Assembly of very low density lipoproteins in rat liver: a study of nascent particles recovered from the rough endoplasmic reticulum

Effect of liver total sphingomyelin synthase deficiency on plasma lipid metabolism

Sphingomyelin (SM) is one major phospholipids on lipoproteins. It is enriched on apolipoprotein B-containing particles, including very low-density lipoprotein (VLDL) and its catabolites, low-density lipoprotein (LDL). SM is synthesized by sphingomyelin synthase 1 and 2 (SMS1 and SMS2) which utilizes ceramide and phosphatidylcholine, as two substrates, to produce SM and diacylglyceride. SMS1 and SMS2 activities are co-expressed in all tested tissues, including the liver where VLDL is produced. Thus, neither Sms1 gene knockout (KO) nor Sms2 KO approach is sufficient to evaluate the effect of SMS on VLDL metabolism. We prepared liver-specific Sms1 KO/global Sms2 KO mice to evaluate the effect of hepatocyte SM biosynthesis in lipoprotein metabolism. We found that hepatocyte total SMS depletion significantly reduces cellular sphingomyelin levels. Also, we found that the deficiency induces cellular glycosphingolipid levels which is specifically related with SMS1 but not SMS2 deficiency. To our surprise, hepatocyte total SMS deficiency has marginal effect on hepatocyte ceramide, diacylglyceride, and phosphatidylcholine levels. Importantly, total SMS deficiency decreases plasma triglyceride but not apoB levels and reduces larger VLDL concentration. The reduction of triglyceride levels also was observed when the animals were on a high fat diet. Our results show that hepatocyte total SMS blocking can reduce VLDL-triglyceride production and plasma triglyceride levels. This phenomenon could be related with a reduction of atherogenicity.

From Congenital Disorders of Fat Malabsorption to Understanding Intra-Enterocyte Mechanisms Behind Chylomicron Assembly and Secretion

During the last two decades, a large body of information on the events responsible for intestinal fat digestion and absorption has been accumulated. In particular, many groups have extensively focused on the absorptive phase in order to highlight the critical "players" and the main mechanisms orchestrating the assembly and secretion of chylomicrons (CM) as essential vehicles of alimentary lipids. The major aim of this article is to review understanding derived from basic science and clinical conditions associated with impaired packaging and export of CM. We have particularly insisted on inborn metabolic pathways in humans as well as on genetically modified animal models (recapitulating pathological features). The ultimate goal of this approach is that "experiments of nature" and in vivo model strategy collectively allow gaining novel mechanistic insight and filling the gap between the underlying genetic defect and the apparent clinical phenotype. Thus, uncovering the cause of disease contributes not only to understanding normal physiologic pathway, but also to capturing disorder onset, progression, treatment and prognosis.

Membrane vesiculation induced by proteins of the dengue virus envelope studied by molecular dynamics simulations

Biological membranes are continuously remodeled in the cell by specific membrane-shaping machineries to form, for example, tubes and vesicles. We examine fundamental mechanisms involved in the vesiculation processes induced by a cluster of envelope (E) and membrane (M) proteins of the dengue virus (DENV) using molecular dynamics simulations and a coarse-grained model. We show that an arrangement of three E-M heterotetramers (EM3) works as a bending unit and an ordered cluster of five such units generates a closed vesicle, reminiscent of the virus budding process. In silico mutagenesis of two charged residues of the anchor helices of the envelope proteins of DENV shows that Arg-471 and Arg-60 are fundamental to produce bending stress on the membrane. The fine-tuning between the size of the EM3 unit and its specific bending action suggests this protein unit is an important factor in determining the viral particle size.

Hepatic ABCA1 deficiency is associated with delayed apolipoprotein B secretory trafficking and augmented VLDL triglyceride secretion

ATP binding cassette transporter A1 (ABCA1) is a membrane transporter that facilitates nascent HDL formation. Tangier disease subjects with complete ABCA1 deficiency have <5% of normal levels of plasma HDL, elevated triglycerides (TGs), and defective vesicular trafficking in fibroblasts and macrophages. Hepatocyte-specific ABCA1 knockout mice (HSKO) have a similar lipid phenotype with 20% of normal plasma HDL levels and a two-fold elevation of plasma TGs due to hepatic overproduction of large, triglyceride-enriched VLDL. We hypothesized that enhanced VLDL TG secretion in the absence of hepatocyte ABCA1 is due to altered intracellular trafficking of apolipoprotein B (apoB), resulting in augmented TG addition to nascent VLDL. We found that trafficking of newly synthesized apoB through the secretory pathway was delayed in ABCA1-silenced rat hepatoma cells and HSKO primary hepatocytes, relative to controls. Endoglycosidase H treatment of cellular apoB revealed a likely delay in apoB trafficking in post-ER compartments. The reduced rate of protein trafficking was also observed for an adenoviral-expressed GPI-linked fluorescent fusion protein, but not albumin, suggesting a selective delay of secretory cargoes in the absence of hepatocyte ABCA1. Our results suggest an important role for hepatic ABCA1 in regulating secretory trafficking and modulating VLDL expansion during the TG accretion phase of hepatic lipoprotein particle assembly.

Comprehensive Transcriptome Analyses of the Fructose-Fed Syrian Golden Hamster Liver Provides Novel Insights into Lipid Metabolism

Dyslipidemia has been widely proven to contribute to cardiovascular diseases and other metabolic disorders, especially in insulin resistance and type 2 diabetes. The overproduction of VLDL is a significant characteristic of dyslipidemia, indicating the dysfunction of hepatic lipid metabolism, from triglyceride synthesis to transport. The fructose-fed Syrian golden hamster is an established animal model for the study of VLDL assembly with insulin resistance, however, it remains unknown how VLDL production is regulated at the transcriptional level due to the absence of a complete hamster genome. Here, we performed deep sequencing and constructed an mRNA-miRNA-lncRNA interaction network of Syrian golden hamster liver in order to reveal the global transcription profile and find potential RNA molecular regulation of VLDL production. We identified 4,450 novel multi-exon hamster lncRNAs and 755 miRNAs expressed in liver. Additionally, 146 differentially expressed coding genes, 27 differentially expressed lncRNA genes, as well as 16 differentially expressed miRNAs were identified. We then constructed an mRNA-miRNA-lncRNA interaction network that may potentially regulate VLDL production, and interestingly found several microRNA-centered regulatory networks. In order to verify our interpretation, miR-486 was selected for further experiments. Overexpression or down-regulation of miR-486 in fructose-fed hamsters resulted in altered hepatic expression of proteins involved in VLDL production, and in modulated levels of circulating VLDL. Our findings implicated that miR-486 is a potential regulator of circulating VLDL levels. These results provide new insights and a valuable resource for further study of the molecular mechanisms of VLDL secretion.

Enabling cytoplasmic delivery and organelle targeting by surface modification of nanocarriers

Nanocarriers are designed to specifically accumulate in diseased tissues. In this context, targeting of intracellular compartments was shown to enhance the efficacy of many drugs and to offer new and more effective therapeutic approaches. This is especially true for therapies based on biologicals that must be encapsulated to favor cell internalization, and to avoid intracellular endosomal sequestration and degradation of the payload. In this review, we discuss specific surface modifications designed to achieve cell cytoplasm delivery and to improve targeting of major organelles; we also discuss the therapeutic applications of these approaches. Last, we describe some integrated strategies designed to sequentially overcome the biological barriers that separate the site of administration from the cell cytoplasm, which is the drug's site of action.

The SNARE Sec22b has a non-fusogenic function in plasma membrane expansion

Development of the nervous system requires extensive axonal and dendritic growth during which neurons massively increase their surface area. Here we report that the endoplasmic reticulum (ER)-resident SNARE Sec22b has a conserved non-fusogenic function in plasma membrane expansion. Sec22b is closely apposed to the plasma membrane SNARE syntaxin1. Sec22b forms a trans-SNARE complex with syntaxin1 that does not include SNAP23/25/29, and does not mediate fusion. Insertion of a long rigid linker between the SNARE and transmembrane domains of Sec22b extends the distance between the ER and plasma membrane, and impairs neurite growth but not the secretion of VSV-G. In yeast, Sec22 interacts with lipid transfer proteins, and inhibition of Sec22 leads to defects in lipid metabolism at contact sites between the ER and plasma membrane. These results suggest that close apposition of the ER and plasma membrane mediated by Sec22 and plasma membrane syntaxins generates a non-fusogenic SNARE bridge contributing to plasma membrane expansion, probably through non-vesicular lipid transfer.

Proteomic analysis of the very low density lipoprotein (VLDL) transport vesicles

The VLDL transport vesicle (VTV) mediates the transport of nascent VLDL particles from the ER to the Golgi and plays a key role in VLDL-secretion from the liver. The functionality of VTV is controlled by specific proteins; however, full characterization and proteomic profiling of VTV remain to be carried out. Here, we report the first proteomic profile of VTVs. VTVs were purified to their homogeneity and characterized biochemically and morphologically. Thin section transmission electron microscopy suggests that the size of VTV ranges between 100 nm to 120 nm and each vesicle contains only one VLDL particle. Immunoblotting data indicate VTV concentrate apoB100, apoB48 and apoAIV but exclude apoAI. Proteomic analysis based on 2D-gel coupled with MALDI-TOF identified a number of vesicle-related proteins, however, many important VTV proteins could only be identified using LC-MS/MS methodology. Our data strongly indicate that VTVs greatly differ in their proteome with their counterparts of intestinal origin, the PCTVs. For example, VTV contains Sec22b, SVIP, ApoC-I, reticulon 3, cideB, LPCAT3 etc. which are not present in PCTV. The VTV proteome reported here will provide a basic tool to study the mechanisms underlying VLDL biogenesis, maturation, intracellular trafficking and secretion from the liver.

Intracellular trafficking and secretion of VLDL

Steady increase in the incidence of atherosclerosis is becoming a major concern not only in the United States but also in other countries. One of the major risk factors for the development of atherosclerosis is high concentrations of plasma low-density lipoprotein, which are metabolic products of very low-density lipoprotein (VLDL). VLDLs are synthesized and secreted by the liver. In this review, we discuss various stages through which VLDL particles go from their biogenesis to secretion in the circulatory system. Once VLDLs are synthesized in the lumen of the endoplasmic reticulum, they are transported to the Golgi. The transport of nascent VLDLs from the endoplasmic reticulum to Golgi is a complex multistep process, which is mediated by a specialized transport vesicle, the VLDL transport vesicle. The VLDL transport vesicle delivers VLDLs to the cis-Golgi lumen where nascent VLDLs undergo a number of essential modifications. The mature VLDL particles are then transported to the plasma membrane and secreted in the circulatory system. Understanding of molecular mechanisms and identification of factors regulating the complex intracellular VLDL trafficking will provide insight into the pathophysiology of various metabolic disorders associated with abnormal VLDL secretion and identify potential new therapeutic targets.

Lumenal lipid metabolism: implications for lipoprotein assembly

Overproduction of apolipoprotein B (apoB)-containing lipoproteins by the liver and the intestine is 1 of the hallmarks of insulin resistance and type 2 diabetes and a well-established risk factor of cardiovascular disease. The assembly of apoB lipoproteins is regulated by the availability of lipids that form the neutral lipid core (triacylglycerol and cholesteryl ester) and the limiting lipoprotein monolayer (phospholipids and cholesterol). Although tremendous advances have been made over the past decade toward understanding neutral lipid and phospholipid biosynthesis and neutral lipid storage in cytosolic lipid droplets (LDs), little is known about the mechanisms that govern the transfer of lipids to the lumen of the endoplasmic reticulum for apoB lipidation. ApoB-synthesizing organs can deposit synthesized neutral lipids into at least 3 different types of LDs, each decorated with a subset of specific proteins: perilipin-decorated cytosolic LDs, and 2 types of LDs formed in the lumen of the endoplasmic reticulum, the secretion-destined LDs containing apoB, and resident lumenal LDs coated with microsomal triglyceride transfer protein and exchangeable apolipoproteins. This brief review will address the current knowledge of lumenal lipid metabolism in the context of apoB assembly and lipid storage.

Hepatic ABCA1 and VLDL triglyceride production

Elevated plasma triglyceride (TG) and reduced high density lipoprotein (HDL) concentrations are prominent features of metabolic syndrome (MS) and type 2 diabetes (T2D). Individuals with Tangier disease also have elevated plasma TG concentrations and a near absence of HDL, resulting from mutations in ATP binding cassette transporter A1 (ABCA1), which facilitates the efflux of cellular phospholipid and free cholesterol to assemble with apolipoprotein A-I (apoA-I), forming nascent HDL particles. In this review, we summarize studies focused on the regulation of hepatic very low density lipoprotein (VLDL) TG production, with particular attention on recent evidence connecting hepatic ABCA1 expression to VLDL, LDL, and HDL metabolism. Silencing ABCA1 in McArdle rat hepatoma cells results in diminished assembly of large (>10nm) nascent HDL particles, diminished PI3 kinase activation, and increased secretion of large, TG-enriched VLDL1 particles. Hepatocyte-specific ABCA1 knockout (HSKO) mice have a similar plasma lipid phenotype as Tangier disease subjects, with a two-fold elevation of plasma VLDL TG, 50% lower LDL, and 80% reduction in HDL concentrations. This lipid phenotype arises from increased hepatic secretion of VLDL1 particles, increased hepatic uptake of plasma LDL by the LDL receptor, elimination of nascent HDL particle assembly by the liver, and hypercatabolism of apoA-I by the kidney. These studies highlight a novel role for hepatic ABCA1 in the metabolism of all three major classes of plasma lipoproteins and provide a metabolic link between elevated TG and reduced HDL levels that are a common feature of Tangier disease, MS, and T2D. This article is part of a Special Issue entitled: Triglyceride Metabolism and Disease.

Lysophosphatidylcholine acyltransferase 3 knockdown-mediated liver lysophosphatidylcholine accumulation promotes very low density lipoprotein production by enhancing microsomal triglyceride transfer protein expression

After de novo biosynthesis phospholipids undergo extensive remodeling by the Lands' cycle. Enzymes involved in phospholipid biosynthesis have been studied extensively but not those involved in reacylation of lysophosphopholipids. One key enzyme in the Lands' cycle is fatty acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT), which utilizes lysophosphatidylcholine (LysoPC) and fatty acyl-CoA to produce various phosphatidylcholine (PC) species. Four isoforms of LPCAT have been identified. In this study we found that LPCAT3 is the major hepatic isoform, and its knockdown significantly reduces hepatic LPCAT activity. Moreover, we report that hepatic LPCAT3 knockdown increases certain species of LysoPCs and decreases certain species of PC. A surprising observation was that LPCAT3 knockdown significantly reduces hepatic triglycerides. Despite this, these mice had higher plasma triglyceride and apoB levels. Lipoprotein production studies indicated that reductions in LPCAT3 enhanced assembly and secretion of triglyceride-rich apoB-containing lipoproteins. Furthermore, these mice had higher microsomal triglyceride transfer protein (MTP) mRNA and protein levels. Mechanistic studies in hepatoma cells revealed that LysoPC enhances secretion of apoB but not apoA-I in a concentration-dependent manner. Moreover, LysoPC increased MTP mRNA, protein, and activity. In short, these results indicate that hepatic LPCAT3 modulates VLDL production by regulating LysoPC levels and MTP expression.

Uptake and trafficking of liposomes to the endoplasmic reticulum

Liposomes are vesicular structures consisting of an aqueous core surrounded by a lipid bilayer. Apart from the cytosol and lysosomes, no other intracellular compartment has been successfully targeted using liposomal delivery. Here, we report the development of liposomes capable of specific targeting to the endoplasmic reticulum (ER) and associated membranes. Using competition and inhibitor assays along with confocal microscopy, we have determined that ER liposomes utilize scavenger and low-density lipoprotein receptors for endocytosis and enter cells through a caveolin- and microtubule-dependent mechanism. They traffic intact to the ER, where fusion with the ER membrane occurs after 22-25 min, which was confirmed by fluorescence-dequenching assays. Once inside the ER, tagged lipids intercalate with the ER membrane and are subsequently incorporated into ER-assembling entities, such as the ER-budding viruses hepatitis C virus (HCV) and bovine viral diarrhea virus (BVDV), lipid droplets, and secreted lipoproteins. ER liposomes are superior to cytosolic liposome formulations for the intracellular delivery of aqueous cargo, such as HIV-1 antivirals, and are especially suited for the prolonged delivery of lipids and lipophilic drugs into human cells.

Lack of phosphatidylethanolamine N-methyltransferase alters plasma VLDL phospholipids and attenuates atherosclerosis in mice

OBJECTIVE

Impaired hepatic phosphatidylcholine (PC) synthesis lowers plasma lipids. We, therefore, tested the hypothesis that lack of phosphatidylethanolamine N-methyltransferase (PEMT), a hepatic enzyme catalyzing PC biosynthesis, attenuates the development of atherosclerosis.

METHODS AND RESULTS

Mice deficient in both PEMT and low-density lipoprotein receptors (Pemt(-/-)/Ldlr(-/-) mice) were fed a high-fat/high-cholesterol diet for 16 weeks. Atherosclerotic lesion area was approximately 80% lower (P<0.01) in Pemt(-/-)/Ldlr(-/-) mice than in Pemt(+/+)/Ldlr(-/-) mice, consistent with the atheroprotective plasma lipoprotein profile (ie, significant reduction in very low-density lipoprotein [VLDL]/intermediate-density lipoprotein/low-density lipoprotein-associated phospholipids [approximately 45%], triacylglycerols [approximately 65%], cholesterol [approximately 58%], and cholesteryl esters [approximately 68%]). Plasma apoB was decreased by 40% to 60%, whereas high-density lipoprotein levels were not altered. In addition, PEMT deficiency reduced plasma homocysteine by 34% to 52% in Pemt(-/-)/Ldlr(-/-) mice. The molar ratio of PC/phosphatidylethanolamine in nascent VLDLs produced by Pemt(-/-)/Ldlr(-/-) mice was lower than in VLDLs in Pemt(+/+)/Ldlr(-/-) mice. Furthermore, deletion of PEMT modestly reduced hepatic VLDL secretion in Ldlr(-/-) mice and altered the rate of VLDL clearance from plasma.

CONCLUSIONS

This is the first report showing that inhibition of hepatic phospholipid biosynthesis attenuates atherosclerosis.

Defects in postabsorptive plasma homeostasis of fatty acids in sickle cell disease

BACKGROUND

The chronic hemolytic anemia experienced by sickle cell disease (SCD) patients leads to adverse effects on oxygen transport by the blood and to a decrease in oxygen availability for peripheral tissues. Limited tissue oxygen availability has the potential to modify events of intracellular metabolism and, thus, alter lipid homeostasis.

METHODS

The impact of SCD on plasma fatty acid homeostasis was determined in 8 African American SCD patients and in 6 healthy African American control subjects under postabsorptive conditions and during a 3-hour IV infusion of a nutrient solution containing lipid, glucose, and amino acids.

RESULTS

SCD patients had higher fasting levels of plasma nonesterified fatty acids (NEFA), triglycerides, and phospholipids than healthy controls. Similarly, SCD patients had higher fasting levels of fatty acids in plasma triglycerides and phospholipids than healthy controls. Infusion of nutrients resulted in equivalent plasma NEFA profiles, total NEFA, and triglycerides in SCD patients and controls. However, the plasma phospholipid concentrations and fatty acid composition of plasma triglycerides and phospholipids were significantly higher in SCD patients; in particular, plasma pools of oleic acid were consistently increased in SCD. Plasma free oleic acid levels were elevated basally, leading to increased oleic acid content in triglycerides and phospholipids both post absorptively and during nutrient infusion.

CONCLUSIONS

There is an underlying defect in lipid metabolism associated with SCD best manifested during the fasting state. This abnormality in lipid homeostasis has the potential to alter red blood cell (RBC) membrane fluidity and function in SCD patients.

Coordinate transcriptional repression of liver fatty acid-binding protein and microsomal triglyceride transfer protein blocks hepatic very low density lipoprotein secretion without hepatosteatosis

Unlike the livers of humans and mice, and most hepatoma cells, which accumulate triglycerides when treated with microsomal triglyceride transfer protein (MTP) inhibitors, L35 rat hepatoma cells do not express MTP and cannot secrete very low density lipoprotein (VLDL), yet they do not accumulate triglyceride. In these studies we show that transcriptional co-repression of the two lipid transfer proteins, liver fatty acid-binding protein (L-FABP) and MTP, which cooperatively shunt fatty acids into de novo synthesized glycerolipids and the transfer of lipids into VLDL, respectively, act together to maintain hepatic lipid homeostasis. FAO rat hepatoma cells express L-FABP and MTP and demonstrate the ability to assemble and secrete VLDL. In contrast, L35 cells, derived as a single cell clone from FAO cells, do not express L-FABP or MTP nor do they assemble and secrete VLDL. We used these hepatoma cells to elucidate how a conserved DR1 promoter element present in the promoters of L-FABP and MTP affects transcription, expression, and VLDL production. In FAO cells, the DR1 elements of both L-FABP and MTP promoters are occupied by peroxisome proliferator-activated receptor alpha-retinoid X receptor alpha (RXRalpha), with which PGC-1beta activates transcription. In contrast, in L35 cells the DR1 elements of both L-FABP and MTP promoters are occupied by chicken ovalbumin upstream promoter transcription factor II, and transcription is diminished. The combined findings indicate that peroxisome proliferator-activated receptor alpha-RXRalpha and PGC-1beta coordinately up-regulate L-FABP and MTP expression, by competing with chicken ovalbumin upstream promoter transcription factor II for the DR1 sites in the proximal promoters of each gene. Additional studies show that ablation of L-FABP prevents hepatic steatosis caused by treating mice with an MTP inhibitor. Our findings show that reducing both L-FABP and MTP is an effective means to reduce VLDL secretion without causing hepatic steatosis.

Isolation of very low density lipoprotein phospholipids enriched in ethanolamine phospholipids from rats injected with Triton WR 1339

Phospholipids carried by very low density lipoprotein (VLDL) are hydrolysed in circulation by lipoprotein and hepatic lipases and lecithin-cholesterol acyltransferase. We have previously demonstrated [J.J. Agren, A. Ravandi, A. Kuksis, G. Steiner, Structural and compositional changes in very low density lipoprotein triacylglycerols during basal lipolysis, Eur. J. Biochem. 269 (2002) 6223-6232] that the infusion of Triton WR 1339 (TWR), which inhibits these lipases, leads in 2 h to five-fold increase in VLDL triacylglycerol concentration along with major differences in the composition of their molecular species. The present study demonstrates that the accumulation of triacylglycerols is accompanied by major changes in the content of the VLDL phospholipids, of which the most significant is the enrichment of phosphatidylethanolamine (PtdEtn). This finding coincides with the enrichment in PtdEtn demonstrated in the VLDL of a hepatocytic Golgi fraction but it had not been demonstrated that the Golgi VLDL, along with its unusual phospholipid composition, can be directly transferred to plasma. Aside from providing an easy access to nascent plasma VLDL, the TWR infusion demonstrates that lipoprotein and hepatic lipases are also responsible for the degradation of plasma VLDL PtdEtn, as independently demonstrated for plasma phosphatidylcholine. Our results indicate also, with the exception of lysophosphatidylcholine, that preferential basal hydrolysis no dot lead to major differences in molecular species composition between circulating and newly secreted VLDL phospholipids. The comparison of the molecular species composition of VLDL and liver phospholipids suggests a selective secretion of PtdEtn and sphingomyelin molecular species during VLDL secretion.

Relation of the size and intracellular sorting of apoB to the formation of VLDL 1 and VLDL 2

In this study, we tested the hypothesis that two separate pathways, the two-step process and an apolipoprotein B (apoB) size-dependent lipidation process, give rise to different lipoproteins. Expression of apoB-100 and C-terminally truncated forms of apoB-100 in McA-RH7777 cells demonstrated that VLDL particles can be assembled by apoB size-dependent linear lipidation, resulting in particles whose density is inversely related to the size of apoB. This lipidation results in a LDL-VLDL 2 particle containing apoB-100. VLDL 1 is assembled by the two-step process by apoB-48 and larger forms of apoB but not to any significant amount by apoB-41. The major amount of intracellular apoB-80 and apoB-100 banded with a mean density of 1.10 g/ml. Its formation was dependent on the sequence between apoB-72 and apoB-90. This dense particle, which is retained in the cell, possibly by chaperones or association with the microsomal membrane, is a precursor of secreted VLDL 1. The intracellular LDL-VLDL 2 particles formed during size-dependent lipidation appear to be the precursors of intracellular VLDL 1. We propose that the dense apoB-100 intracellular particle is converted to LDL-VLDL 2 by size-dependent lipidation. LDL-VLDL 2 is secreted or converted to VLDL 1 by the uptake of the major amount of triglycerides.

Apolipoprotein B-containing lipoprotein particle assembly: lipid capacity of the nascent lipoprotein particle

We previously proposed that the N-terminal 1000-residue betaalpha(1) domain of apolipoprotein B (apoB) forms a bulk lipid pocket homologous to that of lamprey lipovitellin. In support of this "lipid pocket" hypothesis, we demonstrated that apoB:1000 (residues 1-1000) is secreted by a stable transformant of McA-RH7777 cells as a monodisperse particle with high density lipoprotein 3 (HDL(3)) density. In contrast, apoB:931 (residues 1-931), missing only 69 residues of the sequence homologous to lipovitellin, was secreted as a particle considerably more dense than HDL(3). In the present study we have determined the stoichiometry of the lipid component of the apoB:931 and apoB:1000 particles. The secreted [(3)H]glycerol-labeled apoB:1000 particles, isolated by nondenaturing gradient gel electrophoresis, contained 50 phospholipid (PL) and 11 triacylglycerol (TAG) molecules/particle. In contrast, apoB:931 particles contained only a few molecules of PL and were devoid of TAG. The unlabeled apoB:1000 particles, isolated by immunoaffinity chromatography, contained 56 PL, 8 TAG, and 7 cholesteryl ester molecules/particle. The surface to core lipid ratio of apoB:1000-containing particles was approximately 4:1 and was not affected by oleate supplementation. Although very small amounts of microsomal triglyceride transfer protein (MTP) were associated with apoB:1000 particles, it never approached a 1:1 molar ratio of MTP to apoB. These results support a model in which (i) the first 1000 amino acid residues of apoB are competent to complete the lipid pocket without a structural requirement for MTP; (ii) a portion, or perhaps all, of the amino acid residues between 931 and 1000 of apoB-100 are critical for the formation of a stable, bulk lipid-containing nascent lipoprotein particle, and (iii) the lipid pocket created by the first 1000 residues of apoB-100 is PL-rich, suggesting a small bilayer type organization and has a maximum capacity on the order of 50 molecules of phospholipid.

The conversion of apoB100 low density lipoprotein/high density lipoprotein particles to apoB100 very low density lipoproteins in response to oleic acid occurs in the endoplasmic reticulum and not in the Golgi in McA RH7777 cells

The site where bulk lipid is added to apoB100 low density lipoproteins (LDL)/high density lipoproteins (HDL) particles to form triglyceride-enriched very low density lipoproteins (VLDL) has not been identified definitively. We employed several strategies to address this question. First, McA RH7777 cells were pulse-labeled for 20 min with [35S]methionine/cysteine and chased for 1 h (Chase I) to allow study of newly synthesized apoB100 LDL/HDL remaining in the endoplasmic reticulum (ER). After Chase I, cells were incubated for another hour (C2) with/without brefeldin A (BFA) and nocodazole (Noc) (to block ER to Golgi trafficking) and with/without oleic acid (OA). OA treatment alone during C2 increased VLDL secretion. This was prevented by the addition of BFA/Noc in C2. When C2 media were replaced by control media for another 1-h chase (C3), VLDL formed during OA treatment in C2 were secreted into C3 medium. Thus, OA-induced conversion of apoB100 LDL/HDL to VLDL during C2 occurred in the ER. Next, newly synthesized apoB100 lipoproteins were trapped in the Golgi by treatment with Noc and monensin during Chase I (C1), and C2 was carried out in the presence of BFA/Noc with/without OA and without monensin. Under these conditions, OA treatment during C2 did not stimulate VLDL secretion. The same pulse/chase protocols were followed by iodixanol subcellular fractionation, extraction of lipoproteins from ER and Golgi, and sucrose gradient separation of extracted lipoproteins. Cells treated with BFA/Noc and OA in C2 had VLDL in the ER. In the absence of OA, only LDL/HDL were present in the ER. The density of Golgi lipoproteins in these cells was not affected by OA. Similar results were obtained when ER were immuno-isolated with anti-calnexin antibodies. In conclusion, apoB100 bulk lipidation, resulting in conversion of LDL/HDL to VLDL, can occur in the ER, but not in the Golgi, in McA RH7777 cells.

Subcellular localization of microsomal triglyceride transfer protein

Microsomal triglyceride transfer protein (MTP) is essential for the assembly of apolipoprotein B-containing lipoproteins. Within the endoplasmic reticulum, it transfers lipid from the membrane to the forming lipoprotein. Recent evidence suggests that it may also function within the Golgi apparatus. To address this hypothesis, we developed a polyclonal antibody to MTP and used it in a series of studies on mouse liver and McArdle-RH7777 (McA) cells. Western blot analysis demonstrated the presence of MTP within mouse hepatic-Golgi apparatus-rich fractions. In addition, in vitro lipid transfer assays demonstrated the presence of triglyceride transfer activity within the Golgi fractions. Immunohistochemical studies with mouse liver demonstrated the presence of MTP within all hepatocytes, but not in nonparenchymal cells. The subcellular location of MTP in McA cells was investigated using confocal microscopy. MTP colocalized with the trans-Golgi network (TGN) 38 and Golgi SNARE (soluble N-ethylmalemide-sensitive factor attachment protein receptor) of 28 kDa (GS28), markers for the trans- and cis-Golgi apparatus, respectively. Morphometric analyses indicated that approximately 17% of the MTP signal colocalized with the TGN38, while 33% of the trans-Golgi marker colocalized with the MTP. Approximately 17% of the MTP signal colocalized with the GS28, whereas 53% of the cis-Golgi marker colocalized with the MTP. The results provide unequivocal evidence for the location of MTP within the Golgi apparatus, and further highlight the importance of this organelle in the assembly of lipoproteins.

A Drosophila microsomal triglyceride transfer protein homolog promotes the assembly and secretion of human apolipoprotein B. Implications for human and insect transport and metabolism

The assembly and secretion of triglyceride-rich lipoproteins in vertebrates requires apolipoprotein B (apoB) and the endoplasmic reticulum-localized cofactor, microsomal triglyceride transfer protein (MTP). Invertebrates, particularly insects, transport the majority of their neutral and polar lipids in lipophorins; however, the assembly of lipophorin precursor particles was presumed to be MTP-independent. A Drosophila melanogaster expressed gene sequence (CG9342), displaying 23% identity with human MTP, was recently identified. When coexpressed in COS cells, CG9342 promoted the assembly and secretion of apoB34 and apoB41 (N-terminal 34 and 41% of human apoB). The apoB34-containing particles assembled by human MTP and CG9342 displayed similar peak densities of approximately 1.169 g/ml and similar lipid compositions. However, CG9342 displayed differential sensitivities to two inhibitors of human MTP and low vesicle-based lipid transfer activity, in vitro. In addition, important predicted structural distinctions exist between the human and Drosophila proteins suggesting overlapping but not identical functional roles. We conclude that CG9342 and human MTP are orthologs that share only a subset of functions, consistent with known differences in intracellular and extracellular aspects of vertebrate and invertebrate lipid transport and metabolism.

Nascent lipidated apolipoprotein B is transported to the Golgi as an incompletely folded intermediate as probed by its association with network of endoplasmic reticulum molecular chaperones, GRP94, ERp72, BiP, calreticulin, and cyclophilin B

We have previously demonstrated that endoplasmic reticulum (ER)-resident molecular chaperones interact with apolipoprotein B-100 (apoB) during its maturation. The initial stages of apoB folding occur while it is bound to the ER membrane, where it becomes partially lipidated to form a primordial intermediate. We determined whether this intermediate is dependent on the assistance of molecular chaperones for its subsequent folding steps. To that end, microsomes were prepared from HepG2 cells and luminal contents were subjected to KBr density gradient centrifugation. Immunoprecipitation of apoB followed by Western blotting showed that the luminal pool floated at a density of 1.12 g/ml and, like the membrane-bound pool, was associated with GRP94, ERp72, BiP, calreticulin, and cyclophilin B. Except for calreticulin, chaperone/apoB ratio in the lumen was severalfold higher than that in the membrane, suggesting a role for these chaperones both in facilitating the release of the primordial intermediate into the ER lumen and in providing stability. Subcellular fractionation on sucrose gradients showed that apoB in the Golgi was associated with the same array of chaperones as the pool of apoB recovered from heavy microsomes containing the ER, except that chaperone/apoB ratio was lower. KBr density gradient fractionation showed that the major pool of luminal apoB in the Golgi was recovered from 1.02 < d < 1.08 g/ml, whereas apoB in ER was recovered primarily from 1.08 < d < 1.2 g/ml. Both fractions were associated with the same spectrum of chaperones. Together with the finding that GRP94 was found associated with sialylated apoB, we conclude that correct folding of apoB is dependent on the assistance of molecular chaperone, which play multiple roles in its maturation throughout the secretory pathway including distal compartments such as the trans-Golgi network.

The assembly of hepatic very low density lipoproteins: evidence of a role for the Golgi apparatus

Previous studies from our laboratory have suggested that the assembly of lipoproteins by the liver is not completed in the rough endoplasmic reticulum but continues while the particles are en route to or within the Golgi apparatus. To investigate further the role of the Golgi apparatus in lipoprotein assembly, mice were injected with [3H]glycerol and killed 7.5 to 45 min after injection. Microsomes and Golgi apparatus-rich fractions were isolated from the livers and separated into membrane and content fractions. TG within microsomal and Golgi membranes were labeled, rapidly reaching peak specific activity within 7.5 min of isotope injection. The specific activity of TG in microsomal membranes decreased to approximately 40% of peak values by 45 min, whereas the specific activity of TG in the Golgi membranes decreased to approximately 30% of peak values by 45 min. To determine whether the turnover of the Golgi membrane TG pool was dependent on microsomal TG transfer protein (MTP), mice were gavaged with an MTP inhibitor, and the labeling experiments were repeated. Inhibition of MTP attenuated the turnover of newly synthesized Golgi membrane TG by approximately 50% and the turnover of microsomal membrane TG by approximately 40%. Based on the rapid turnover of the Golgi membrane TG pool and the attenuation of the turnover of this pool by MTP inhibitor, we propose that lipid is transferred from the Golgi membrane to luminal lipoproteins in an MTP-dependent manner. The results support our hypothesis that assembly of VLDL continues within the Golgi apparatus.

Intracellular assembly of very low density lipoproteins containing apolipoprotein B100 in rat hepatoma McA-RH7777 cells

Previous studies with McA-RH7777 cells showed a 15-20-min temporal delay in the oleate treatment-induced assembly of very low density lipoproteins (VLDL) after apolipoprotein (apo) B100 translation, suggesting a post-translational process. Here, we determined whether the post-translational assembly of apoB100-VLDL occurred within the endoplasmic reticulum (ER) or in post-ER compartments using biochemical and microscopic techniques. At steady state, apoB100 distributed throughout ER and Golgi, which were fractionated by Nycodenz gradient centrifugation. Pulse-chase experiments showed that it took about 20 min for newly synthesized apoB100 to exit the ER and to accumulate in the cis/medial Golgi. At the end of a subsequent 20-min chase, a small fraction of apoB100 accumulated in the distal Golgi, and a large amount of apoB100 was secreted into the medium as VLDL. VLDL was not detected either in the lumen of ER or in that of cis/medial Golgi where apoB100 was membrane-associated and sensitive to endoglycosidase H treatment. In contrast, VLDL particles were found in the lumen of the distal Golgi where apoB100 was resistant to endoglycosidase H. Formation of lumenal VLDL almost coincided with the appearance of VLDL in the medium, suggesting that the site of VLDL assembly is proximal to the site of secretion. When microsomal triglyceride transfer protein activity was inactivated after apoB had exited the ER, VLDL formation in the distal Golgi and its subsequent secretion was unaffected. Lipid analysis by tandem mass spectrometry showed that oleate treatment increased the masses of membrane phosphatidylcholine (by 68%) and phosphatidylethanolamine (by 27%) and altered the membrane phospholipid profiles of ER and Golgi. Taken together, these results suggest that VLDL assembly in McA-RH7777 cells takes place in compartments at the distal end of the secretory pathway.

Inhibition of phosphatidylcholine synthesis via the phosphatidylethanolamine methylation pathway impairs incorporation of bulk lipids into VLDL in cultured rat hepatocytes

Inhibition of phosphatidylcholine (PC) synthesis via the phosphatidylethanolamine (PE) methylation pathway was shown to decrease the secretion of VLDL from primary rat hepatocytes (Nishimaki-Mogami et al. 1996. BIOCHIM: Biophys. Acta. 1304: 21-31). To understand further the role of PE methylation, we determined the effect of bezafibrate, an inhibitor of PE methylation, on VLDL assembly within the microsomal lumen. Bezafibrate was shown to decrease VLDL (triacylglycerol) secretion only when cellular PE methylation was active in the presence of methionine. Pulse-chase experiments showed that bezafibrate treatment did not impair the movement of [(35)S]apolipoprotein (apo)B-48 from microsomal membranes into the lumen. However, bezafibrate treatment resulted in reduced VLDL-[(35)S]apoB-48 and increased [(35)S]apoB-48-containing particles in the HDL density range (HDL-[(35)S]apoB-48) within the lumen. Inhibition of PE methylation by bezafibrate or 3-deazaadenosine after the completion of HDL-[(35)S]apoB-48 assembly effectively decreased VLDL-[(35)S]apoB-48 secretion with a concomitant increase in HDL-[(35)S]apoB-48 secretion. These findings suggest that inhibition of PC synthesis via the PE methylation pathway impairs the stage of bulk triacylglycerol incorporation during the assembly of VLDL.

Localization of microsomal triglyceride transfer protein in the Golgi: possible role in the assembly of chylomicrons

Although a critical role of microsomal transfer protein (MTP) has been recognized in the assembly of nascent apolipoprotein B (apoB)-containing lipoproteins, it remains unclear where and how MTP transfers lipids in the secretory pathway during the maturational process of apoB lipidation. The aims of this study were to determine whether MTP functions in the secretory pathway as well as in the endoplasmic reticulum and whether its large 97-kDa subunit interacts with the small 58-kDa protein disulfide isomerase (PDI) subunit and apoB, particularly in the Golgi apparatus. Using a high resolution immunogold approach combined with specific polyclonal antibodies, the large and small subunits of MTP were observed over the rough endoplasmic reticulum and the Golgi. Double immunocytochemical detection unraveled the colocalization of MTP and PDI as well as MTP and apoB in these same subcellular compartments. To confirm the spatial contact of these proteins, Golgi fractions were isolated, homogenized, and incubated with an anti-MTP large subunit antibody. Immunoprecipitates were applied on SDS-PAGE and then transferred on to nitrocellulose. Immunoblotting the membrane with PDI and apoB antibodies confirmed the colocalization of these proteins with MTP. Furthermore, MTP activity assay disclosed a substantial triglyceride transfer in the Golgi fractions. The occurrence of membrane-associated apoB in the Golgi, coupled with its interaction with active MTP, suggests an important role for the Golgi in the biogenesis of apoB-containing lipoproteins.

Dipyridamole prevents the coconut oil-induced hypercholesterolemia. A study on lipid plasma and lipoprotein composition

For a better understanding of the hypolipidemic function of dipyridamole, we have studied the comparative effects of diet supplementation with 10% coconut oil with and without dipyridamole on the lipid plasma and lipoprotein composition in chicks. This study was performed under postprandial and food-deprivation (12h) conditions. Coconut oil induced a clear hypercholesterolemia under both feeding conditions. Simultaneous administration of dipyridamole maintained total and esterified cholesterol at levels similar to those observed in control animals sacrificed under postprandial conditions. Under these conditions, our results also show that dipyridamole significantly reduced cholesterol levels in all the chick plasma lipoproteins that were increased by coconut oil administration. Nevertheless, it should be emphasised that the levels of total cholesterol found in intermediate- and very-low-density lipoproteins were lower than in control. All chemical components of these fractions were significantly decreased by dipyridamole. The effects were not significant in chicks deprived of food. In conclusion, our results show that the hypercholesterolemia induced by coconut oil was prevented by dipyridamole. To our knowledge, this is one of the first reports on the antihypercholesterolemic effects of dipyridamole.

Dipyridamole reduces cholesterol and triglyceride levels in plasma and lipoproteins from young chicks fed a saturated fat-enriched diet

We have studied the value of dipyridamole for the prevention of hypercholesterolemia caused by saturated fat. This study shows that supplementation of 10-20% coconut oil to the diet induced a significant hypercholesterolemia under postprandial conditions. Simultaneous administration of dipyridamole and 10% coconut oil for 2 weeks maintained plasma cholesterol and triglycerides at levels similar to control, counteracting the increase induced by coconut oil. Dipyridamole significantly reduced cholesterol levels in all the chick plasma lipoproteins that were increased by coconut oil administration. However, it should be emphasized that cholesterol and other chemical components were more drastically reduced by dipyridamole in intermediate- and very-low-density lipoproteins than in other lipoprotein fractions.

Differential metabolism of human VLDL according to content of ApoE and ApoC-III

We studied the metabolism of very low density lipoprotein (VLDL) and intermediate density lipoprotein (IDL) particles that did or did not have apolipoprotein E (apoE) in 12 normolipidemic women by endogenously labeling plasma apolipoprotein B. The plasma was separated into bound (E+) and unbound (E-) fractions by use of a monoclonal antibody (1D7), and the fractions were ultracentrifuged to yield E+ and E- subfractions of light and dense VLDL and IDL. VLDL E+ and IDL E+ were produced mainly by the liver. VLDL E+ and IDL E+ had lower fractional catabolic rates and much higher apolipoprotein C-III (apoC-III) content than did the corresponding E- particles. Most light VLDL apoE+ underwent lipolysis to dense VLDL E+ with reduced apoC-III content, which was removed from the circulation without conversion to IDL. In contrast, most light VLDL apoE-, poor in apoC-III, was removed from the circulation, and a smaller proportion underwent lipolysis to dense VLDL E-. Most dense VLDL E- underwent lipolysis to IDL E-. The rate constant for lipolysis of dense VLDL to IDL was greater for E- than for E+, and the rate constant for clearance from plasma was greater for dense VLDL E+ than for E-. In conclusion, metabolism of human VLDL particles is influenced by their content of apoE, further modulated by the coexistence of apoC-III.

Microsomal triglyceride transfer protein binding and lipid transfer activities are independent of each other, but both are required for secretion of apolipoprotein B lipoproteins from liver cells

Recent studies indicate that microsomal triglyceride transfer protein (MTP) and apolipoprotein B (apoB) interact physically via two specific binding sites located within the amino-terminal globular region of apoB100. The first site is thought to be within the first 5.8% of the amino-terminal sequence, and the second site is between 9 and 16% of the amino-terminal sequence. It is not clear from prior studies whether these sites have unique or overlapping functions. Furthermore, there are no data differentiating between lipid transfer and potential chaperone functions of MTP. In the present study we have attempted to further characterize the physiologic interaction between apoB and MTP and to determine the relationship between the binding and lipid transfer aspects of the interaction. HepG2 cells were transiently transfected with apoB cDNAs, and MTP binding to apoB polypeptides was determined by two-step immunoprecipitation. MTP bound equally well to apoB polypeptides with (apoB13, 16,beta, apoB34, and apoB42) or without (apoB16, apoB13, and 16 or apoB13, 13, and 16) beta sheet domains. When proteasomal degradation of newly synthesized apoB polypeptides was blocked, MTP binding to all of the apoB polypeptides was only modestly affected by lipid availability and was independent of MTP-associated lipid transfer. Furthermore, MTP did not bind directly to a portion of the first beta sheet domain. We created two apoB constructs (apoB16del and apoB34del) by deleting the first 210 amino acids of apoB16 and apoB34. These apoB polypeptides, therefore, lacked the putative first MTP binding site. MTP binding to apoB16del and apoB34del was decreased significantly. However, the secretion of apoB16del was not different from apoB16, whereas the secretion of apoB34del was impaired significantly. Our results indicate that the interaction between MTP and apoB involves independent binding and lipid transfer activities but that both activities are required for the secretion of apolipoprotein B from liver cells.

Very-low-density lipoprotein assembly and secretion

The assembly of apolipoprotein B (apoB) into VLDL is broadly divided into two steps. The first involves transfer of lipid by the microsomal triglyceride transfer protein (MTP) to apoB during translation. The second involves fusion of apoB-containing precursor particles with triglyceride droplets to form mature VLDL. ApoB and MTP are homologs of the egg yolk storage protein, lipovitellin. Homodimerization surfaces in lipovitellin are reutilized in apoB and MTP to achieve apoB-MTP interactions necessary for first step assembly. Structural modeling predicts a small lipovitellin-like lipid binding cavity in MTP and a transient lipovitellin-like cavity in apoB important for nucleation of lipid sequestration. The formation of triglyceride droplets in the endoplasmic reticulum requires MTP however, their fusion with apoB may be MTP-independent. Second step assembly is modulated by phospholipase D and A2. Phospholipases may prime membrane transport steps required for second step fusion and/or channel phospholipids into a pathway for VLDL triglyceride production. The enzymology of VLDL triglyceride synthesis is still poorly understood; however, it appears that ACAT2 is the sole source of cholesterol esters for VLDL and chylomicron assembly. VLDL production is controlled primarily at the level of presecretory degradation. Recently, it was discovered that the LDL receptor modulates VLDL production through its interactions with nascent VLDL in the secretory pathway.

APOLIPOPROTEIN B: mRNA editing, lipoprotein assembly, and presecretory degradation

Apolipoprotein (apo)B circulates in two distinct forms, apoB100 and apoB48. Human liver secretes apoB100, the product of a large mRNA encoding 4536 residues. The small intestine of all mammals secretes apoB48, which arises following C-to-U deamination of a single cytidine base in the nuclear apoB transcript, introducing a translational stop codon. This process, referred to as apoB RNA editing, operates through a multicomponent enzyme complex that contains a single catalytic subunit, apobec-1, in addition to other protein factors that have yet to be cloned. ApoB RNA editing also exhibits stringent cis-acting requirements that include both structural and sequence-specific elements-specifically efficiency elements that flank the minimal cassette, an AU-rich RNA context, and an 11-nucleotide mooring sequence-located in proximity to a suitably positioned (usually upstream) cytidine. C-to-U RNA editing may become unconstrained under circumstances where apobec-1 is overexpressed, in which case multiple cytidines in apoB RNA, as well as in other transcripts, undergo C-to-U editing. ApoB RNA editing is eliminated following targeting of apobec-1, establishing that there is no genetic redundancy in this function. Under physiological circumstances, apoB RNA editing exhibits developmental, hormonal, and nutritional regulation, in some cases related to transcriptional regulation of apobec-1 mRNA. ApoB and the microsomal triglyceride transfer protein (MTP) are essential for the assembly and secretion of apoB-containing lipoproteins. MTP functions by transferring lipid to apoB during its translation and by transporting triglycerides into the endoplasmic reticulum to form apoB-free lipid droplets. These droplets fuse with nascent apoB-containing particles to form mature, very low-density lipoproteins or chylomicrons. In cultured hepatic cells, lipid availability dictates the rate of apoB production. Unlipidated or underlipidated forms of apoB are subjected to presecretory degradation, a process mediated by retrograde transport from the lumen of the endoplasmic reticulum to the cytosol, coupled with multiubquitination and proteasomal degradation. Although control of lipid secretion in vivo is primarily achieved at the level of lipoprotein particle size, regulation of apoB production by presecretory degradation may be relevant in some dyslipidemic states.

Short- and long-term effects of atorvastatin, lovastatin and simvastatin on the cellular metabolism of cholesteryl esters and VLDL secretion in rat hepatocytes

The short- and long-term in vitro effects of the hydroxymethylglutaryl-CoA reductase inhibitor atorvastatin, compared with lovastatin and simvastatin on VLDL secretion, and on the formation and the neutral and acid lysosomal hydrolysis of cholesteryl esters was investigated in rat liver hepatocytes maintained in suspension (2 or 4 h) or cultured in monolayers (24 h). All statins time-dependently reduced [14C]oleate incorporation into cholesteryl esters, but when exogenous cholesterol was added only atorvastatin caused an immediate transient decrease in hepatocyte ACAT activity. Activity of the lysosomal, microsomal and cytosolic CEH isoforms was unaffected by the hepatocyte treatments. Statins reduced free and esterified cholesterol mass in hepatocyte microsomes after 2 h, and this was followed by a modest decline in VLDL cholesteryl esters, whilst secretion of VLDL apoB and triglycerides was unaltered. However, after 24 h of treatment, statins caused generalized 20-40% decreases in the secretion of VLDL apoB, cholesterol and triglycerides, with the reduction in apoB48 secretion being significantly superior to that caused in apoB100. The mean diameter of secreted VLDL was not modified by either duration or drug treatment. Additional studies with subcellular fractions demonstrated that statins have a direct selective effect on the enzymes governing the cholesterol-cholesteryl ester cycle, with the exception of the microsomal CEH. Atorvastatin, lovastatin and simvastatin inhibited ACAT activity in microsomes by 50% at doses of 250, 100 and 50 microM, respectively. The cytosolic CEH elicited a biphasic profile of activity with activations up to 100 microM statin and inhibitions above 250 microM, and the lysosomal CEH was only inhibited by atorvastatin at a dose of 100 microM or more. We conclude that a prolonged, but not a short, limited availability of hepatocyte cholesterol derived from the endogenous synthesis reduces VLDL secretion, and that reactivity of statins at the cellular level are more similar than reactivity at the subcellular level as regards the cholesterol-cholesteryl ester cycle.

Human, but not bovine, oxidized cerebral spinal fluid lipoproteins disrupt neuronal microtubules

Cerebral spinal fluid (CSF) lipoproteins have become a focus of research since the observation that inheritance of particular alleles of the apolipoprotein E gene affects the risk of Alzheimer's disease (AD). There is evidence of increased lipid peroxidation in CSF lipoproteins from patients with AD, but the biological significance of this observation is not known. A characteristic of the AD brain is a disturbance of the neuronal microtubule organization. We have shown previously that 4-hydroxy-2(E)-nonenal, a major product of lipid peroxidation, causes disruption of neuronal microtubules and therefore tested whether oxidized CSF lipoproteins had the same effect. We exposed Neuro 2A cells to human CSF lipoproteins and analyzed the microtubule organization by immunofluorescence. In vitro oxidized human CSF lipoproteins caused disruption of the microtubule network, while their native (nonoxidized) counterparts did not. Microtubule disruption was observed after short exposures (1 h) and lipoprotein concentrations were present in CSF (20 microg/mL), conditions that did not result in loss of cell viability. Importantly, adult bovine CSF lipoproteins, oxidized under identical conditions, had no effect on the microtubule organization of Neuro 2A cells. Comparison of human and bovine CSF lipoproteins revealed similar oxidation-induced modifications of apolipoproteins E and A-I as analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. Fatty acid analysis revealed substantially lower amounts of unsaturated fatty acids in bovine CSF lipoproteins, when compared to their human counterparts. Our data therefore indicate that oxidized human CSF lipoproteins are detrimental to neuronal microtubules. This effect is species-specific, since equally oxidized bovine CSF lipoproteins left the neuronal microtubule organization unchanged.

The assembly of very low density lipoproteins in rat hepatoma McA-RH7777 cells is inhibited by phospholipase A2 antagonists

In McA-RH7777 cells, the oleate-stimulated assembly and secretion of very low density lipoproteins (VLDL) was associated with enhanced deacylation of phospholipids, which was markedly decreased by inactivation of the cellular phospholipase A(2). Treatment of the cells with antagonists or antisense oligonucleotide of the Ca(2+)-independent phospholipase A(2) (iPLA(2)) significantly inhibited secretion of apoB100-VLDL and triglyceride. Similar inhibitory effect of the iPLA(2) antagonists was observed on apoB48-VLDL secretion, but secretion of high density lipoprotein particles (such as apoAI- and apoB48-high density lipoprotein) or proteins in general was unaffected. The iPLA(2) antagonist did not affect the synthesis of apoB100 or triglyceride, nor did it affect the activities of phospholipase D, phosphatidate phosphohydrolase, or microsomal triglyceride transfer protein. Inactivation of iPLA(2) resulted in impaired apoB100-VLDL assembly as shown by decreased apoB100-VLDL and triglyceride within the microsomal lumen, with concomitant increase in apoB100 association with the microsomal membranes. The inhibitory effect of iPLA(2) antagonists on apoB100-VLDL assembly/secretion could be abated by pretreatment of cells with oleate. Analysis of molecular species of microsomal phosphatidylcholine and phosphatidylethanolamine by electron spray tandem mass spectrometry revealed that the enrichment of oleoyl moieties was altered by the treatment of iPLA(2) antagonist. These results suggest that the oleate-induced VLDL assembly/secretion may depend upon the establishment of membrane glycerolipids enriched in oleoyl chain, a process mediated by the iPLA(2) activity.

Morphofunctional changes in hepatopancreatobiliary organs in experimental dyslipoproteinemia

Dyslipoproteinemia was induced and corrected in 40 Chinchilla rabbits. Lipid metabolism disorders were corrected by Fishant vaseline-pectin emulsion and partial ileoshunting. Experimental dyslipoproteinemia initiated destructive dystrophic processes in the hepatopancreatobiliary organs manifesting by liver stenosis, gallbladder cholesterosis, and focal structural and functional disorders in pancreatic parenchyma. Correction of dyslipoproteinemia normalized the status of hepatopancreatobiliary organs, decreased the severity of destructive processes, and prevented their further development.

The assembly and secretion of apolipoprotein B-48-containing very low density lipoproteins in McA-RH7777 cells

We have used an extraction procedure, which released membrane-bound apoB-100, to study the assembly of apoB-48 VLDL (very low density lipoproteins). This procedure released apoB-48, but not integral membrane proteins, from microsomes of McA-RH7777 cells. Upon gradient ultracentrifugation, the extracted apoB-48 migrated in the same position as the dense apoB-48-containing lipoprotein (apoB-48 HDL (high density lipoprotein)) secreted into the medium. Labeling studies with [(3)H]glycerol demonstrated that the HDL-like particle extracted from the microsomes contains both triglycerides and phosphatidylcholine. The estimated molar ratio between triglyceride and phosphatidylcholine was 0.70 +/- 0.09, supporting the possibility that the particle has a neutral lipid core. Pulse-chase experiments indicated that microsomal apoB-48 HDL can either be secreted as apoB-48 HDL or converted to apoB-48 VLDL. These results support the two-step model of VLDL assembly. To determine the size of apoB required to assemble HDL and VLDL, we produced apoB polypeptides of various lengths and followed their ability to assemble VLDL. Small amounts of apoB-40 were associated with VLDL, but most of the nascent chains associated with VLDL ranged from apoB-48 to apoB-100. Thus, efficient VLDL assembly requires apoB chains of at least apoB-48 size. Nascent polypeptides as small as apoB-20 were associated with particles in the HDL density range. Thus, the structural requirements of apoB to form HDL-like first-step particles differ from those to form second-step VLDL. Analysis of proteins in the d < 1.006 g/ml fraction after ultracentrifugation of the luminal content of the cells identified five chaperone proteins: binding protein, protein disulfide isomerase, calcium-binding protein 2, calreticulin, and glucose regulatory protein 94. Thus, intracellular VLDL is associated with a network of chaperones involved in protein folding. Pulse-chase and subcellular fractionation studies showed that apoB-48 VLDL did not accumulate in the rough endoplasmic reticulum. This finding indicates either that the two steps of apoB lipoprotein assembly occur in different compartment or that the assembled VLDL is transferred rapidly out of the rough endoplasmic reticulum.

Inactivation of microsomal triglyceride transfer protein impairs the normal redistribution but not the turnover of newly synthesized glycerolipid in the cytosol, endoplasmic reticulum and Golgi of primary rat hepatocytes

The requirements for microsomal triglyceride transfer protein (MTP) during the turnover and transfer of glycerolipids from intracellular compartments into secretory very low-density lipoprotein (VLDL) were studied by pre-labelling lipids with [(3)H]glycerol and [(14)C]oleate in primary cultures of rat hepatocytes. The intracellular redistribution of pre-labelled glycerolipids was then compared at the end of subsequent chase periods during which the MTP inhibitor BMS-200150 was either present or absent in the medium. Inhibition of MTP resulted in a decreased output of VLDL triacylglycerol (TAG) and a delayed removal of labelled TAG from the cytosol and from the membranes of the smooth endoplasmic reticulum (SER), the cis- and the trans-Golgi. Inactivation of MTP did not decrease the bulk lipolytic turnover of cellular TAG as reflected by changes in its [(3)H]glycerol:[(14)C]oleate ratios. However, a larger proportion of the resultant TAG fatty acids was re-esterified and remained with the membranes of the various subcellular fractions rather than emerging as VLDL. The effects of BMS-200150 on the pattern of phospholipid (PL) mechanism and redistribution suggested that inhibition of MTP prevented the normal lipolytic transfer of PL-derived fatty acids out of the SER, cis- and trans-Golgi membrane pools. Finally, changes in the (14)C specific radioactivities of the cytosolic and membrane pools of TAG suggested that inhibition of MTP prevented a normal influx of relatively unlabelled fatty acids into these pools during the chase period.

The activity of microsomal triglyceride transfer protein is essential for accumulation of triglyceride within microsomes in McA-RH7777 cells. A unified model for the assembly of very low density lipoproteins

Previously, based on distinct requirement of microsomal triglyceride transfer protein (MTP) and kinetics of triglyceride (TG) utilization, we concluded that assembly of very low density lipoproteins (VLDL) containing B48 or B100 was achieved through different paths (Wang, Y. , McLeod, R. S., and Yao, Z. (1997) J. Biol. Chem. 272, 12272-12278). To test if the apparent dual mechanisms were accounted for by apolipoprotein B (apoB) length, we studied VLDL assembly using transfected cells expressing various apoB forms (e.g. B64, B72, B80, and B100). For each apoB, enlargement of lipoprotein to form VLDL via bulk TG incorporation was induced by exogenous oleate, which could be blocked by MTP inhibitor BMS-197636 treatment. While particle enlargement was readily demonstrable by density ultracentrifugation for B64- and B72-VLDL, it was not obvious for B80- and B100-VLDL unless the VLDL was further resolved by cumulative rate flotation into VLDL(1) (S(f) > 100) and VLDL(2) (S(f) 20-100). BMS-197636 diminished B100 secretion in a dose-dependent manner (0.05-0.5 microM) and also blocked the particle enlargement from small to large B100-lipoproteins. These results yield a unified model that can accommodate VLDL assembly with all apoB forms, which invalidates our previous conclusion. To gain a better understanding of the MTP action, we examined the effect of BMS-197636 on lipid and apoB synthesis during VLDL assembly. While BMS-197636 (0.2 microM) entirely abolished B100-VLDL(1) assembly/secretion, it did not affect B100 translation or translocation across the microsomal membrane, nor did it affect TG synthesis and cell TG mass. However, BMS-197636 drastically decreased accumulation of [(3)H]glycerol-labeled TG and TG mass within microsomal lumen. The decreased TG accumulation was not a result of impaired B100-VLDL assembly, because in cells treated with brefeldin A (0.2 microgram/ml), the assembly of B100-VLDL was blocked yet lumenal TG accumulation was normal. Thus, MTP plays a role in facilitating accumulation of TG within microsomes, a prerequisite for the post-translational assembly of TG-enriched VLDL.

Manipulation of cholesterol and cholesteryl ester synthesis has multiple effects on the metabolism of apolipoprotein B and the secretion of very-low-density lipoprotein by primary hepatocyte cultures

Inhibition of esterified and non-esterified cholesterol synthesis by lovastatin in primary rat hepatocytes suppressed the net synthesis and very-low-density lipoprotein (VLDL) secretion of apolipoprotein B (apoB)-48 and apoB-100. Lovastatin did not alter the rates of apoB-48 and apoB-100 post-translational degradation. 25-Hydroxycholesterol, which inhibited non-esterified cholesterol synthesis but increased the synthesis of cholesteryl ester, showed differential effects on the metabolism of apoB-48 and apoB-100. Whereas the secretion of apoB-48 VLDL was suppressed there was no effect on the secretion of apoB-100 VLDL. The post-translational degradation of apoB-48, but not of apoB-100, was enhanced by 25-hydroxycholesterol. The net synthesis rates of apoB-48 and apoB-100 were unaffected by 25-hydroxycholesterol. The inhibitory effect of lovastatin alone on the net synthesis of apoB-48 and apoB-100 was reversed by the simultaneous presence of 25-hydroxycholesterol, suggesting a role for newly synthesised cholesteryl ester. Prevention of the reversal effect by the acyl-CoA: cholesterol acyltransferase (ACAT) inhibitor YM 17E supported this interpretation. In the presence of lovastatin, restoration of the net synthesis of apoB by 25-hydroxycholesterol was not accompanied by an increased VLDL output of apoB-48 and apoB-100. However, under these conditions there was an increased post-translational degradation of apoB-48 and apoB-100. These results suggest that interference with intracellular cholesterol and cholesteryl ester metabolism interrupts VLDL assembly at sites of both apoB net synthesis and post-translational degradation.