fat calories were 55% and carbohydrate 30%; the high fat-high cholesterol diet was identical with the high fat diet except approximately 750 or approximately 1,500 mg/d of cholesterol were added. Each diet was eaten for five weeks at home and for the sixth week at the general Clinical Research Center. Fasting (12-14 hours) plasmas were collected every two weeks for lipoprotein-lipid and apoprotein quantitation. At the end of each period, fasting and 4-hour postprandial samples were analyzed also by zonal ultracentrifugation and gel permeation chromatography.(ABSTRACT TRUNCATED AT 250 WORDS)