Carboxyl-terminal truncation impairs lipid recruitment by apolipoprotein B100 but does not affect secretion of the truncated apolipoprotein B-containing lipoproteins

Missense mutation Q384K in the APOB gene affecting the large lipid transfer module of apoB reduces the secretion of apoB-100 in the liver without reducing the secretion of apoB-48 in the intestine

BACKGROUND

Molecular genetic testing of patients with hypobetalipoproteinemia may identify a genetic cause that can form the basis for starting proper therapy. Identifying a genetic cause may also provide novel data on the structure-function relationship of the mutant protein.

OBJECTIVE

To identify a genetic cause of hypobetalipoproteinemia in a patient with levels of low density lipoprotein cholesterol at the detection limit of 0.1 mmol/l.

METHODS

DNA sequencing of the translated exons with flanking intron sequences of the genes adenosine triphosphate-binding cassette transporter 1, angiopoietin-like protein 3, apolipoprotein B, apolipoprotein A1, lecithin-cholesterol acyltransferase, microsomal triglyceride transfer protein and proprotein convertase subtilisin/kexin type 9.

RESULTS

The patient was homozygous for mutation Q384K (c.1150C>A) in the apolipoprotein B gene, and this mutation segregated with hypobetalipoproteinemia in the family. Residue Gln384 is located in the large lipid transfer module of apoB that has been suggested to be important for lipidation of apolipoprotein B through interaction with microsomal triglyceride transfer protein. Based on measurements of serum levels of triglycerides and apolipoprotein B-48 after an oral fat load, we conclude that the patient was able to synthesize apolipoprotein B-48 in the intestine in a seemingly normal fashion.

CONCLUSION

Our data indicate that mutation Q384K severely reduces the secretion of apolipoprotein B-100 in the liver without reducing the secretion of apolipoprotein B-48 in the intestine. Possible mechanisms for the different effects of this and other missense mutations affecting the large lipid transfer module on the two forms of apoB are discussed.

Novel protein-truncating variant in the APOB gene may protect from coronary artery disease and adverse cardiovascular events

Background and aims

Genetic testing is still rarely used for the diagnosis of dyslipidemia, even though gene variants determining plasma lipids levels are not uncommon.

Methods

Starting from a a pilot-analysis of targeted Next Generation Sequencing (NGS) of 5 genes related to familial hypercholesterolemia (LDLR, APOB, PCSK9, HMGCR, APOE) within a cardiovascular cohort in subjects with extreme plasma concentrations of low-density lipoprotein (LDL) cholesterol, we discovered and characterized a novel point mutation in the APOB gene, which was associated with very low levels of apolipoprotein B (ApoB) and LDL cholesterol.

Results

APOB c.6943 G > T induces a premature stop codon at the level of exon 26 in the APOB gene and generates a protein which has the 51% of the mass of the wild type ApoB-100 (ApoB-51), with a truncation at the level of residue 2315. The premature stop codon occurs after the one needed for the synthesis of ApoB-48, allowing chylomicron production at intestinal level and thus avoiding potential nutritional impairments. The heterozygous carrier of APOB c.6943G > T, despite a very high-risk profile encompassing all the traditional risk factors except for dyslipidemia, had normal coronary arteries by angiography and did not report any major adverse cardiovascular event during a 20-years follow-up, thereby obtaining advantage from the gene variant as regards protection against atherosclerosis, apparently without any metabolic retaliation.

Conclusions

Our data support the use of targeted NGS in well-characterized clinical settings, as well as they indicate that.a partial block of ApoB production may be well tolerated and improve cardiovascular outcomes.

Hsp40s play distinct roles during the initial stages of apolipoprotein B biogenesis

Apolipoprotein B (ApoB) is the primary component of atherogenic lipoproteins, which transport serum fats and cholesterol. Therefore, elevated levels of circulating ApoB are a primary risk factor for cardiovascular disease. During ApoB biosynthesis in the liver and small intestine under nutrient-rich conditions, ApoB cotranslationally translocates into the endoplasmic reticulum (ER) and is lipidated and ultimately secreted. Under lipid-poor conditions, ApoB is targeted for ER Associated Degradation (ERAD). Although prior work identified select chaperones that regulate ApoB biogenesis, the contributions of cytoplasmic Hsp40s are undefined. To this end, we screened ApoB-expressing yeast and determined that a class A ER-associated Hsp40, Ydj1, associates with and facilitates the ERAD of ApoB. Consistent with these results, a homologous Hsp40, DNAJA1, functioned similarly in rat hepatoma cells. DNAJA1 deficient cells also secreted hyperlipidated lipoproteins, in accordance with attenuated ERAD. In contrast to the role of DNAJA1 during ERAD, DNAJB1-a class B Hsp40-helped stabilize ApoB. Depletion of DNAJA1 and DNAJB1 also led to opposing effects on ApoB ubiquitination. These data represent the first example in which different Hsp40s exhibit disparate effects during regulated protein biogenesis in the ER, and highlight distinct roles that chaperones can play on a single ERAD substrate.

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.

Can modulators of apolipoproteinB biogenesis serve as an alternate target for cholesterol-lowering drugs?

Understanding the molecular defects underlying cardiovascular disease is necessary for the development of therapeutics. The most common method to lower circulating lipids, which reduces the incidence of cardiovascular disease, is statins, but other drugs are now entering the clinic, some of which have been approved. Nevertheless, patients cannot tolerate some of these therapeutics, the drugs are costly, and/or the treatments are approved for only rare forms of disease. Efforts to find alternative treatments have focused on other factors, such as apolipoproteinB (apoB), which transports cholesterol in the blood stream. The levels of apoB are regulated by endoplasmic reticulum (ER) associated degradation as well as by a post ER degradation pathway in model systems, and we suggest that these events provide novel therapeutic targets. We discuss first how cardiovascular disease arises and how cholesterol is regulated, and then summarize the mechanisms of action of existing treatments for cardiovascular disease. We then review the apoB biosynthetic pathway, focusing on steps that might be amenable to therapeutic interventions.

Relationship between Amphipathic β Structures in the β1 Domain of Apolipoprotein B and the Properties of the Secreted Lipoprotein Particles in McA-RH7777 Cells

Our previous studies demonstrated that the first 1000 amino acid residues (the βα₁ domain) of human apolipoprotein (apo) B-100, termed apoB:1000, are required for the initiation of lipoprotein assembly and the formation of a monodisperse stable phospholipid (PL)-rich particle. The objectives of this study were (a) to assess the effects on the properties of apoB truncates undergoing sequential inclusion of the amphipathic β strands in the 700 N-terminal residues of the β₁ domain of apoB-100 and (b) to identify the subdomain in the β₁ domain that is required for the formation of a microsomal triglyceride transfer protein (MTP)-dependent triacylglycerol (TAG)-rich apoB-containing particle. Characterization of particles secreted by stable transformants of McA-RH7777 cells demonstrated the following. (1) The presence of amphipathic β strands in the 200 N-terminal residues of the β₁ domain resulted in the secretion of apoB truncates (apoB:1050 to apoB:1200) as both lipidated and lipid-poor particles. (2) Inclusion of residues 300-700 of the β₁ domain led to the secretion of apoB:1300, apoB:1400, apoB:1500, and apoB:1700 predominantly as lipidated particles. (3) Particles containing residues 1050-1500 were all rich in PL. (4) There was a marked increase in the lipid loading capacity and TAG content of apoB:1700-containing particles. (5) Only the level of secretion of apoB:1700 was markedly diminished by MTP inhibitor BMS-197636. These results suggest that apoB:1700 marks the threshold for the formation of a TAG-rich particle and support the concept that MTP participates in apoB assembly and secretion at the stage where particles undergo a transition from PL-rich to TAG-rich.

Phosphatidylcholine biosynthesis and lipoprotein metabolism

Phosphatidylcholine (PC) is the major phospholipid component of all plasma lipoprotein classes. PC is the only phospholipid which is currently known to be required for lipoprotein assembly and secretion. Impaired hepatic PC biosynthesis significantly reduces the levels of circulating very low density lipoproteins (VLDLs) and high density lipoproteins (HDLs). The reduction in plasma VLDLs is due in part to impaired hepatic secretion of VLDLs. Less PC within the hepatic secretory pathway results in nascent VLDL particles with reduced levels of PC. These particles are recognized as being defective and are degraded within the secretory system by an incompletely defined process that occurs in a post-endoplasmic reticulum compartment, consistent with degradation directed by the low-density lipoprotein receptor and/or autophagy. Moreover, VLDL particles are taken up more readily from the circulation when the PC content of the VLDLs is reduced, likely due to a preference of cell surface receptors and/or enzymes for lipoproteins that contain less PC. Impaired PC biosynthesis also reduces plasma HDLs by inhibiting hepatic HDL formation and by increasing HDL uptake from the circulation. These effects are mediated by elevated expression of ATP-binding cassette transporter A1 and hepatic scavenger receptor class B type 1, respectively. Hepatic PC availability has recently been linked to the progression of liver and heart disease. These findings demonstrate that hepatic PC biosynthesis can regulate the amount of circulating lipoproteins and suggest that hepatic PC biosynthesis may represent an important pharmaceutical target. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Intestinal lymphatic transport for drug delivery

Intestinal lymphatic transport has been shown to be an absorptive pathway following oral administration of lipids and an increasing number of lipophilic drugs, which once absorbed, diffuse across the intestinal enterocyte and while in transit associate with secretable enterocyte lipoproteins. The chylomicron-associated drug is then secreted from the enterocyte into the lymphatic circulation, rather than the portal circulation, thus avoiding the metabolically-active liver, but still ultimately returning to the systemic circulation. Because of this parallel and potentially alternative absorptive pathway, first-pass metabolism can be reduced while increasing lymphatic drug exposure, which opens the potential for novel therapeutic modalities and allows the implementation of lipid-based drug delivery systems. This review discusses the physiological features of the lymphatics, enterocyte uptake and metabolism, links between drug transport and lipid digestion/re-acylation, experimental model (in vivo, in vitro, and in silico) of lymphatic transport, and the design of lipid- or prodrug-based drug delivery systems for enhancing lymphatic drug transport.

Apolipoprotein B100 biogenesis: a complex array of intracellular mechanisms regulating folding, stability, and lipoprotein assemblyThis paper is one of a selection of papers published in this special issue entitled “Canadian Society of Biochemistry, Molecular & Cellular Biology 52nd Annual Meeting — Protein Folding: Principles and Diseases” and has undergone the Journal's usual peer review process

Apolipoprotein B100 (apoB) is a large amphipathic lipid-binding protein that is synthesized by hepatocytes and used to assemble and stabilize very low density lipoproteins (VLDL). It may have been derived through evolution from other lipid-associating proteins such as microsomal triglyceride transfer protein or vitellogenin. The correct folding of apoB requires assistance from chaperone proteins in co-translational lipidation, disulfide bond formation, and glycosylation. Any impairment in these processes results in co-translational targeting of the misfolded apoB molecule for proteasomal degradation. In fact, most of the regulation of apoB production is mediated by intracellular degradation. ApoB that misfolds post-translationally, perhaps as a result of oxidative stress, may be eliminated through autophagy. This review focuses on the proposed pentapartite domain structure of apoB, the role that each domain plays in the binding of lipid species and regulation of apoB synthesis, and the process of VLDL assembly. The factors involved in the recognition, ubiquitination, and proteasomal delivery of defective apoB molecules are also discussed.

Nonsynonymous mutations within APOB in human familial hypobetalipoproteinemia: evidence for feedback inhibition of lipogenesis and postendoplasmic reticulum degradation of apolipoprotein B

Five nontruncating missense APOB mutations, namely A31P, G275S, L324M, G912D, and G945S, were identified in heterozygous carriers of familial hypobetalipoproteinemia (FHBL) in the Italian population. To test that the FHBL phenotype was a result of impaired hepatic secretion of mutant apoB proteins, we performed transfection studies using McA-RH7777 cells stably expressing wild type or mutant forms of human apolipoprotein B-48 (apoB-48). All mutant proteins displayed varied impairment in secretion, with G912D the least affected and A31P barely secreted. Although some A31P was degraded by proteasomes, a significant proportion of it (although inappropriately glycosylated) escaped endoplasmic reticulum (ER) quality control and presented in the Golgi compartment. Degradation of the post-ER A31P was achieved by autophagy. Expression of A31P also decreased secretion of endogenous apoB and triglycerides, yet the impaired lipoprotein secretion did not lead to lipid accumulation in the cells or ER stress. Rather, expression of genes involved in lipogenesis was down-regulated, including liver X receptor alpha, sterol regulator element-binding protein 1c, fatty acid synthase, acetyl-CoA carboxylase 1, stearoyl-CoA desaturase 1, and lipin-1. These results suggest that feedback inhibition of hepatic lipogenesis in conjunction with post-ER degradation of misfolded apoB proteins can contribute to reduce fat accumulation in the FHBL liver.

Microsomal triglyceride transfer protein in plasma and cellular lipid metabolism

PURPOSE OF REVIEW

This review summarizes recent advances about the role of microsomal triglyceride transfer protein in plasma and tissue lipid homeostasis.

RECENT FINDINGS

Microsomal triglyceride transfer protein emerged as a phospholipid transfer protein and acquired triacylglycerol transfer activity during evolution from invertebrates to vertebrates. These activities are proposed to participate in 'nucleation' and 'desorption' steps during the biosynthesis of primordial apoB-containing lipoproteins. Microsomal triglyceride transfer protein also transfers phospholipids to the glycolipid antigen presentation molecule CD1d. Under physiologic conditions, plasma apoB-containing lipoproteins and microsomal triglyceride transfer protein expression exhibit diurnal variations synchronized by food and light. Microsomal triglyceride transfer protein is regulated at the transcriptional level. HNF4alpha is critical for its transcription. Other transcription factors along with coactivators and corepressors modulate microsomal triglyceride transfer protein expression. Reductions in microsomal triglyceride transfer protein mRNA and activity are related to steatosis in HCV-3 infected patients. CCl4 induces steatosis by enhancing proteasomal degradation of microsomal triglyceride transfer protein and can be partially avoided by inhibiting this degradation. Chemical antagonists cause hepatosteatosis, but this was not seen in the absence of fatty acid binding protein.

SUMMARY

Microsomal triglyceride transfer protein is a target to lower plasma lipids and to reduce inflammation in certain immune disorders. More knowledge is required, however, regarding its regulation and its role in the biosynthesis of apoB-containing lipoproteins and CD1d.

The physiological and molecular regulation of lipoprotein assembly and secretion

Triglycerides are insoluble in water and yet are transported at milligram per millilitre concentrations in the bloodstream. This is made possible by the ability of the liver and intestine to assemble lipid-protein emulsions (i.e. lipoproteins), which transport hydrophobic molecules. The assembly of triglyceride-rich lipoproteins requires the coordination of protein and lipid synthesis, which occurs on the cytoplasmic surface of the endoplasmic reticulum (ER), and their concerted assembly and translocation into the luminal ER secretory pathway as nascent lipoprotein particles. The availability of lipid substrate for triglyceride production and the machinery for lipoprotein assembly are highly sensitive to nutritional, hormonal, and genetic modulation. Disorders in lipid metabolism or an imbalance between lipogenesis and lipoprotein assembly can lead to hyperlipidemia and/or hepatic steatosis. We selectively review recently-identified machinery, such as transcription factors and nuclear hormone receptors, which provide new clues to the regulation of lipoprotein secretion.

Missense mutations in APOB within the betaalpha1 domain of human APOB-100 result in impaired secretion of ApoB and ApoB-containing lipoproteins in familial hypobetalipoproteinemia

Familial hypobetalipoproteinemia (FHBL) is associated with mutations in the APOB gene. We reported the first missense APOB mutation, R463W, in an FHBL kindred (Burnett, J. R., Shan, J., Miskie, B. A., Whitfield, A. J., Yuan, J., Tran, K., Mc-Knight, C. J., Hegele, R. A., and Yao, Z. (2003) J. Biol. Chem. 278, 13442-13452). Here we identified a second nonsynonymous APOB mutation, L343V, in another FHBL kindred. Heterozygotes for L343V (n = 10) had a mean plasma apoB at 0.31 g/liter as compared with 0.80 g/liter in unaffected family members (n = 22). The L343V mutation impaired secretion of apoB-100 and very low density lipoproteins. The secretion efficiency was 20% for B100wt and 10% for B100LV and B100RW. Decreased secretion of mutant apoB-100 was associated with increased endoplasmic reticulum retention and increased binding to microsomal triglyceride transfer protein and BiP. Reduced secretion efficiency was also observed with B48LV and B17LV. Biochemical and biophysical analyses of apoB domain constructs showed that L343V and R463W altered folding of the alpha-helical domain within the N terminus of apoB. Thus, proper folding of the alpha-helical domain of apoB-100 is essential for efficient secretion.

Endothelial progenitor cells display clonal restriction in multiple myeloma

BACKGROUND

In multiple myeloma (MM), increased neoangiogenesis contributes to tumor growth and disease progression. Increased levels of endothelial progenitor cells (EPCs) contribute to neoangiogenesis in MM, and, importantly, covary with disease activity and response to treatment. In order to understand the mechanisms responsible for increased EPC levels and neoangiogenic function in MM, we investigated whether these cells were clonal by determining X-chromosome inactivation (XCI) patterns in female patients by a human androgen receptor assay (HUMARA). In addition, EPCs and bone marrow cells were studied for the presence of clonotypic immunoglobulin heavy-chain (IGH) gene rearrangement, which indicates clonality in B cells; thus, its presence in EPCs would indicate a close genetic link between tumor cells in MM and endothelial cells that provide tumor neovascularization.

METHODS

A total of twenty-three consecutive patients who had not received chemotherapy were studied. Screening in 18 patients found that 11 displayed allelic AR in peripheral blood mononuclear cells, and these patients were further studied for XCI patterns in EPCs and hair root cells by HUMARA. In 2 patients whose EPCs were clonal by HUMARA, and in an additional 5 new patients, EPCs were studied for IGH gene rearrangement using PCR with family-specific primers for IGH variable genes (VH).

RESULTS

In 11 patients, analysis of EPCs by HUMARA revealed significant skewing (> or = 77% expression of a single allele) in 64% (n = 7). In 4 of these patients, XCI skewing was extreme (> or = 90% expression of a single allele). In contrast, XCI in hair root cells was random. Furthermore, PCR amplification with VH primers resulted in amplification of the same product in EPCs and bone marrow cells in 71% (n = 5) of 7 patients, while no IGH rearrangement was found in EPCs from healthy controls. In addition, in patients with XCI skewing in EPCs, advanced age was associated with poorer clinical status, unlike patients whose EPCs had random XCI.

CONCLUSION

Our results suggest that EPCs in at least a substantial subpopulation of MM patients are related to the neoplastic clone and that this is an important mechanism for upregulation of tumor neovascularization in MM.

Phospholipid Transfer Activity of Microsomal Triacylglycerol Transfer Protein Is Sufficient for the Assembly and Secretion of Apolipoprotein B Lipoproteins

Human microsomal triacylglycerol transfer protein (hMTP) is essential for apolipoprotein B (apoB)-lipoprotein assembly and secretion and is known to transfer triacylglycerols, cholesterol esters, and phospholipids. To understand the relative importance of each lipid transfer activity, we compared the ability of hMTP and its Drosophila ortholog (dMTP) to assemble apoB lipoproteins and to transfer various lipids. apoB48 secretion was induced when co-expressed with either hMTP or dMTP in COS cells, and oleic acid supplementation further augmented secretion without altering particle density. C-terminal epitope-tagged dMTP (dMTP-FLAG) facilitated the secretion of apoB polypeptides in the range of apoB48 to apoB72 but was approximately 50% as efficient as hMTP-FLAG. Comparison of lipid transfer activities revealed that although phospholipid transfer was similar in both orthologs, dMTP was unable to transfer neutral lipids. We conclude that the phospholipid transfer activity of MTP is sufficient for the assembly and secretion of primordial apoB lipoproteins and may represent its earliest function evolved for the mobilization of lipid in invertebrates. Identification of MTP inhibitors, which selectively affect transfer of a specific lipid class, may have therapeutic potential.

Attenuated secretion of very low density lipoproteins from McA-RH7777 cells treated with eicosapentaenoic acid is associated with impaired utilization of triacylglycerol synthesized via phospholipid remodeling

In McA-RH7777 cells stably expressing human apolipoprotein (apo) B100, treatment with oleic acid (18:1(n-9)) promoted whereas treatment with eicosapentaenoic acid (EPA, 20:5(n-3)) attenuated assembly and secretion of VLDL. Under conditions where the cells were cultured in the presence of 20% serum, EPA (0.4 mM) had marginal effect on the secretion of total apoB100 (determined by pulse-chase analysis) but decreased (by 50%) secretion of triacylglycerol (TG), indicating that the inhibitory effect of EPA was exerted primarily on TG-rich VLDL. Analysis of phospholipid mass and species by tandem mass spectrometry showed increased phosphatidylethanolamine (PE) in EPA-treated cells, the increase was significant in the distal Golgi membranes (by 170%) and endoplasmic reticulum (by 116%). Lipid pulse-chase studies showed a major distinction between phospholipid species containing 20:5(n-3) and 18:1(n-9), which in turn was associated with distinct compartmentalization of TG containing 20:5(n-3) or 18:1(n-9) between cytosol and microsomes and their recruitment during VLDL assembly. Thus, 18:1-TG was secreted as VLDL but 20:5-TG was not. These results suggest that EPA attenuation of VLDL secretion is associated with impaired utilization of TG derived from phospholipid remodeling.

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 transport of triglycerides through the secretory pathway of hepatocytes is impaired in apolipoprotein E deficient mice

BACKGROUND/AIMS

Apolipoprotein E (apoE)-deficient mice develop hepatic steatosis and secrete reduced levels of VLDL-TG.

METHODS AND RESULTS

We examined the effects of apoE-deficiency on intracellular lipid homeostasis and secretion of triglycerides (TG). We show that intracellular TG turnover and activities of diacylglycerol acyltransferase (DGAT) and microsomal triglyceride transfer protein (MTP) are similar in Apoe(-/-) and wild type mice. In addition, apoB synthesis was not decreased in Apoe(-/-) cells. Thus, the accumulation of lipid in these cells is not attributable to perturbed TG turnover, apoB synthesis, and the activities of DGAT and MTP. Inhibition of MTP had a more profound impact on the secretion of VLDL-TG from wild type hepatocytes than Apoe(-/-) hepatocytes, indicating that MTP was more limiting for the production of VLDL-TG from wild type cells. In marked contrast to the MTP-deficient model of fatty liver, electron microscopy of lipid-stained liver sections of Apoe(-/-) mice revealed an accumulation of lipid in numerous small, putative ER-derived vesicles and in the cytosol. No abnormalities were observed in the Golgi of Apoe(-/-) mice.

CONCLUSIONS

These results suggest that the removal of lipids from the early or intermediary compartments of the secretory pathway of hepatocytes is impaired in Apoe(-/-) mice.

Amino acid sequences within the β1 domain of human apolipoprotein B can mediate rapid intracellular degradation

Apolipoprotein B (apoB)-48 contains a region termed the beta1 domain that is predicted to be composed of extensive amphipathic beta-strands. Analysis of truncated apoB variants revealed that sequences between the carboxyl termini of apoB-37 and apoB-42 governed the secretion efficiency and intracellular stability of apoB. Although apoB-37, apoB-34, and apoB-29 were stable and secreted efficiently, apoB-42 and apoB-100 were secreted poorly and were degraded by an acetyl-leucyl-leucyl-norleucinal (ALLN)-sensitive pathway. Amino acid sequence analysis suggested that a segment between the carboxyl termini of apoB-38 and apoB-42 was 63% homologous to fatty acid binding proteins (FABPs), which contain orthogonal beta-sheets. To test the hypothesis that sequences from the beta1 domain are involved in apoB degradation, fusion proteins were created that contained apoB-29 linked to fragments derived from the beta1 domain of apoB or to liver FABP. Fusion proteins containing the beta1 domain segments apoB-34-42 or apoB-37-42 were degraded rapidly, whereas other fusion proteins were stable and secreted efficiently. Degradation was ALLN-sensitive, and the apoB-34-42 segment increased the association of the apoB protein with the cytosolic surface of the microsomal membrane. Our data suggest that the presence of specific sequences in the beta1 domain of human apoB increases degradation by promoting the cytosolic exposure of the protein, although not all regions of the beta1 domain are functionally equivalent.

Hepatic secretion of apoB-100 is impaired in hypobetalipoproteinemic mice with an apoB-38.9-specifying allele

Apolipoprotein B (apoB) truncation-specifying mutations cause familial hypobetalipoproteinemia (FHBL). Lipoprotein kinetics studies have shown that production rates of apoB-100 are reduced by 70-80% in heterozygous FHBL humans, instead of the expected 50%. To develop suitable mouse models to study the underlying mechanism, apoB-38.9-only (Apob(38.9/38.9)) mice were crossbred with Apobec-1 knockout (Apobec-1(-/-)) mice or apoB-100-only (Apob(100/100)) mice to produce two lines of apoB-38.9 heterozygous mice that produce only apoB-38.9 and apoB-100, namely Apobec-1(-/-)/Apob(38.9/+) and Apob(38.9/100) mice. In vivo rates of apoB-100 secretion were measured using [35S]Met/Cys to label proteins and Triton WR-1339 to block apoB-100 VLDL lipolysis/uptake. Rates of secretion were reduced by 80%, rather than the expected 50%, in both Apobec-1(-/-)/Apob(38.9/+) and Apob(38.9/100) mice compared with those of the respective Apobec-1(-/-)/Apob(+/+) and Apob(100/100) control mice. Continuous labeling and pulse-chase experiments in primary hepatocyte cultures revealed that rates of apoB-100 synthesis by Apobec-1(-/-)/Apob(38.9/+) and Apob(38.9/100) hepatocytes were reduced to the expected 50% of those of the respective controls, but the efficiency of secretion of apoB-100 was significantly lower in apoB-38.9 heterozygous hepatocytes. The greater-than-expected decreases in apoB-100 production rates of FHBL heterozygous humans appear to be attributable to a defect in secretion rather than in the synthesis of apoB-100 from the unaffected apoB allele.

Identification of the lipoprotein initiating domain of apolipoprotein B

We have explored the minimum sequence requirement for the initiation of apolipoprotein B (apoB)-mediated triglyceride-rich lipoprotein assembly. A series of apoB COOH-terminal truncation mutants, spanning a range from apoB34 (amino acid residues 1-1544 of apoB100) to apoB19 (residues 1-862) were transfected into COS cells with and without coexpression of the microsomal triglyceride transfer protein (MTP). ApoB34, -25, -23, -21, -20.5, and -20.1 underwent efficient conversion to buoyant lipoproteins when coexpressed with MTP. ApoB19.5 (amino acids 1-884) also directed MTP-dependent particle assembly, although at reduced efficiency. When apoB19.5 was truncated by another 22 amino acids to form apoB19, MTP-dependent lipoprotein assembly was abolished. Analysis of the lipid stoichiometry of secreted lipoproteins revealed that all apoB truncation mutants formed spherical particles containing a hydrophobic core. Even highly truncated assembly-competent forms of apoB, such as apoB19.5 and 20.1, formed lipoproteins with surface:core lipid ratios of <1. We conclude that the translation of the first approximately 884 amino acids of apoB completes a domain capable of initiating nascent lipoprotein assembly. The composition of lipids recruited into lipoproteins by this initiating domain is consistent with formation of small emulsion particles, perhaps by simultaneous desorption of both polar and neutral lipids from a saturated bilayer.

A novel nontruncating APOB gene mutation, R463W, causes familial hypobetalipoproteinemia

Familial hypobetalipoproteinemia (FHBL), an autosomal co-dominant disorder, is associated with reduced plasma concentrations (<5th percentile for age and sex) of apolipoprotein (apo) B and beta-migrating lipoproteins. To date, only mutations in APOB encoding prematurely truncated apoB have been found in FHBL. We discovered a novel APOB gene mutation, namely R463W, in an extended Christian Lebanese FHBL kindred. Heterozygotes for R463W had the typical FHBL phenotype, whereas homozygotes had barely detectable apoB-100. The effect of the R463W mutation on apoB secretion was examined using transfected McA-RH7777 cells that expressed one of two recombinant human apoBs, namely B48 and B17. In both cases, the mutant proteins (B48RW and B17RW) were retained within the endoplasmic reticulum and were secreted poorly compared with their wild-type counterparts. Pulse-chase analysis showed that secretion efficiencies of B48RW and B17RW were, respectively, 45 and 40% lower than those of the wild-types. Substitution of Arg(463) with Ala in apoB-17 (B17RA) decreased secretion efficiency by approximately 50%, but substitution with Lys (B17RK) had no effect on secretion, indicating that the positive charge was important. Molecular modeling of apoB predicted that Arg(463) was in close proximity to Glu(756) and Asp(456). Substitution of Glu(756) with Gln (B17EQ) had no effect on secretion, but substitution of Asp(456) with Asn (B17DN) decreased secretion to the same extent as B17RW. In co-transfection experiments, the mutant B17RW showed increased binding to microsomal triglyceride transfer protein as compared with wild-type B17. Thus, the naturally occurring R463W mutant reveals a key local domain governing assembly and secretion of apoB-containing lipoproteins.

The N-linked oligosaccharides at the amino terminus of human apoB are important for the assembly and secretion of VLDL

We determined the role of N-linked glycosylation of apolipoprotein B (apoB) in the assembly and secretion of lipoproteins using transfected rat hepatoma McA-RH7777 cells expressing human apoB-17, apoB-37, and apoB-50, three apoB variants with different ability to recruit neutral lipids. Substituting Asn residue with Gln at the single glycosylation site within apoB-17 (N(158)) decreased its secretion efficiency to a level equivalent to that of wild-type apoB-17 treated with tunicamycin, but had little effect on its synthesis or intracellular distribution. When selective N-to-Q substitution was introduced at one or more of the five N-linked glycosylation sites within apoB-37 (N(158), N(956), N(1341), N(1350), and N(1496)), secretion efficiency of apoB-37 from transiently transfected cells was variably affected. When all five N-linked glycosylation sites were mutated within apoB-37, the secretion efficiency and association with lipoproteins were decreased by >50% as compared with wild-type apoB-37. Similarly, mutant apoB-50 with all of its N-linked glycosylation sites mutagenized showed decreased secretion efficiency and decreased lipoprotein association in both d < 1.02 and d > 1.02 g/ml fractions. The inability of mutant apoB-37 and apoB-50 to associate with very low-density lipoproteins was attributable to impaired assembly and was not due to the limitation of lipid availability. The decreased secretion of mutant apoB-17 and apoB-37 was not accompanied by accumulation within the cells, suggesting that the proportion of mutant apoB not secreted was rapidly degraded. However unlike apoB-17 or apoB-37, accumulation of mutant apoB-50 was observed within the endoplasmic reticulum and Golgi compartments. These data imply that the N-glycans at the amino terminus of apoB play an important role in the assembly and secretion of lipoproteins containing the carboxyl terminally truncated apoB.

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.

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.

Variation in the human ApoB signal peptide modulates ApoB17 translocation

The functional effects of the common 27- or 24-amino-acid (aa) variants in the human apoB signal peptide (SP) on intracellular and secreted apoB17 were investigated in vitro. Only in the presence of oleate was a significant difference in intracellular and secreted SP27-B17 compared to SP24-B17 observed (P = 0.01 and P < 0.0007, respectively), although in the presence or absence of oleate mRNA levels from the two constructs were similar. After fractionation, oleate treatment enhanced microsomal SP27-B17 by 150% (P < 0.0005) with a modest but significant effect on SP24-B17 (32% P = 0.007). Oleate stimulated SP24-B17 accumulation in the nonmicrosomal fraction. The data suggest that the presence of oleate leads to inefficient translocation of the 24-amino-acid signal peptide, possibly resulting in increased retrograde translocation into the cytoplasm and reduced intracellular and secreted levels compared to the "wildtype" 27 aa SP. This implies a direct role of the SP variants in the regulation of apoB intracellular metabolism.

A targeted apolipoprotein B-38.9-producing mutation causes fatty livers in mice due to the reduced ability of apolipoprotein B-38.9 to transport triglycerides

Nonphysiological truncations of apolipoprotein (apo) B-100 cause familial hypobetalipoproteinemia (FHBL) in humans and mice. An elucidation of the mechanisms underlying the FHBL phenotypes may provide valuable information on the metabolism of apo B-containing lipoproteins and the structure-function relationship of apo B. To generate a faithful mouse model of human FHBL, a subtle mutation was introduced into the mouse apo B gene by targeting embryonic stem cells using homologous recombination followed by removal of the selection marker gene by Cre-loxP-mediated site-specific recombination. The engineered mice bear a premature stop codon at residue 1767 and a 42-base pair loxP inserted into intron 24 of the apo B gene, thus closely resembling the apo B-38.9-producing mutation in humans. Apo B-38.9 was the sole apo B protein in homozygote (apob(38.9/38.9)) plasma. In heterozygotes (apob(+/)(38. 9)), apo B-100 and apo B-48 were reduced by 75 and 40%, respectively, and apo B-38.9 represented 20% of total circulating apo B. Hepatic apo B-38.9 mRNA levels were reduced by 40%. In cultured apob(+/)(38. 9) hepatocytes, apo B-100 was produced in trace quantities, and the synthesis rate of apo B-38.9 relative to apo B-48 was reduced by 40%. However, almost equimolar amounts of apo B-38.9 and apo B-48 were secreted into the media. Pulse-chase studies revealed that apo B-38. 9 was secreted at a faster rate and more efficiently than apoB-48. Nevertheless, both apob(+/)(38.9) and apob(38.9/38.9) mice had reduced hepatic triglyceride secretion rates and fatty livers. Thus, low mRNA levels or defective secretion of apo B-38.9 may not be responsible for the FHBL phenotypes caused by the apo B-38.9 mutation. Rather, a reduced capacity of apo B-38.9 for triglyceride transport may account for the fatty livers in these mice.

A study of fatty liver disease and plasma lipoproteins in a kindred with familial hypobetalipoproteinemia due to a novel truncated form of apolipoprotein B (APO B-54.5)

BACKGROUND/AIMS

Familial hypobetalipoproteinemia (FHBL) is a co-dominant disorder characterized by reduced plasma levels of low-density lipoproteins. It can be caused by mutations in the gene encoding apolipoprotein B-100 (apo B), leading to the formation of truncated apo Bs which have a reduced capacity to export lipids from the hepatocytes as lipoprotein constituents. Case reports suggest the occurrence of liver disease in FHBL, but there are no studies of liver involvement in FHBL with defined apo B gene mutations. The presence of fatty liver disease was investigated in a large FHBL kindred.

METHODS

Plasma lipoprotein and apolipoprotein analysis, liver function tests, and apo B gene sequence were performed in 16 members of a FHBL kindred. The presence of fatty liver was assessed by ultrasound and computed tomography scanning.

RESULTS

The proband, a non-obese heavy drinker male with hypobetalipoproteinemia, had steatohepatitis with fibrosis. He was heterozygous for a novel non-sense mutation of apo B gene producing a truncated apo B of 2745 amino acids (designated apo B-54.5, having half the size of normal apo B-100). Seven other members of his kindred carried apo B-54.5. Although all of them were hypolipidemic, their lipid levels showed a large inter-individual variability not accounted for by polymorphisms of genes involved in apo B metabolism. Four carriers (two heavy drinkers and two teetotallers), irrespective of their plasma lipid levels, had ultrasonographic evidence of fatty liver. In the other four carriers no evidence of fatty liver was found.

CONCLUSIONS

In this kindred apo B-54.5 predisposes to fatty liver, which however may require some additional factors to become clinically relevant.

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.

Palmitoylation of apolipoprotein B is required for proper intracellular sorting and transport of cholesteroyl esters and triglycerides

Apolipoprotein B (apoB) is an essential component of chylomicrons, very low density lipoproteins, and low density lipoproteins. ApoB is a palmitoylated protein. To investigate the role of palmitoylation in lipoprotein function, a palmitoylation site was mapped to Cys-1085 and removed by mutagenesis. Secreted lipoprotein particles formed by nonpalmitoylated apoB were smaller and denser and failed to assemble a proper hydrophobic core. Indeed, the relative concentrations of nonpolar lipids were three to four times lower in lipoprotein particles containing mutant apoB compared with those containing wild-type apoB, whereas levels of polar lipids isolated from wild-type or mutant apoB lipoprotein particles appeared identical. Palmitoylation localized apoB to large vesicular structures corresponding to a subcompartment of the endoplasmic reticulum, where addition of neutral lipids was postulated to occur. In contrast, nonpalmitoylated apoB was concentrated in a dense perinuclear area corresponding to the Golgi compartment. The involvement of palmitoylation as a structural requirement for proper assembly of the hydrophobic core of the lipoprotein particle and its intracellular sorting represent novel roles for this posttranslational modification.

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.

Overexpression of apolipoprotein E3 in transgenic rabbits causes combined hyperlipidemia by stimulating hepatic VLDL production and impairing VLDL lipolysis

The differential effects of overexpression of human apolipoprotein (apo) E3 on plasma cholesterol and triglyceride metabolism were investigated in transgenic rabbits expressing low (<10 mg/dL), medium (10 to 20 mg/dL), or high (>20 mg/dL) levels of apoE3. Cholesterol levels increased progressively with increasing levels of apoE3, whereas triglyceride levels were not significantly affected at apoE3 levels up to 20 mg/dL but were markedly increased at levels of apoE3 >20 mg/dL. The medium expressers had marked hypercholesterolemia (up to 3- to 4-fold over nontransgenics), characterized by an increase in low density lipoprotein (LDL) cholesterol, while the low expressers had only slightly increased plasma cholesterol levels. The medium expressers displayed an 18-fold increase in LDL but also had a 2-fold increase in hepatic very low density lipoprotein (VLDL) triglyceride production, an 8-fold increase in VLDL apoB, and a moderate decrease in the ability of the VLDL to be lipolyzed. However, plasma clearance of VLDL was increased, likely because of the increased apoE3 content. The increase in LDL appears to be due to an enhanced competition of VLDL for LDL receptor binding and uptake, resulting in the accumulation of LDL. The combined hyperlipidemia of the apoE3 high expressers (>20 mg/dL) was characterized by a 19-fold increase in LDL cholesterol but also a 4-fold increase in hepatic VLDL triglyceride production associated with a marked elevation of plasma VLDL triglycerides, cholesterol, and apoB100 (4-, 9-, and 25-fold over nontransgenics, respectively). The VLDL from the high expressers was much more enriched in apoE3 and markedly depleted in apoC-II, which contributed to a >60% inhibition of VLDL lipolysis. The combined effects of stimulated VLDL production and impaired VLDL lipolysis accounted for the increases in plasma triglyceride and VLDL concentrations in the apoE3 high expressers. The hyperlipidemic apoE3 rabbits have phenotypes similar to those of familial combined hyperlipidemia, in which VLDL overproduction is a major biochemical feature. Overall, elevated expression of apoE3 appears to determine plasma lipid levels by stimulating hepatic VLDL production, enhancing VLDL clearance, and inhibiting VLDL lipolysis. Thus, the differential expression of apoE may, within a rather narrow range of concentrations, play a critical role in modulating plasma cholesterol and triglyceride levels and may represent an important determinant of specific types of hyperlipoproteinemia.

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.

Identification of domains in apolipoprotein B100 that confer a high requirement for the microsomal triglyceride transfer protein

The microsomal triglyceride transfer protein (MTP) is required for the assembly and secretion of apoB-containing lipoproteins. To investigate the role of MTP in lipoprotein assembly, we determined the ability of carboxyl-terminally truncated forms of apoB to be secreted from cells treated with the MTP inhibitor 4'-bromo-3'-methylmetaqualone (Benoist, F., Nicodeme, E., and Grand-Perret, T. (1996) Eur. J. Biochem. 240, 713-720). In Caco-2 and mhAT3F cells that produce apoB100 and apoB48, the inhibitor preferentially blocked apoB100 secretion. When the inhibitor was tested on McA-RH7777 cells stably transfected with cDNAs encoding human apoB100, apoB72, apoB53, apoB29, and apoB18, the secretion of apoB100, apoB72, and apoB53 was preferentially impaired relative to apoB48 and shorter forms. To delineate the region between apoB48 and apoB53 that has a high requirement for MTP, we used puromycin to generate a range of truncated forms of apoB in HepG2 cells. The secretion of apoB53 and longer forms of apoB was markedly affected by low concentrations of the MTP inhibitor (approximately 1 microM), whereas apoB51 and smaller forms of apoB were only affected at higher concentrations (> 10 microM). The size-related sensitivity to MTP inhibitor was not due to late processing or retention, since the same result was observed when nascent lipoproteins were isolated from the endoplasmic reticulum. The MTP inhibitor did not alter the density of the secreted lipoproteins, indicating that each apoB polypeptide requires a minimally defined amount of lipid to attain a secretable conformation. Our results suggest that the folding of the domain between apoB51 and apoB53 has a high requirement for lipid. This domain is predicted to form amphipathic alpha-helices and to bind lipid reversibly. It proceeds and is followed by rigid amphipathic beta-sheets that are predicted to associate with lipid irreversibly. We speculate that these domains enable apoB to switch from a stable lipid-poor conformation in apoB48 to another lipid-rich conformation in apoB100 during lipoprotein assembly.

Intracellular translocation and stability of apolipoprotein B are inversely proportional to the length of the nascent polypeptide

We have studied the relationship between the length of apolipoprotein B (apoB) and its intracellular translocation and stability using McArdle RH7777 (McA-RH7777) cells expressing recombinant human apoB variants, ranging in size from B15 to B100. The translocational status of apoB was assessed based on trypsin sensitivity of apoB using isolated microsomes as well as permeabilized cells. In isolated microsomes, shorter apoB variants (</=B48) were 75-100% resistant to exogenous trypsin digestion, whereas apoB variants larger than B48 were less than 40% trypsin-resistant. Experiments with hepatic microsomes isolated from rat or transgenic mice expressing human B48 and B100 also confirmed the high trypsin accessibility of B100 compared with B48. In permeabilized cells, apoB variants shorter than B48 were relatively resistant to exogenous trypsin (percentage of trypsin-resistant apoB greater than 70%) in contrast to recombinant human B72 and B100, which were only 55 and 42% trypsin-resistant, respectively. The trypsin sensitivity of human B100 was comparable with that of endogenous rat B100 in McA-RH7777 cells as well as endogenous B100 in HepG2 cells (percentages of trypsin-resistant cells were as follows: for human B100 construct, 42 +/- 7.5%; for endogenous McA-RH7777 B100, 52 +/- 2.9%; and for endogenous HepG2 B100, 46 +/- 6.3%). Overall, an inverse correlation between the length of apoB and its resistance to exogenous trypsin was evident irrespective of the model system examined. An inverse relationship was also observed between the size of apoB and its co-translational resistance to proteasomal degradation. Truncated apoB constructs were relatively insensitive to proteasome inhibition by MG132 co-translationally (during the pulse) compared with the full-length B100, which was highly sensitive (apoB recovered in the presence of MG132 as a percentage of control was as follows: B15, 127%; B29, 94%; B48, 110%; B72, 140%; B100, 282%). Post-translationally (over a 2-h chase), a similar inverse relationship was found, with B100 being the least stable in comparison with truncated apoB variants. In summary, as the size of the nascent apoB chain increases, there appears to be a greater cytosolic exposure of the polypeptide, leading to a higher sensitivity to proteasomal degradation.

Sequences within the amino terminus of ApoB100 mediate its noncovalent association with apo(a)

Although sequences within the C terminus of apolipoprotein B (apoB) have been implicated in the formation of covalent lipoprotein(a) [Lp(a)] particles, sequences in apoB that mediate initial noncovalent interaction with apo(a) remain to be characterized. To address this question, we have used an affinity chromatography method in which 2 recombinant forms of apo(a) [r-apo(a); either a 17-kringle form (17K) or a derivative containing apo(a) kringle IV types 5-8] have been immobilized onto Sepharose beads. Conditioned media from rat hepatoma (McA-RH7777) cell lines stably expressing various carboxyl-terminally truncated forms of human apoB (ranging from full-length apoB to apoB15) were applied to the r-apo(a) affinity columns; the columns were subsequently washed and eluted with epsilon-aminocaproic acid (epsilon-ACA). Specific binding was quantified by Western blot analysis of column fractions. Of the apoB truncations examined, apoB94, apoB42, apoB37, and apoB29 exhibited complete specific binding to 17K r-apo(a). Only approximately 50% binding was observed for apoB18, whereas essentially no detectable binding was observed with apoB15. In all cases, similar results were obtained when the r-apo(a) kringle IV types 5-8-Sepharose column was used. Additionally, substitution of proline for epsilon-ACA as the eluent resulted in similar column profiles with either r-apo(a) affinity column. We also demonstrated that apoB48 present in chylomicrons bound completely to the 17K column in an epsilon-ACA-dependent manner. Taken together, these results represent the first demonstration that N-terminal sequences in apoB between amino acid residues 680 (apoB15) and 781 (apoB18) are essential for noncovalent association with apo(a) and that these sequences interact with domain(s) present within apo(a) kringle IV types 5-8.

Functional analysis of disulfide linkages clustered within the amino terminus of human apolipoprotein B

We tested the involvement of N-terminal six disulfide bonds (Cys-1 through Cys-12) of human apolipoprotein (apo) B in the assembly and secretion of lipoproteins using two C-terminal-truncated apoB variants, namely B50 and B18. In transfected rat hepatoma McA-RH7777 cells, B50 could assemble very low density lipoproteins (VLDL), and B18 was secreted as high density lipoproteins. When all 12 cysteine residues were substituted with alanines in B50, the mutant protein (B50C1-12) lost its ability to assemble lipid and was degraded intracellularly. However, mutation had no effect on B50C1-12 translation or translocation across the microsomal membrane. Post-translational degradation of B50C1-12 was partially inhibited by the proteasome inhibitor MG132. To determine which cysteines were critical in VLDL assembly and secretion, we prepared three additional mutant B50s, each containing four selected Cys-to-Ala substitutions in tandem (i.e. Cys-1 to Cys-4, Cys-5 to Cys-8, and Cys-9 to Cys-12). Expression of these mutants showed that disruption of disulfide bond formation within Cys-5 to Cys-8 diminished apoB secretion, whereas within Cys-1 to Cys-4 or Cys-9 to Cys-12 had lesser or no effect. In another two mutants in which only one disulfide bond (i.e. between Cys-5 and Cys-6 or between Cys-7 and Cys-8) was eliminated, only secretion of B50 with mutations at Cys-7 and Cys-8 was decreased. Thus, the disulfide bond involving Cys-7 and Cys-8 is most important for VLDL assembly and secretion. In addition, assembly and secretion of VLDL containing endogenous B100 or B48 were impaired in cells transfected with B50s containing Cys-7 and Cys-8 mutation. The Cys-to-Ala substitution abolished recognition of B50 by MB19, a conformational antibody with an epitope at the N terminus of human apoB. The Cys-to-Ala substitution also attenuated secretion of B18, but the effect of the mutation on B18 secretion was less evident than on B50.

Functional expression of the chicken low density lipoprotein receptor-related protein in a mutant chinese hamster ovary cell line restores toxicity of Pseudomonas exotoxin A and degradation of alpha2-macroglobulin

The low density lipoprotein receptor-related protein (LRP) is responsible for the clearance of several physiological ligands including a complex of proteinase and alpha2-macroglobulin (alpha2M) and for the entrance of Pseudomonas exotoxin A (PEA) into cells. We have prepared expression plasmids for the full-length chicken LRP (designated LRP100) and two intermediates encoding 25 and 67% of the receptor (designated LRP25 and LRP67, respectively) using overlapping cDNA fragments. LRP25 and LRP67 encode the N-terminal 22 and 64%, respectively, of LRP100 plus the transmembrane and intracellular domains. Transient transfection of these plasmids into COS-7 cells yielded recombinant proteins of expected molecular mass and immunoreactivity. However, LRP100 was incompletely processed into alpha- (515-kDa) and beta- (85-kDa) chains and was poorly transported from the endoplasmic reticulum to the Golgi compartment. Stable transformants of LRP100, LRP67, and LRP25 were generated in a mutant Chinese hamster ovary cell line that lacked expression of endogenous LRP and was resistant to PEA. All forms of recombinant LRP proteins were transported from the endoplasmic reticulum to the Golgi apparatus in Chinese hamster ovary cells as shown by their sensitivity to endoglycosidase H and resistance to neuraminidase. Cell surface iodination and subcellular fractionation studies indicated that all three LRP variants were expressed on the plasma membrane. Furthermore, expression of the three LRP variants restored, to various degrees, sensitivity to PEA and the ability to degrade methylamine-activated alpha2M (alpha2M*). These data suggest that deletion of large internal portions of LRP, including the processing site, does not prevent transport of LRP to the plasma membrane, nor does it abolish the interaction of LRP with alpha2M* or PEA. This LRP expression system may allow for the characterization of domains within LRP responsible for its multifunctionality.

Measurement of apolipoprotein B in various cell lines: correlation between intracellular levels and rates of secretion

We have standardized simple but sensitive enzyme-linked immunoassays to understand a relationship between intracellular levels and secretion rates of apoB. The assays were based on commercially available antibodies and were specific to human apoB. A monoclonal antibody, 1D1, was immobilized on microtiter wells and incubated with different amounts of low density lipoproteins to obtain a standard curve. Conditioned media were added to other wells in parallel, and the amount of apoB was quantitated from a linear regression curve. To standardize conditions for the measurement of intracellular apoB, cells were homogenized and solubilized with different concentrations of taurocholate. We found that 0.5% taurocholate was sufficient to solubilize all the apoB in HepG2, Caco-2, and McA-RH7777 cells. Next, a standard curve was prepared in the presence of taurocholate and used to determine intracellular levels of apoB in different cell lines. The intracellular levels (pmol/mg cell protein) and the rates of secretion (pmol/mg/h) of apoB100 were positively correlated (r2 = 0.81, P = 0.0009) in HepG2 cells. Furthermore, a positive correlation (r2 = 0.88, P < 0.0001) was found between intracellular and secreted apoB42 in stably transfected McA-RH7777 cells. In contrast, no correlation was observed for human apoB28 and apoB18 in stably transfected cells that were secreted either partially associated or completely unassociated with lipoproteins. These studies indicated that the rate of secretion of lipid-associated apoB, but not the lipid-free apoB, was tightly controlled.

Activation of a cryptic splice-site in intron 24 leads to the formation of apolipoprotein B-27.6

Apo B expression is confined to the intestine and liver, and its secretion from these tissues is dependent on the expression of a lipid transfer protein, microsomal triglyceride transfer protein (MTP). Previously, we reported a model system for the study of apolipoprotein (apo B) biogenesis using heterologous expression in COS cells (Patel SB, Grundy SM. J. Lipid Res. 1995;36:2090-2103). We now report the characterization of the effects of a T-->C transition in the splice-site at +2 of intron 24 previously reported by Talmud et al. (J. Lipid Res. 1994;35:468-77). Using our heterologous expression system, we show that the mutation led to aberrant processing of intron 24, but normal processing of intron 25. The resultant translation of this mutant mRNA produced a truncated apo B protein of the size of apo B-27.6. Reverse transcription, polymerase chain reaction and sequencing of the amplified products were used to show that a cryptic donor splice-site within intron 24 was utilized, resulting in the generation of a novel hydrophilic 29 amino acid carboxyl-terminal tail. Co-expression of apo B-27.6 with microsomal triglyceride transfer protein (MTP) showed that this protein could bind MTP and resulted in the secretion of a lipoprotein particle with a buoyant density in the range 1.16-1.25 g/ml. These results indicate that this splice-site mutation leads to an activation of a downstream cryptic splice-site within intron 24, causing an insertion of 40 bases of intron 24 sequences into the mature RNA. This leads to a frame-shift of translation resulting in addition of 29 new amino acids at the carboxyl-terminus, before an in-frame stop translation codon is encountered, truncating the apo B at B-27.6.

Co-translational degradation of apolipoprotein B100 by the proteasome is prevented by microsomal triglyceride transfer protein. Synchronized translation studies on HepG2 cells treated with an inhibitor of microsomal triglyceride transfer protein

We studied the effect of inhibition of microsomal triglyceride transfer protein (MTP) on apolipoprotein (apo) B100 translation and secretion using HepG2 cells. The MTP-mediated lipid transfer activity was reduced using a specific MTP inhibitor. ApoB100 translation was synchronized by treatment with puromycin prior to L-[35S]methionine pulse-chase labeling. During the first 4 min of chase, synthesis of apoB polypeptides the size of 100-200 kDa was insensitive to the inhibitor, suggesting that inhibition of MTP did not affect the initiation of apoB100 translation. After 15 min of chase, the 100-200-kDa species were chased into polypeptides larger than 320 kDa (i.e. apoB65 or 65% of full-length apoB100) in both control and inhibitor-treated cells. However, the amount of these polypeptides decreased (by 36% for apoB65-75, by 64% for apoB75-85, by 76% for apoB85-95, and by 77% for apoB100) upon MTP inhibition. No accumulation of smaller polypeptides was observed, but total immunoprecipitable apoB radioactivity was decreased suggesting that apoB could undergo co-translational degradation when MTP activity was reduced. Inhibitors of the multicatalytic proteinase complex (proteasome) such as lactacystin or MG-115 could prevent apoB co-translational degradation. Nevertheless, MG-115 could not avoid the MTP inhibitor decreasing apoB100 secretion but rather induced the accumulation of secretion-incompetent apoB100 in the cell. These results indicate that MTP activity is required during the elongation of apoB100 polypeptides, particularly at the sequences downstream of carboxyl terminus of apoB65. Co-translational degradation might constitute a more general mechanism of early quality control for large or complex proteins.

Normal activity of microsomal triglyceride transfer protein is required for the oleate-induced secretion of very low density lipoproteins containing apolipoprotein B from McA-RH7777 cells

The requirement of the activity of microsomal triglyceride transfer protein (MTP) for very low density lipoprotein (VLDL) secretion was determined using McA-RH7777 cells stably transfected with human apoB48 (hB48). Secretion of VLDL containing hB48 (hB48-VLDL) by the transfected cells was induced by exogenous oleate (0.4 mM), and oleate-dependent VLDL secretion was selectively inhibited by brefeldin A (0.2 microg/ml). Two protocols were used to determine the effect of MTP inhibition on VLDL secretion. In the first protocol, cell protein and lipid were labeled with radioactive amino acids and oleate prior to MTP inhibition (using 5 microM of the photoaffinity inhibitor BMS-192951 to reduce MTP activity by 65-70%), and secretion of prelabeled apoB and triacylglycerol (TG) associated with lipoproteins was monitored during oleate-supplemented chase. In control cells, a 6-fold increase in incorporation of prelabeled TG into hB48-VLDL was observed after oleate supplement, while incorporation of prelabeled TG into VLDL containing endogenous rat apoB100 (rB100-VLDL) was unaffected. Inhibition of MTP activity abolished the oleate-induced utilization of prelabeled TG (by 80%) and hB48 (by 70%) for hB48-VLDL secretion but decreased utilization of pre-existing TG (by <25%) and B100 (by 45%) for rB100-VLDL secretion to a lesser extent. Inhibition of MTP did not affect incorporation of prelabeled TG or hB48 into high density lipoproteins containing hB48 (hB48-HDL). In the second protocol, MTP was inactivated prior to metabolic labeling of protein and lipid, and secretion of newly labeled apoB and TG as lipoproteins was monitored after oleate supplement. Under this condition, MTP inhibition decreased incorporation of newly labeled TG (by 80%) and hB48 (80%) into hB48-VLDL but did not affect their incorporation into hB48-HDL. Additionally, MTP inhibition decreased incorporation of newly labeled TG (by 50%) and rB100 (by 90%) into rB100-VLDL. Thus, normal activity of MTP is required for the oleate-induced secretion of hB48-VLDL from McA-RH7777 cells.