Apolipoprotein B sequence requirements for hepatic very low density lipoprotein assembly. Evidence that hydrophobic sequences within apolipoprotein B48 mediate lipid recruitment

Association of Apolipoprotein B and Adiponectin Receptor 1 Genes with Carcass, Bone Integrity and Performance Traits in a Paternal Broiler Line

Apolipoprotein B (APOB) and Adiponectin Receptor 1 (ADIPOR1) are related to the regulation of feed intake, fat metabolism and protein deposition and are candidate genes for genomic studies in birds. In this study, associations of two single nucleotide polymorphisms (SNPs) g.102A>T (APOB) and g.729C>T (ADIPOR1) with carcass, bone integrity and performance traits in broilers were investigated. Genotyping was performed on a paternal line of 1,454 broilers. The SNP detection was carried out by PCR-RFLP technique using the restriction enzymes HhaI for the SNP g.729C>T and MslI for the SNP g.102A>T. The association analyses of the two SNPs with 85 traits were performed using the restricted maximum likelihood (REML) and Generalized Quasi-Likelihood Score (GQLS) methods. For REML the model included the random additive genetic effect of animal and fixed effects of sex, hatch and SNP genotypes. In the GQLS method, a logistic regression was used to associate the genotypes with phenotypes adjusted for fixed effects of sex and hatch. The SNP g.729C>T in the ADIPOR1 gene was associated with thickness of the femur and breast skin yield. Thus, the ADIPOR1 gene seems implicated in the metabolism and/or fat deposition and bone integrity in broilers.

Apolipoprotein B100 quality control and the regulation of hepatic very low density lipoprotein secretion

Apolipoprotein B (apoB) is the main protein component of very low density lipoprotein (VLDL) and is necessary for the assembly and secretion of these triglyceride (TG)-rich particles. Following release from the liver, VLDL is converted to low density lipoprotein (LDL) in the plasma and increased production of VLDL can therefore play a detrimental role in cardiovascular disease. Increasing evidence has helped to establish VLDL assembly as a target for the treatment of dyslipidemias. Multiple factors are involved in the folding of the apoB protein and the formation of a secretion-competent VLDL particle. Failed VLDL assembly can initiate quality control mechanisms in the hepatocyte that target apoB for degradation. ApoB is a substrate for endoplasmic reticulum associated degradation (ERAD) by the ubiquitin proteasome system and for autophagy. Efficient targeting and disposal of apoB is a regulated process that modulates VLDL secretion and partitioning of TG. Emerging evidence suggests that significant overlap exists between these degradative pathways. For example, the insulin-mediated targeting of apoB to autophagy and postprandial activation of the unfolded protein response (UPR) may employ the same cellular machinery and regulatory cues. Changes in the quality control mechanisms for apoB impact hepatic physiology and pathology states, including insulin resistance and fatty liver. Insulin signaling, lipid metabolism and the hepatic UPR may impact VLDL production, particularly during the postprandial state. In this review we summarize our current understanding of VLDL assembly, apoB degradation, quality control mechanisms and the role of these processes in liver physiology and in pathologic states.

Improved recovery and identification of membrane proteins from rat hepatic cells using a centrifugal proteomic reactor

Despite their importance in many biological processes, membrane proteins are underrepresented in proteomic analysis because of their poor solubility (hydrophobicity) and often low abundance. We describe a novel approach for the identification of plasma membrane proteins and intracellular microsomal proteins that combines membrane fractionation, a centrifugal proteomic reactor for streamlined protein extraction, protein digestion and fractionation by centrifugation, and high performance liquid chromatography-electrospray ionization-tandem MS. The performance of this approach was illustrated for the study of the proteome of ER and Golgi microsomal membranes in rat hepatic cells. The centrifugal proteomic reactor identified 945 plasma membrane proteins and 955 microsomal membrane proteins, of which 63 and 47% were predicted as bona fide membrane proteins, respectively. Among these proteins, >800 proteins were undetectable by the conventional in-gel digestion approach. The majority of the membrane proteins only identified by the centrifugal proteomic reactor were proteins with ≥ 2 transmembrane segments or proteins with high molecular mass (e.g. >150 kDa) and hydrophobicity. The improved proteomic reactor allowed the detection of a group of endocytic and/or signaling receptor proteins on the plasma membrane, as well as apolipoproteins and glycerolipid synthesis enzymes that play a role in the assembly and secretion of apolipoprotein B100-containing very low density lipoproteins. Thus, the centrifugal proteomic reactor offers a new analytical tool for structure and function studies of membrane proteins involved in lipid and lipoprotein metabolism.

Missense mutation in APOC3 within the C-terminal lipid binding domain of human ApoC-III results in impaired assembly and secretion of triacylglycerol-rich very low density lipoproteins: evidence that ApoC-III plays a major role in the formation of lipid precursors within the microsomal lumen

Hepatic assembly of triacylglycerol (TAG)-rich very low density lipoproteins (VLDL) is achieved through recruitment of bulk TAG (presumably in the form of lipid droplets within the microsomal lumen) into VLDL precursor containing apolipoprotein (apo) B-100. We determined protein/lipid components of lumenal lipid droplets (LLD) in cells expressing recombinant human apoC-III (C3wt) or a mutant form (K58E, C3KE) initially identified in humans that displayed hypotriglyceridemia. Although expression of C3wt markedly stimulated secretion of TAG and apoB-100 as VLDL(1), the K58E mutation (located at the C-terminal lipid binding domain) abolished the effect in transfected McA-RH7777 cells and in apoc3-null mice. Metabolic labeling studies revealed that accumulation of TAG in LLD was decreased (by 50%) in cells expressing C3KE. A Fat Western lipid protein overlay assay showed drastically reduced lipid binding of the mutant protein. Substituting Lys(58) with Arg demonstrated that the positive charge at position 58 is crucial for apoC-III binding to lipid and for promoting TAG secretion. On the other hand, substituting both Lys(58) and Lys(60) with Glu resulted in almost entire elimination of lipid binding and loss of function in promoting TAG secretion. Thus, the lipid binding domain of apoC-III plays a key role in the formation of LLD for hepatic VLDL assembly and secretion.

Ubiquitination regulates the assembly of VLDL in HepG2 cells and is the committing step of the apoB-100 ERAD pathway

Apolipoprotein B-100 (apoB-100) is degraded by endoplasmic reticulum-associated degradation (ERAD) when lipid availability limits assembly of VLDLs. The ubiquitin ligase gp78 and the AAA-ATPase p97 have been implicated in the proteasomal degradation of apoB-100. To study the relationship between ERAD and VLDL assembly, we used small interfering RNA (siRNA) to reduce gp78 expression in HepG2 cells. Reduction of gp78 decreased apoB-100 ubiquitination and cytosolic apoB-ubiquitin conjugates. Radiolabeling studies revealed that gp78 knockdown increased secretion of newly synthesized apoB-100 and, unexpectedly, enhanced VLDL assembly, as the shift in apoB-100 density in gp78-reduced cells was accompanied by increased triacylglycerol (TG) secretion. To explore the mechanisms by which gp78 reduction might enhance VLDL assembly, we compared the effects of gp78 knockdown with those of U0126, a mitogen-activated protein kinase/ERK kinase1/2 inhibitor that enhances apoB-100 secretion in HepG2 cells. U0126 treatment increased secretion of both apoB100 and TG and decreased the ubiquitination and cellular accumu-lation of apoB-100. Furthermore, p97 knockdown caused apoB-100 to accumulate in the cell, but if gp78 was concomitantly reduced or assembly was enhanced by U0126 treatment, cellular apoB-100 returned toward baseline. This indicates that ubiquitination commits apoB-100 to p97-mediated retrotranslocation during ERAD. Thus, decreasing ubiquitination of apoB-100 enhances VLDL assembly, whereas improving apoB-100 lipidation decreases its ubiquitination, suggesting that ubiquitination has a regulatory role in VLDL assembly.

Recent progress in understanding protein and lipid factors affecting hepatic VLDL assembly and secretion

Excess lipid induced metabolic disorders are one of the major existing challenges for the society. Among many different causes of lipid disorders, overproduction and compromised catabolism of triacylglycerol-rich very low density lipoproteins (VLDL) have become increasingly prevalent leading to hyperlipidemia worldwide. This review provides the latest understanding in different aspects of VLDL assembly process, including structure-function relationships within apoB, mutations in APOB causing hypobetalipoproteinemia, significance of modulating microsomal triglyceride-transfer protein activity in VLDL assembly, alterations of VLDL assembly by different fatty acid species, and hepatic proteins involved in vesicular trafficking, and cytosolic lipid droplet metabolism that contribute to VLDL assembly. The role of lipoprotein receptors and exchangeable apolipoproteins that promote or diminish VLDL assembly and secretion is discussed. New understanding on dysregulated insulin signaling as a consequence of excessive triacylglycerol-rich VLDL in the plasma is also presented. It is hoped that a comprehensive view of protein and lipid factors that contribute to molecular and cellular events associated with VLDL assembly and secretion will assist in the identification of pharmaceutical targets to reduce disease complications related to hyperlipidemia.

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.

Surface study of apoB1694-1880, a sequence that can anchor apoB to lipoproteins and make it nonexchangeable

Apolipoprotein B (apoB) is a nonexchangeable apolipoprotein. During lipoprotein assembly, it recruits phospholipids and triacylglycerols (TAG) into TAG-rich lipoprotein particles. It remains bound to secreted lipoproteins during lipid metabolism in plasma. The beta1 region (residues 827-1880) of apoB has a high amphipathic beta strand (AbetaS) content and is proposed to be one region anchoring apoB to lipoproteins. The AbetaS-rich region between apoB37 and apoB41 (residues 1694-1880) was cloned, expressed, and purified. The interfacial properties were studied at the triolein/water (TO/W) and air/water (A/W) interfaces. ApoB[37-41] is surface-active and adsorbs to the TO/W interface. After adsorption the unbound apoB[37-41] was removed from the aqueous phase. Adsorbed apoB[37-41] did not desorb and could not be forced off by increasing the surface pressure up to 23 mN/m. ApoB[37-41] adsorbed on the TO/W interface was completely elastic when compressed and expanded by +/-13% of its area. On an A/W interface, the apoB[37-41] monolayer became solid when compressed to 4 mN/m pressure indicating extended beta-sheet formation. It could be reversibly compressed and expanded between low pressure and its collapse pressure (35 mN/m). Our studies confirm that the AbetaS structure of apoB[37-41] is a lipid-binding motif that can irreversibly anchor apoB to lipoproteins.

Insect lipoprotein biogenesis depends on an amphipathic β cluster in apolipophorin II/I and is stimulated by microsomal triglyceride transfer protein

Lipoproteins transport lipids in the circulation of an evolutionally wide diversity of animals. The pathway for lipoprotein biogenesis has been revealed to a large extent in mammals only, in which apolipoprotein B (apoB) acquires lipids via the assistance of microsomal triglyceride transfer protein (MTP) and binds them by means of amphipathic protein structures. To investigate whether this is a common mechanism for lipoprotein biogenesis in animals, we studied the structural elements involved in the assembly of the insect lipoprotein, lipophorin. LOCATE sequence analysis predicted that the insect lipoprotein precursor, apolipophorin II/I (apoLp-II/I), contains clusters of amphipathic alpha-helices and beta-strands, organized along the protein as N-alpha(1)-beta-alpha(2)-C, reminiscent of a truncated form of apoB. Recombinant expression of a series of C-terminal truncation variants of Locusta migratoria apoLp-II/I in an insect cell (Sf9) expression system revealed that the formation of a buoyant high density lipoprotein requires the amphipathic beta cluster. Coexpression of apoLp-II/I with the MTP homolog of Drosophila melanogaster affected insect lipoprotein biogenesis quantitatively as well as qualitatively, as the secretion of apoLp-II/I proteins was increased several-fold and the buoyant density of the secreted lipoprotein decreased concomitantly, indicative of augmented lipidation. Based on these findings, we propose that, despite specific modifications, the assembly of lipoproteins involves MTP as well as amphipathic structures in the apolipoprotein carrier, both in mammals and insects. Thus, lipoprotein biogenesis in animals appears to rely on structural elements that are of early metazoan origin.

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.

MTP inhibition as a treatment for dyslipidaemias: time to deliver or empty promises?

The development of cholesterol-lowering drugs, including a statins, bile acid sequestrants and cholesterol absorption inhibitors has expanded the options for cardiovascular prevention. Recent treatment guidelines emphasise that individuals at substantial risk for atherosclerotic coronary heart disease should meet defined lipid targets. Combination therapy with drugs that have different and complementary mechanisms of action is often needed to achieve these goals. Existing approaches to the treatment of hypercholesterolaemia are still ineffective in halting the progression of coronary artery disease in some patients despite combination therapies. Other patients are resistant to, or intolerant of, conventional pharmacotherapy and remain at high-risk of atherosclerotic cardiovascular disease, so that alternative approaches are needed. New agents, including inhibitors of microsomal triglyceride transfer protein (MTP), may play a future role, either alone or in combination, in the treatment of hyperlipidaemias. This review focuses on novel approaches to treat dyslipidaemias via the inhibition of MTP. Patients most suitable for use of MTP inhibitors include those with hepatic hypersecretion of apoB, including the metabolic syndrome, Type 2 diabetes mellitus and familial combined hyperlipidaemia, as well as homozygous and heterozygous familial hypercholesterolaemia. However, certain safety issues with these agents need resolving, particularly fatty liver disease.

The assembly of triacylglycerol-rich lipoproteins: an essential role for the microsomal triacylglycerol transfer protein

Raised plasma triacylglycerol is an independent risk factor for cardiovascular disease, and an understanding of factors which regulate the synthesis and degradation of lipoproteins which carry triacylglycerol in the blood may lead to novel approaches to the treatment of hypertriacylglycerolaemia. An active microsomal triacylglycerol transfer protein (MTP) is essential for the assembly of particles which transport triacylglycerol through the circulation. After absorption in the intestine, dietary fat and fat-soluble vitamins are incorporated into chylomicrons in the intestinal epithelial cells, and these lipoproteins reach the bloodstream via the lymphatic system. Patients with the rare genetic disorder, abetalipoproteinaemia, in which MTP activity is absent, present clinically with fat-soluble vitamin and essential fatty acid deficiency, indicating a key role for MTP in the movement of fat into the body. The triacylglycerol-rich lipoprotein found in fasting blood, VLDL, is assembled in the liver by an MTP-dependent process similar to chylomicron assembly, and transports triacylglycerol to extra-hepatic tissues such as adipose tissue and heart. In the absence of MTP activity, VLDL are not synthesized and only extremely low levels of triacylglycerol are present in the blood. Dietary components, including fat, cholesterol and ethanol, can modify the expression of the MTP gene and, hence, MTP activity. The present review summarizes current knowledge of the role of MTP in the assembly and secretion of triacylglycerol-rich lipoproteins, and the regulation of its activity in both animal and cell systems.

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.

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.

Interfacial properties of amphipathic β strand consensus peptides of apolipoprotein B at oil/water interfaces

The region between residues 968 and 1882 of apolipoprotein B (apoB-21 to apoB-41) is rich in amphipathic beta strands (AbetaSs) and promotes the assembly of primordial triacylglyceride (TAG)-rich lipoproteins. To understand the importance of AbetaS in recruiting TAG, the interfacial properties of two AbetaS consensus peptides, P12 and P27, were studied at dodecane/water (DD/W) and triolein/water (TO/W) interfaces. P12 (acetyl-LSLSLNADLRLK-amide) and P27 (acetyl-LSLSLNADLRLKNGNLSLSLNADLRLK-amide), when added into the aqueous phase surrounding a suspended oil drop (dodecane or triolein), decreased the interfacial tension (gamma) in a concentration-dependent manner. At the DD/W interface, 1 x 10(-5) M P12 decreased gamma to approximately 20 mN/m and 6.6 x 10(-6) M P27 decreased gamma to approximately 13 mN/m. At the TO/W interface, 1.5 x 10(-5) M P12 decreased gamma to approximately 14 mN/m and 9.0 x 10(-6) M P27 decreased gamma to approximately 12 mN/m. The surface area of both peptides was between 11.2 and 15.1 angstroms2 per residue, consistent with beta sheets lying flat on DD/W and TO/W interfaces. P12 and P27 are almost purely elastic on DD/W, TO/W, and air/water interfaces. When P12 and P27 were compressed beyond the equilibrium gamma to as low as 4 mN/m, they could not be readily desorbed from either interface. These properties probably help in assembling nascent TAG-rich lipoproteins, and AbetaS may anchor apoB to beta lipoproteins.

Conjugated linoleic acids, atherosclerosis, and hepatic very-low-density lipoprotein metabolism

Conjugated linoleic acids (CLAs) are isomeric forms of the 18:2 fatty acid that contain conjugated sites of unsaturation. Although CLAs are minor components of the diet, they have many reported biological activities. For nearly a decade, the potential for CLA to modify the atherosclerotic process has been examined in animal models, and studies of supplementation of the human diet with CLA were started with the anticipation that such an intervention could also reduce the risk of cardiovascular disease. Central to the hypothesis is the expectation that dietary modification could alter plasma lipid and lipoprotein metabolism toward a more cardioprotective profile. This review examines the evidence in support of the hypothesis and the mechanistic studies that lend support for a role of CLA in hepatic lipid and lipoprotein metabolism. Although there are still limited studies in strong support of a role for CLA in the reduction of early atherosclerotic lesions, there has been considerable progress in defining the mechanisms of CLA action. CLA could primarily modulate the metabolism of fatty acids in the liver. The tools are now available to examine isomer-specific effects of CLA on hepatic lipid and lipoprotein metabolism and the potential of CLA to modify hepatic gene expression patterns. Additional animal and cell culture studies will increase our understanding of these unusual fatty acids and their potential for health benefits in humans.

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.

Assembly and secretion of very low density lipoproteins containing apolipoprotein B48 in transfected McA-RH7777 cells. Lack of evidence that palmitoylation of apolipoprotein B48 is required for lipoprotein secretion

We examined the role of S-linked palmitoylation of human apolipoprotein (apo) B in the assembly and secretion of very low density lipoproteins using recombinant human apoB48. There are four free cysteine residues (Cys(1085), Cys(1396), Cys(1478), and Cys(1635)) within apoB48 that potentially can be palmitoylated. All four cysteine residues were substituted with serine by site-specific mutagenesis. The mutant protein was expressed in transfected rat hepatoma McA-RH7777 cells. Metabolic labeling of the stably transfected cells with iodopalmitic acid analog showed that the mutant apoB48 lacked palmitoylation. The lack of palmitoylation had little impact on the ability of apoB48 to assemble and secrete very low density lipoproteins or high density lipoproteins. Immunocytochemistry experiments using confocal microscopy failed to reveal any major alterations in the intracellular distribution of the mutant apoB48 at steady state. Pulse-chase analysis combined with subcellular fractionation showed no apparent deficiency in the movement of the mutant apoB48 protein from the endoplasmic reticulum to cis/medial Golgi. However, the mutant apoB48 lacking palmitoylation showed retarded movement toward the distal Golgi and increased association (>2-fold) with the membranes of the secretory compartments. A marginal decrease (by 15-20%) in secretion efficiency as compared with that of wild type apoB48 was also observed. These results suggest that lack of palmitoylation may influence the partitioning of apoB48 between microsomal membranes and microsomal lumen, but it does not compromise the ability of apoB48 to assemble lipoproteins.

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.

Amino terminal 38.9% of apolipoprotein B-100 is sufficient to support cholesterol-rich lipoprotein production and atherosclerosis

OBJECTIVE

Carboxyl terminal truncation of apolipoprotein (apo)B-100 and apoB-48 impairs their capacity for triglyceride transport, but the ability of the resultant truncated apoB to transport cholesterol and to support atherosclerosis has not been adequately studied. The atherogenicity of apoB-38.9 was determined in this study by using our apoB-38.9-only (Apob38.9/38.9) mice.

METHODS AND RESULTS

ApoB-38.9-lipoproteins (Lp-B38.9) circulate at very low levels in Apob38.9/38.9 mice as small LDLs or HDLs. Disruption of apoE gene in these mice caused accumulation of large amounts of betaVLDL-like LpB-38.9 in plasma. These betaVLDL particles were more enriched with cholesteryl esters but poor in triglycerides compared with the apoB-48-betaVLDL of the apoB-wild-type/apoE-null (Apob+/+/Apoe-/-) mice. Likewise, apoB-38.9-VLDL secreted by cultured Apob38.9/38.9 mouse hepatocytes also had higher ratios of total cholesterol to triglycerides than apoB-48-VLDL secreted by the apoB-48-only hepatocytes. Thus, despite its impaired triglyceride-transporting capacity, apoB-38.9 has a relatively intact capacity for cholesterol transport. Spontaneous aortic atherosclerotic lesions were examined in apoB-38.9-only/apoE-null (Apob38.9/38.9/Apoe-/-) mice at ages 9 and 13 months. Extensive lesions were found in the Apob38.9/38.9/Apoe-/- mice as well as in their Apob+/38.9/Apoe-/- and Apob+/+/Apoe-/- littermates.

CONCLUSIONS

Deleting the C-terminal 20% from apoB-48 does not impair its ability to transport cholesterol and to support atherosclerosis, thus narrowing the "atherogenic region" of apoB.

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.

Hypobetalipoproteinemic mice with a targeted apolipoprotein (Apo) B-27.6-specifying mutation: in vivo evidence for an important role of amino acids 1254-1744 of ApoB in lipid transport and metabolism of the apoB-containing lipoprotein

Carboxyl-terminal deletion of apoB-100 may impair its triglyceride (TG)-transporting capability and alter its catabolism. Here, we compare our newly generated apoB gene (Apob)-targeted apoB-27.6-bearing mice to our previously reported apoB-38.9 mice to understand further the relationship between the size of a truncated apoB variant and its function/metabolism in vivo. The apoB-27.6-specifying mutation produces a premature stop codon six amino acids (aa) downstream of the last codon of mouse Apob exon 24 (corresponding to aa 1254 of human apoB-100). ApoB-27.6 transcripts were 3- and 5-fold more abundant than apoB wild type and apoB-38.9 transcripts in the liver. Likewise, hepatic secretion rates of apoB-27.6 were 7-fold higher than those of apoB-48 and apoB-38.9. In contrast, apoB-27.6 heterozygotes (Apob(27.6/+)) had lower hepatic TG secretion rates and higher liver TG contents than both apoB-38.9 heterozygotes (Apob(38.9/+)) and apoB wild type mice (Apob(+/+)). ApoB-27.6 was secreted by Apob(27.6/+) hepatocytes as dense high density lipoprotein particles. Moreover, despite its high secretion rates, apoB-27.6 was barely detectable in plasma. Disruption of apoE gene in Apob(38.9/+) and Apob(27.6/+) dramatically increased plasma levels of apoB-38.9 as well as apoB-48 but caused no change in plasma apoB-27.6 concentrations. Finally, the birth rate of apoB-27.6 homozygotes (Apob(27.6/27.6)) from intercrosses of Apob(27.6/+) was 7-fold lower than that of Apob(38.9/38.9) from Apob(38.9/+) intercrosses (1.8% versus 12%). Crossbreeding of Apob(27.6/27.6) and Apob(38.9/38.9) produced viable Apob(27.6/38.9) offspring, but Apob(27.6/27.6) intercrosses produced no offspring. Together, these results demonstrate in vivo that the apoB-27.6-apoB-38.9 peptide segment (aa 1254-1744) plays a critical role, not only in supporting hepatic TG-secretion and in modulating catabolism of apoB-containing lipoproteins, but also in normal mouse embryonic development.

Apolipoprotein B metabolism: tracer kinetics, models, and metabolic studies

The study of apolipoprotein (apo) B metabolism is central to our understanding of lipoprotein metabolism. However, the assembly and secretion of apoB-containing lipoproteins is a complex process. Specialized techniques, developed and applied to in vitro and in vivo studies of apoB metabolism, have provided insights into the mechanisms involved in the regulation of this process. Moreover, these studies have important implications for understanding both the pathophysiology as well as the therapeutic options for the dyslipidemias. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the applications of kinetic analysis and multicompartmental modeling to the study of apoB metabolism. New developments and significant advances over the last decade are discussed.

Lipoprotein secretion and triglyceride stores in the heart

The genes for apolipoprotein B and microsomal triglyceride transfer protein are expressed in mouse and human heart tissue. Why the heart would express these "lipoprotein assembly" genes has been unclear. Here we demonstrate that the beating mouse heart actually secretes spherical lipoproteins. Moreover, increased cardiac production of lipoproteins (e.g., in mice that express a human apolipoprotein B transgene) was associated with increased triglyceride secretion from the heart and decreased stores of triglycerides within the heart. Increased cardiac production of lipoproteins also reduced the pathological accumulation of triglycerides that occurs in the hearts of mice lacking long-chain acyl coenzyme A dehydrogenase. In contrast, blocking heart lipoprotein secretion (e.g., in heart-specific microsomal triglyceride transfer protein knockout mice) increased cardiac triglyceride stores. Thus, heart lipoprotein secretion helps regulate cardiac triglyceride stores and may protect the heart from the detrimental effects of surplus lipids.

Proteolytic degradation and impaired secretion of an apolipoprotein A-I mutant associated with dominantly inherited hypoalphalipoproteinemia

We have devised a combined in vivo, ex vivo, and in vitro approach to elucidate the mechanism(s) responsible for the hypoalphalipoproteinemia in heterozygous carriers of a naturally occurring apolipoprotein A-I (apoA-I) variant (Leu(159) to Arg) known as apoA-I Finland (apoA-I(FIN)). Adenovirus-mediated expression of apoA-I(FIN) decreased apoA-I and high density lipoprotein cholesterol concentrations in both wild-type C57BL/6J mice and in apoA-I-deficient mice expressing native human apoA-I (hapoA-I). Interestingly, apoA-I(FIN) was degraded in the plasma, and the extent of proteolysis correlated with the most significant reductions in murine apoA-I concentrations. ApoA-I(FIN) had impaired activation of lecithin:cholesterol acyltransferase in vitro compared with hapoA-I, but in a mixed lipoprotein preparation consisting of both hapoA-I and apoA-I(FIN) there was only a moderate reduction in the activation of this enzyme. Importantly, secretion of apoA-I was also decreased from primary apoA-I-deficient hepatocytes when hapoA-I was co-expressed with apoA-I(FIN) following infection with recombinant adenoviruses, a condition that mimics secretion in heterozygotes. Thus, this is the first demonstration of an apoA-I point mutation that decreases LCAT activation, impairs hepatocyte secretion of apoA-I, and makes apoA-I susceptible to proteolysis leading to dominantly inherited hypoalphalipoproteinemia.

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.

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.

Specificity of lipid incorporation is determined by sequences in the N-terminal 37 of apoB

The N-terminal 17% of apolipoprotein B (apoB-17) is secreted lipid-poor while apoB-41 particles are secreted with a triacylglycerol (TAG)-rich core. Thus, the sequence between apoB-17 and apoB-41 is necessary for the assembly of TAG-rich lipoproteins. To delineate this region, C127 cells were permanently transfected to secrete the N-terminal 29, 32.5, or 37% of apoB. Density gradient centrifugation showed that secreted apoB-29, apoB-32.5, and apoB-37 had peak densities of 1.25, 1.22, and 1.16 g/mL and percent lipid of particle weights of 30, 37, and 49%, respectively. Calculated anhydrous particle diameters were: apoB-29 = 81 A, apoB-32.5 = 88 A, and apoB-37 = 101 A. Immunoprecipitated particles labeled with [(3)H]oleate showed that, as apoB length increased from apoB-29 to apoB-32.5 and apoB-37, the number of TAG (core) molecules per apoB particle increased almost 16-fold from 8 to 32 to 124, while phospholipids and diacylglycerols (surface lipids) increased only slightly from 71 to 87 to 97 molecules, respectively. Thus, sequences in the C-terminus of apoB-29 bind phospholipids and diacylglycerols, sequences between apoB-29 and apoB-32.5 augment TAG binding and sequences between apoB-32.5 and apoB-41 account for the marked incorporation of TAG at a rate of approximately 1 TAG per 2 amino acids. Cryoelectron micrographs of isolated apoB-37 particles revealed mostly spherical particles of approximately 110 A (11.0 nm) with an electron lucent center, consistent with these particles having a TAG core. We suggest that the predicted amphipathic beta-sheets beginning at apoB-29, starts to preferentially recruit core lipids into apoB and propose that the consistent presence of DAG in the secreted particles may have a role in fission of the nascent lipoprotein particles from the endoplasmic reticulum membrane.

Disulfide bonds are required for folding and secretion of apolipoprotein B regardless of its lipidation state

Apolipoprotein (apo) B-100, an essential protein for the assembly and secretion of very low density lipoproteins depends on lipid binding (lipidation) for its secretion. Seven of its 8 disulfides are clustered within the N-terminal 21%. The role of these disulfides in the secretion of lipidated or unlipidated truncated forms of apoB was studied in C127 cells expressing apoB-17, apoB-29, or apoB-41. These cells do not express microsomal triglyceride transfer protein yet secrete apoB-41 on triacylglycerol-rich lipoproteins while apoB-29 and apoB-17 are secreted with little or no lipid, respectively. Dithiothreitol utilized in pulse-chase studies prevented the cotranslational formation of disulfides and when added posttranslationally reduced native disulfides. As a result, the secretion of reduced apoB forms was blocked and they were retained in the cells. Reduced apoB polypeptides were rescued following removal of dithiothreitol, as they underwent post-translational disulfide bonding, attained their mature form, and were subsequently secreted. Together the data suggest that in C127 cells the formation of native disulfides is critical for the folding and secretion of apoB independent of its length, its requirement for lipidation or microsomal triglyceride transfer protein expression. Therefore, these cells provide an appropriate model to study the folding of apoB in great detail.

Well-defined regions of apolipoprotein B-100 undergo conformational change during its intravascular metabolism

Apolipoprotein B (apoB)-100-containing lipoproteins are secreted from the liver as large triglyceride-rich very low density lipoproteins (VLDLs) into the circulation, where they are transformed, through the action of lipases and plasma lipid transfer proteins, into smaller, less buoyant, cholesteryl ester-rich low density lipoproteins (LDLs). As a consequence of this intravascular metabolism, apoB-containing lipoproteins are heterogeneous in size, in hydrated density, in surface charge, and in lipid and apolipoprotein composition. To identify specific regions of apoB that may undergo conformational changes during the intravascular transformation of VLDLs into LDLs, we have used a panel of 29 well-characterized anti-apoB monoclonal antibodies to determine whether individual apoB epitopes are differentially expressed in VLDL, intermediate density lipoprotein (IDL), and LDL subfractions isolated from 6 normolipidemic subjects. When analyzed in a solid-phase radioimmunoassay, the expression of most epitopes was remarkably similar in VLDLs, IDLs, and LDLs. Two epitopes that are close to the apoB LDL receptor-binding site show an increased expression in large (1.019 to 1.028 g/mL), medium (1.028 to 1.041 g/mL), and small (1.041 to 1.063 g/mL) LDLs compared with VLDLs and IDLs, and 2 epitopes situated between apoB residues 4342 and 4536 are significantly more immunoreactive in small and medium-sized LDLs compared with VLDLs, IDLs, and large LDLs. Therefore, as VLDL is converted to LDL, conformational changes identified by monoclonal antibodies occur at precise points in the metabolic cascade and are limited to well-defined regions of apoB structure. These conformational changes may correspond to alterations in apoB functional activities.

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.

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.

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.

Translocation efficiency, susceptibility to proteasomal degradation, and lipid responsiveness of apolipoprotein B are determined by the presence of beta sheet domains

Apolipoprotein (apo) B100 is an atypical secretory protein in that its translocation across the endoplasmic reticulum membrane is inefficient, resulting in the partial translocation and exposure of apoB100 on the cytoplasmic surface of the endoplasmic reticulum. Cytosolic exposure leads to the association of nascent apoB with heat shock protein 70 and to its predisposition to ubiquitination and proteasomal degradation. The basis for the inefficient translocation of apoB100 remains unclear and controversial. To test the hypothesis that beta sheet domains present in apoB100 contribute to its inefficient translocation, we created human apoB chimeric constructs apoB13,16 and apoB13,13,16, which contain amino-terminal alpha globular domains but no beta sheet domains, and apoB13,16,beta, which has an amphipathic beta sheet domain of apoB100 inserted into apoB13,16. These constructs, along with carboxyl-terminal truncations of apoB100, apoB34 and apoB42, were used to transfect HepG2 and Chinese hamster ovary cells. In contrast to the lack of effect of proteinase K on apoB13,16 and apoB13,13,16, the levels of apoB34, apoB42, and apoB13,16,beta were decreased by 70-85% after proteinase K-induced proteolysis in both HepG2 and Chinese hamster ovary cells. Either oleic acid or proteasomal inhibitors (N-acetyl-leucinyl-leucinyl-norleucinal and lactacystin) significantly increased the cell levels of apoB13,16,beta, apoB34, apoB42, and full-length apoB100 but had no effect on the cell levels of apoB13,16 and apoB13,13,16. When HepG2 cells were incubated with a microsomal triglyceride transfer protein inhibitor, the cellular levels of apoB13,16,beta, apoB34, and apoB42 were decreased by 70-80%, whereas the levels of apoB13,16 and apoB13,13,16 were unaffected. The effects of microsomal triglyceride transfer protein inhibition were reversed by lactacystin. Our results clearly demonstrate that the translocation efficiency, susceptibility to proteasomal degradation, and lipid responsiveness of apoB were determined by the presence of a lipid binding beta sheet domain. It is possible that beta sheet domains may at least transiently facilitate the interaction of apoB with the lipid bilayer surrounding the translocation channel.

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.

The role of factors that regulate the synthesis and secretion of very-low-density lipoprotein by hepatocytes

Lipoproteins are particles that contribute to overall metabolic homeostasis by transporting hydrophobic lipids in the blood plasma to and from different tissues in the body. Very-low-density lipoprotein (VLDL) is the principal vehicle for the transport of endogenous triglyceride (TG), and, ultimately, through its metabolic product, low-density lipoprotein (LDL), of cholesterol as well. It is synthesized mainly in hepatocytes, with small amounts also being produced by enterocytes in the fasting state. The mechanism of VLDL assembly is complex and is regulated at different levels by a variety of factors. The main structural protein of VLDL is called apolipoprotein B-100 (Apo B). Apo B formation and degradation therefore represent two major points of regulation of VLDL secretion. Hepatic levels of lipids such as phosphatidylcholine (PC), cholesteryl ester (CE), fatty acids (FA), and TG also affect VLDL synthesis. There are different views as to the specific mechanism by which each lipid class affects VLDL particle formation. In general, PC appears to promote the translocation of apo B from the cytosol to the lumen of the endoplasmic reticulum, a step that is crucial in the early stages of VLDL assembly. Apo B degradation is suppressed, and therefore VLDL secretion is enhanced, in the presence of elevated CE levels. For TG to be incorporated into the lipoprotein, it requires the action of a protein called microsomal triglyceride transfer protein (MTP). MTP might have a preference for TG comprised of FA with a certain degree of saturation. It becomes apparent that changes in diet that are accompanied by variations in the type of fats that are ingested affect VLDL formation and secretion. Regulation also occurs post-prandially in response to elevations in plasma insulin levels. Acute elevations in insulin inhibit VLDL secretion by promoting the degradation of apo B. This action is consistent with insulin's anabolic properties as it allows for the hepatic storage of lipid rather than for its distribution in VLDL to other tissues for fuel. Many studies have attempted to unravel the mechanisms of VLDL formation and secretion. The fact that so many factors are involved complicates the issue. The purpose of this article is to describe the relationship between different factors involved in VLDL assembly and secretion so that a better understanding of its metabolic regulation may be achieved.

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.

Inhibition of cholesterol esterification by DuP 128 decreases hepatic apolipoprotein B secretion in vivo: effect of dietary fat and cholesterol

To further test the hypothesis that newly synthesized cholesteryl esters regulate hepatic apolipoprotein B (apoB) secretion into plasma, apoB kinetic studies were carried out in seven control miniature pigs and in seven animals after 21 days intravenous administration of the acyl coenzyme A:cholesterol acyltransferase (ACAT) inhibitor DuP 128 (2.2 mg/kg/day). Pigs were fed a fat (34% of calories; polyunsaturated/monounsaturated/saturated ratio, 1:1:1) and cholesterol (400 mg/day; 0.1%; 0.2 mg/kcal) containing pig chow based diet. DuP 128 significantly reduced total plasma triglyceride and very low density lipoprotein (VLDL) triglyceride concentrations by 36 and 31%, respectively (P<0.05). Autologous 131I-VLDL and 125I-LDL were injected simultaneously into each pig and apoB kinetic data was analyzed using multicompartmental analysis (SAAM II). The VLDL apoB pool size decreased by 26% (0.443 vs. 0.599 mg/kg; P<0. 001) which was due entirely to a 28% reduction in VLDL apoB production or secretion rate (1.831 vs. 2.548 mg/kg/h; P=0.006). The fractional catabolic rate (FCR) for VLDL apoB was unchanged. The LDL apoB pool size and production rate were unaffected by DuP 128 treatment. Hepatic microsomal ACAT activity decreased by 51% (0.44 vs. 0.90 nmol/min/mg; P<0.001). Although an increase in hepatic free cholesterol and subsequent decrease in both LDL receptor expression and LDL apoB FCR might be expected, this did not occur. The concentration of hepatic free cholesterol decreased 12% (P=0.008) and the LDL apoB FCR were unaffected by DuP 128 treatment. In addition, DuP 128 treatment did not alter the concentration of hepatic triglyceride or the activity of diacylglycerol acyltransferase, indicating a lack of effect of DuP 128 on hepatic triglyceride metabolism. In our previous studies, DuP 128 treatment of miniature pigs fed a low fat, cholesterol free diet, decreased VLDL apoB secretion by 65% resulting in a reduction in plasma apoB of 60%. We conclude that in miniature pigs fed a high fat, cholesterol containing diet, the inhibition of hepatic cholesteryl ester synthesis by DuP 128 decreases apoB secretion into plasma, but the effect is attenuated relative to a low fat, cholesterol free diet.

The intracellular triacylglycerol/fatty acid cycle: a comparison of its activity in hepatocytes which secrete exclusively apolipoprotein (apo) B100 very-low-density lipoprotein (VLDL) and in those which secrete predominantly apoB48 VLDL

Hamster hepatocytes, like human hepatocytes, secrete triacylglycerol (TAG) as very-low-density lipoprotein (VLDL) in association with apolipoprotein (apo) B100, whereas in the rat, TAG is secreted predominantly in association with apoB48. Nevertheless, in hepatocytes from both species, a minimum of between 60% and 70% [69. 1+/-1.4% (hamster), 60.6+/-2.5% (rat)] of the VLDL TAG was secreted following lipolysis and re-esterification of intracellular TAG. The fractional rates of hepatocellular TAG turnover (lipolysis and re-esterification) were similar in both species [1.83+/-0.28 pools/24 h (hamster), 1.39+/-0.23 pools/24 h (rat)]. Comparison of the relative changes in the 3H and 14C specific radioactivities of the VLDL and cellular TAG, pre-labelled with [3H]glycerol and [4C]oleate, suggested that fatty acids released by lipolysis either were recruited directly into a VLDL assembly pool or were recycled to the cellular pool following re-esterification. Recycling in the hamster was somewhat greater than in the rat (66.1+/-5.7% versus 53. 7+/-4.8% of TAG lipolysed respectively). Similarly, a larger proportion of newly synthesized TAG was retained within the cell, rather than secreted as VLDL, in the hamster compared with the rat (37.9+/-2.8% versus 20+/-3.8%, P<0.01). These factors may have contributed to the somewhat lower rate of VLDL TAG secretion in the hamster hepatocytes compared with those from the rat (43.3+/-4.2 versus 96.4+/-3.4 microg/24 h per mg of cell protein). Rat hepatocytes were more sensitive to inhibition of VLDL secretion by insulin than were those from hamster. In neither case did insulin affect total or fractional TAG turnover. The results suggest that assembly of both apoB100 VLDL and apoB48 VLDL is associated with efficient intracellular TAG lipolysis.