Hypoalbuminemia increases lysophosphatidylcholine in low-density lipoprotein of normocholesterolemic subjects.
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BACKGROUND: A phospholipid, lysophosphatidylcholine (LPC), is the major determinant of the atherosclerotic properties of oxidized low-density lipoprotein (LDL). Under normal circumstances most LPC is bound to albumin. We hypothesized that lipoprotein LPC concentrations are increased in hypoalbuminemic patients with the nephrotic syndrome, irrespective of their lipid levels. To test this hypothesis, we selected nephrotic and control subjects with matched LDL cholesterol levels. METHODS: Lipoproteins and the albumin-rich lipoprotein-deficient fractions were separated by ultracentrifugation and their phospholipid composition was analyzed by thin-layer chromatography. RESULTS: Nephrotic subjects (albumin 23 +/- 2 g/liter and LDL cholesterol 3.1 +/- 0.2 mmol/liter) had a LDL LPC concentration that was increased (P < 0.05) to 66 +/- 7 vs. 35 +/- 6 micromol/liter in matched controls (albumin 42 +/- 5 g/liter and LDL cholesterol 3.1 +/- 0.2 mmol/liter). LPC in very low-density lipoprotein plus intermediate-density lipoprotein (VLDL + IDL) in these subjects was also increased to 33 +/- 7 vs. 9 +/- 2 micromol/liter in controls (P < 0.05). Conversely, LPC was decreased to 19 +/- 4 micromol/liter in the albumin-containing fraction of these hypoalbuminemic patients, as compared to 46 +/- 10 micromol/liter in the controls (P < 0.05). LPC was also low (14 +/- 4 micromol/liter) in the albumin-containing fraction of hypoalbuminemic, hypocholesterolemic patients with nonrenal diseases. In hyperlipidemic nephrotic subjects (albumin 21 +/- 2 g/liter and LDL cholesterol 5.7 +/- 0.5 mmol/liter) the LPC levels in LDL and VLDL + IDL were further increased, to 95 +/- 20 and 56 +/- 23 micromol/liter, respectively (P < 0.05). CONCLUSION: These findings suggest that in the presence of hypoalbuminemia in combination with proteinuria, LPC shifts from albumin to VLDL, IDL and LDL. This effect is independent of hyperlipidemia. Increased LPC in lipoproteins may be an important factor in the disproportionate increase in cardiovascular disease in nephrotic patients with hypoalbuminemia. (+info)
Liver-specific inactivation of the abetalipoproteinemia gene completely abrogates very low density lipoprotein/low density lipoprotein production in a viable conditional knockout mouse.
(2/2383)
Conventional knockout of the microsomal triglyceride transfer protein large subunit (lMTP) gene is embryonic lethal in the homozygous state in mice. We have produced a conditional lMTP knockout mouse by inserting loxP sequences flanking exons 5 and 6 by gene targeting. Homozygous floxed mice were born live with normal plasma lipids. Intravenous injection of an adenovirus harboring Cre recombinase (AdCre1) produced deletion of exons 5 and 6 and disappearance of lMTP mRNA and immunoreactive protein in a liver-specific manner. There was also disappearance of plasma apolipoprotein (apo) B-100 and marked reduction in apoB-48 levels. Wild-type mice showed no response, and heterozygous mice, an intermediate response, to AdCre1. Wild-type mice doubled their plasma cholesterol level following a high cholesterol diet. This hypercholesterolemia was abolished in AdCre1-treated lMTP-/- mice, the result of a complete absence of very low/intermediate/low density lipoproteins and a slight reduction in high density lipoprotein. Heterozygous mice showed an intermediate lipoprotein phenotype. The rate of accumulation of plasma triglyceride following Triton WR1339 treatment in lMTP-/- mice was <10% that in wild-type animals, indicating a failure of triglyceride-rich lipoprotein production. Pulse-chase experiments using hepatocytes isolated from wild-type and lMTP-/- mice revealed a failure of apoB secretion in lMTP-/- animals. Therefore, the liver-specific inactivation of the lMTP gene completely abrogates apoB-100 and very low/intermediate/low density lipoprotein production. These conditional knockout mice are a useful in vivo model for studying the role of MTP in apoB biosynthesis and the biogenesis of apoB-containing lipoproteins. (+info)
Apolipoprotein B in the rough endoplasmic reticulum: translation, translocation and the initiation of lipoprotein assembly.
(3/2383)
Apolipoprotein (apo) B and the microsomal triglyceride transfer protein are essential for the hepatic assembly and secretion of triglyceride-rich VLDL. To understand how apoB initiates the process of lipoprotein formation, interest has focused on the biogenesis of its amino terminal globular domain (alpha1 domain). When only this domain is expressed in hepatoma cells, no lipoprotein particle will form. However, proper folding of the alpha1 domain is essential for the internal lipophilic regions of apoB to engage in cotranslational lipid recruitment. The essential function of this domain may be related to its capacity to promote a specific physical interaction with the microsomal triglyceride transfer protein, necessary for apoB's proper folding and lipidation. Alternatively, this domain may promote an autonomous lipid recruitment step that nucleates microsomal triglyceride transfer protein-dependent lipid sequestration by apoB. Forms of apoB that fail to initiate particle assembly or forms associated with aberrant underlipidated particles are targeted for intracellular turnover. Two sites of apoB degradation have been identified. In hepatocarcinoma-derived cells, misassembled apoB may undergo progressive reverse translocation from the endoplasmic reticulum lumen to the cytosol, a process that is mechanistically coupled to polyubiquitination and proteasome-mediated degradation on the cytosolic side of the membrane. Alternatively, studies in primary hepatocytes reveal that apoB may undergo sorting to a post-endoplasmic reticulum compartment for presecretory degradation. In either case, the balance between assembly and presecretory degradation of apoB may represent a control point for the production of hepatic VLDL. (+info)
Assembly of very low density lipoprotein: a two-step process of apolipoprotein B core lipidation.
(4/2383)
The liver plays a primary role in lipid metabolism. Important functions include the synthesis and incorporation of hydrophobic lipids, triacylglycerols and cholesteryl esters into the core of water-miscible particles called lipoproteins and the secretion of these particles into the circulation for transport to distant tissues. In this article, we present a brief overview of one aspect of the assembly process of very low density lipoproteins, namely, possible mechanisms for combining core lipids with apolipoprotein B. This is a complex process in which apolipoprotein B interacts with core lipids to form very low density lipoproteins by a two-step process that can be dissociated biochemically. (+info)
Estrogen induction of VLDLy assembly in egg-laying hens.
(5/2383)
The yolk of a 60-g chicken egg contains 6 g of triacylglycerols transported to the oocyte from the liver of the laying hen in apolipoprotein (apo) B-containing particles. With the onset of egg production, estrogen shifts hepatocytic lipoprotein production from generic VLDL to VLDLy (yolk targeted). These VLDLy are triacylglycerol-rich particles; they are reduced in size by one half, are resistant to lipoprotein lipase and are taken up intact by oocyte receptors. The VLDLy pathway for apoB provides sufficient energy for the caloric requirements of chick development. VLDLy size reduction occurs in spite of surplus liver triacylglycerols and is necessary for VLDL particles to pass through the granulosa basal lamina and reach the receptors located on the oocyte surface. New ultrastructural data show that some proximal tubule cells of bird kidney secrete generic VLDL, perhaps providing energy and other VLDL-associated nutrients to tissues bypassed by VLDLy. Birds are an apoB100-only species, providing a natural in vivo model with which to investigate mechanisms of apoB100 VLDL assembly. Preliminary studies of liver lipoprotein assembly intermediates isolated from the biosynthetic membranes (endoplasmic reticulum) of the laying hen are consistent with the presence of both putative first- and second-step precursor particles of VLDLy. These findings suggest that the two-step mechanism of apoB core lipidation is an ancient development in apoB biology, handed down to mammals from oviparous ancestors. (+info)
Competition of Abeta amyloid peptide and apolipoprotein E for receptor-mediated endocytosis.
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The genetic polymorphism of apolipoprotein E (apoE) is associated with the age of onset and relative risk of Alzheimer's disease (AD). In contrast to apoE3, the wild type allele, apoE4 confers an increased risk of late-onset AD. We demonstrate that the beta-amyloid peptide isoforms Abeta (1-28), Abeta (1-40), and Abeta (1-43) compete for the cellular metabolism of apoE3 and apoE4 containing beta-very low density lipoproteins. An antibody raised against Abeta (1-28) cross-reacted with recombinant apoE. Epitope mapping revealed positive amino acid clusters as common epitopes of Abeta (13 through 17; HHQKL) and apoE (residues 144 through 148; LRKRL), both regions known to be heparin binding domains. Abeta in which amino acids 13 through 17 (HHQKL) were replaced by glycine (GGQGL) failed to compete with the cellular uptake of apoE enriched betaVLDL. These observations indicate that Abeta and apoE are taken up into cells by a common pathway involving heparan sulfate proteoglycans. (+info)
Identification of a novel Arg-->Cys mutation in the LDL receptor that contributes to spontaneous hypercholesterolemia in pigs.
(7/2383)
We previously carried out genetic and metabolic studies in a partially inbred herd of pigs carrying cholesterol-elevating mutations. Quantitative pedigree analysis indicated that apolipoprotein (apo)B and a second major gene were responsible for the hypercholesterolemia in these animals. In this study, we assessed LDL receptor function by three different methods: ligand blots of liver membranes using beta-very low density lipoprotein (VLDL) as a ligand; low density lipoprotein (LDL)-dependent proliferation of T-lymphocytes; and direct binding of 125I-labeled LDL to cultured skin fibroblasts. All three methods demonstrated that LDL receptor ligands bound with decreased affinity to the LDL receptor in these animals. In skin fibroblasts from the hypercholesterolemic pigs, the Kd of binding was about 4-fold higher than in cells from normal pigs. The cDNA of the pig LDL receptor from normal and hypercholesterolemic pigs was isolated and sequenced. We identified a missense mutation that results in an Arg'Cys substitution at the position corresponding to Arg94 of the human LDL receptor. The mutation is in the third repeat of the ligand binding domain of the receptor. By single-stranded conformational polymorphism (SSCP) analysis, we studied the relationship between LDL receptor genotype and plasma cholesterol phenotype. In contrast to humans, the hypercholesterolemia associated with the LDL receptor mutation in pigs was expressed as a recessive trait. The LDL receptor mutation made a far more significant contribution to hypercholesterolemia than did the apoB mutation, consistent with observations made in human subjects with apoB mutations. Within each genotypic group (mutated apoB or mutated receptor), there was a wide range in plasma cholesterol. As the animals were on a well-controlled low-fat diet, this suggests that there are additional genetic factors that influence the penetrance of cholesterol-elevating mutations. (+info)
Dietary fish oils inhibit early events in the assembly of very low density lipoproteins and target apoB for degradation within the rough endoplasmic reticulum of hamster hepatocytes.
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Dietary fish oils inhibited secretion and stimulated intracellular degradation of apolipoprotein (apo)B in hamster hepatocytes, while dietary sunflower oils stimulated secretion and had no effect on degradation of apoB. To investigate the intracellular site at which fish oils act, we have made use of our previous observations that inhibition of degradation by N-acetyl-leucyl-leucyl-norleucinal (ALLN) results in accumulation of apoB in the trans -Golgi membrane and does not stimulate secretion, while inhibition of degradation by o-phenanthroline results in accumulation of apoB in the rough endoplasmic reticulum membrane and stimulates secretion. Thus, ALLN protects apoB which has been diverted from secretion and o -phenanthroline protects apoB which is targetted for secretion. Addition of o -phenantholine to the incubation medium of hepatocytes from fish oil-fed hamsters inhibited degradation of apoB and stimulated its secretion in particles of the density of VLDL, while addition of ALLN had no effect. These observations suggest that dietary fish oils reversibly inhibit early steps in the assembly of very low density lipoprotein precursors and target apoB for degradation in the rough endoplasmic reticulum. (+info)