Murine mammary-derived cells secrete the N-terminal 41% of human apolipoprotein B on high density lipoprotein-sized lipoproteins containing a triacylglycerol-rich core

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.

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.

Apolipoprotein B-containing lipoprotein assembly in microsomal triglyceride transfer protein-deficient McA-RH7777 cells

Microsomal triglyceride transfer protein (MTP) is required for the assembly and secretion of apolipoprotein (apo) B-containing lipoproteins. Previously, we demonstrated that the N-terminal 1,000 residues of apoB (apoB:1000) are necessary for the initiation of apoB-containing lipoprotein assembly in rat hepatoma McA-RH7777 cells and that these particles are phospholipid (PL) rich. To determine if the PL transfer activity of MTP is sufficient for the assembly and secretion of primordial apoB:1000-containing lipoproteins, we employed microRNA-based short hairpin RNAs (miR-shRNAs) to silence Mttp gene expression in parental and apoB:1000-expressing McA-RH7777 cells. This approach led to 98% reduction in MTP protein levels in both cell types. Metabolic labeling studies demonstrated a drastic 90-95% decrease in the secretion of rat endogenous apoB100-containing lipoproteins in MTP-deficient McA-RH7777 cells compared with cells transfected with negative control miR-shRNA. A similar reduction was observed in the secretion of rat endogenous apoB48 under the experimental conditions employed. In contrast, MTP absence had no significant effect on the synthesis, lipidation, and secretion of human apoB:1000-containing particles. These results provide strong evidence in support of the concept that in McA-RH7777 cells, acquisition of PL by apoB:1000 and initiation of apoB-containing lipoprotein assembly, a process distinct from the conventional first-step assembly of HDL-sized apoB-containing particles, do not require MTP. This study indicates that, in hepatocytes, a factor(s) other than MTP mediates the formation of the PL-rich primordial apoB:1000-containing initiation complex.

Reconstituting initial events during the assembly of apolipoprotein B-containing lipoproteins in a cell-free system

The synthesis of apolipoprotein B (apoB) dictates the formation of chylomicrons and very low-density lipoproteins, two major lipoprotein precursors in the human plasma. Despite its biological significance, the mechanism of the assembly of these apoB-containing lipoproteins remains elusive. An essential obstacle is the lack of systems that allow fine dissection of key components during assembly, including nascent apoB peptide, lipids in defined forms, chaperones, and microsomal triglyceride transfer protein (MTP). In this study, we used a prokaryotic cell-free expression system to reconstitute early events in the assembly of apoB-containing lipoprotein that involve the N-terminal domains of apoB. Our study shows that N-terminal domains larger than 20.5% of apoB (B20.5) have an intrinsic ability to remodel vesicular phospholipid bilayers into discrete protein-lipid complexes. The presence of appropriate lipid substrates during apoB translation plays a pivotal role for successful lipid recruitment, and similar lipid recruitment fails to occur if the lipids are added posttranslationally. Cotranslational presence of MTP can dramatically promote the folding of B6.4-20.5 and B6.4-22. Furthermore, apoB translated in the presence of MTP retains its phospholipid recruitment capability posttranslationally. Our data suggest that during the synthesis of apoB, the N-terminal domain has a short window for intrinsic phospholipid recruitment, the time frame of which is predetermined by the environment where apoB synthesis occurs. The presence of MTP prolongs this window of time by acting as a chaperone. The absence of either proper lipid substrate or MTP may result in the improper folding of apoB and, consequently, its degradation.

The N-terminal domain of apolipoprotein B-100: structural characterization by homology modeling

Background

Apolipoprotein B-100 (apo B-100) stands as one of the largest proteins in humans. Its large size of 4536 amino acids hampers the production of X-ray diffraction quality crystals and hinders in-solution NMR analysis, and thus necessitates a domain-based approach for the structural characterization of the multi-domain full-length apo B.

Results

The structure of apo B-17 (the N-terminal 17% of apolipoprotein B-100) was predicted by homology modeling based on the structure of the N-terminal domain of lipovitellin (LV), a protein that shares not only sequence similarity with B17, but also a functional aspect of lipid binding and transport. The model structure was first induced to accommodate the six disulfide bonds found in that region, and then optimized using simulated annealing.

Conclusion

The content of secondary structural elements in this model structure correlates well with the reported data from other biophysical probes. The overall topology of the model conforms with the structural outline corresponding to the apo B-17 domain as seen in the EM representation of the complete LDL structure.

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.

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.

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.

2000 George Lyman Duff Memorial Lecture: atherosclerosis is a liver disease of the heart

The production of apolipoprotein B (apoB)-containing lipoproteins by the liver is regulated by a complex series of processes involving apoB being cotranslationally translocated across the endoplasmic reticulum and assembled into a lipoprotein particle. The translocation of apoB across the endoplasmic reticulum is facilitated by the intraluminal chaperone, microsomal triglyceride transfer protein (MTP). MTP facilitates the translocation and folding of apoB, as well as the addition of lipid to lipid-binding domains (which consist of amphipathic beta sheets and alpha helices). In the absence of MTP or sufficient lipid, apoB exhibits translocation arrest. Thus, apoB translation, translocation, and assembly with lipids to form a core-containing lipoprotein particle occur as concerted processes. Abrogation of >/=1 of these processes diverts apoB into a degradation pathway that is dependent on conjugation with ubiquitin and proteolysis by the proteasome. The nascent core-containing lipoprotein particle that forms within the lumen of the endoplasmic reticulum can be "enlarged" to form a mature very low density lipoprotein particle. Additional studies show that the assembly and secretion of apoB-containing lipoproteins are linked to the cholesterol/bile acid synthetic pathway controlled by cholesterol 7alpha-hydroxylase. Studies in cultured cells and transgenic mice indicate that the expression of cholesterol 7alpha-hydroxylase indirectly regulates the expression of lipogenic enzymes through changes in the cellular content of mature sterol response element binding proteins. Oxysterols and bile acids may also act via the ligand-activated nuclear receptors LXR and FXR to link the metabolic pathways controlling energy balance and lipid metabolism to nutritional state.

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

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

Translocational status of ApoB in the presence of an inhibitor of microsomal triglyceride transfer protein

Despite numerous studies demonstrating that microsomal triglyceride transfer protein (MTP) activity is critical to apoB secretion, there is still controversy as to whether MTP directly facilitates the translocation of apoB across the membrane of the endoplasmic reticulum (ER) through either the recruitment of lipids and/or chaperone activity. In the present study, a specific inhibitor of MTP (BMS 197636) was utilized in HepG2 cells to investigate whether a direct relationship exists between the translocation of apoB across the ER membrane and the lipid-transferring activity of MTP. Inhibition of MTP (with 10 and 50 nmol/L of the inhibitor) did not significantly affect the translocation of newly synthesized apoB (P = 0.77) or the translocational efficiency of the steady-state apoB mass (P = 0.45), despite a 49% decrease in apoB secretion and increased proteosomal degradation. These results compared well with subcellular fractionation experiments which showed no significant change in the fraction of apoB accumulated in the lumen of isolated microsomes in MTP-treated cells (P = 0.35). In summary, MTP lipid transfer activity does not appear to influence translocational status of apoB, but its inhibition is associated with an increased susceptibility to proteasome-mediated degradation and reduced assembly and secretion of apoB lipoprotein particles.

Progress towards understanding the role of microsomal triglyceride transfer protein in apolipoprotein-B lipoprotein assembly

The microsomal triglyceride transfer protein (MTP) is necessary for the proper assembly of the apolipoprotein B containing lipoproteins, very low density lipoprotein and chylomicrons. Recent research has significantly advanced our understanding of the role of MTP in these pathways at the molecular and cellular level. Biochemical studies suggest that initiation of lipidation of the nascent apolipoprotein B polypeptide may occur through a direct association with MTP. This early lipidation may be required to allow the nascent polypeptide to fold properly and therefore avoid ubiquitination and degradation. Concerning the addition of core neutral lipids in the later stages of lipoprotein assembly, cell culture studies show that MTP lipid transfer activity is not required for this to occur for apolipoprotein B-100 containing lipoproteins. Likewise, MTP does not appear to directly mediate addition of core neutral lipid to nascent apoB-48 particles. However, new data indicate that MTP is required to produce triglyceride rich droplets in the smooth endoplasmic reticulum which may supply the core lipids for conversion of nascent, dense apoB-48 particles to mature VLDL. In addition, assembly of dense apolipoprotein B-48 containing lipoproteins has been observed in mouse liver in the absence of MTP. As a result of these new data, an updated model for the role of MTP in lipoprotein assembly is proposed.

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.

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.

Cell and molecular biology of the assembly and secretion of apolipoprotein B-containing lipoproteins by the liver

Triglycerides are one of the most efficient storage forms of free energy. Because of their insolubility in biological fluids, their transport between cells and tissues requires that they be assembled into lipoprotein particles. Genetic disruption of the lipoprotein assembly/secretion pathway leads to several human disorders associated with malnutrition and developmental abnormalities. In contrast, patients displaying inappropriately high rates of lipoprotein production display increased risk for the development of atherosclerotic cardiovascular disease. Insights provided by diverse experimental approaches describe an elegant biological adaptation of basic chemical interactions required to overcome the thermodynamic dilemma of producing a stable emulsion vehicle for the transport and tissue targeting of triglycerides. The mammalian lipoprotein assembly/secretion pathway shows an absolute requirement for: (1) the unique amphipathic protein: apolipoprotein B, in a form that is sufficiently large to assemble a lipoprotein particle containing a neutral lipid core; and, (2) a lipid transfer protein (microsomal triglyceride transfer protein-MTP). In the endoplasmic reticulum apolipoprotein B has two distinct metabolic fates: (1) entrance into the lipoprotein assembly pathway within the lumen of the endoplasmic reticulum; or, (2) degradation in the cytoplasm by the ubiquitin-dependent proteasome. The destiny of apolipoprotein B is determined by the relative availability of individual lipids and level of expression of MTP. The dynamically varied expression of cholesterol-7alpha-hydroxylase indirectly influences the rate of lipid biosynthesis and the assembly and secretion lipoprotein particles by the liver.

Assembly and secretion of chylomicrons by differentiated Caco-2 cells. Nascent triglycerides and preformed phospholipids are preferentially used for lipoprotein assembly

To develop a cell culture model for chyclomicron (CM) assembly, the apical media of differentiated Caco-2 cells were supplemented with oleic acid (OA) together with either albumin or taurocholate (TC). The basolateral media were subjected to sequential density gradient ultracentrifugations to obtain large CM, small CM, and very low density lipoproteins (VLDL), and the distribution of apoB in these fractions was quantified. In the absence of OA, apoB was secreted as VLDL/LDL size particles. Addition of OA (>/=0.8 mM) with TC, but not with albumin, resulted in the secretion of one-third of apoB as CM. Lipid analysis revealed that half of the secreted phospholipids (PL) and triglycerides (TG) were associated with CM. In CM, TG were 7-11-fold higher than PL indicating that CM were TG-rich particles. Secreted CM contained apoB100, apoB48, and other apolipoproteins. Secretion of large CM was specifically inhibited by Pluronic L81, a detergent known to inhibit CM secretion in animals. These studies demonstrate that differentiated Caco-2 cells assemble and secrete CM in a manner similar to enterocytes in vivo. Next, experiments were performed to identify the sources of lipids used for lipoprotein assembly. Cells were labeled with [3H]glycerol for 12 h, washed, and supplemented with OA, TC, and [14C] glycerol for various times to induce CM assembly and to radiolabel nascent lipids. TG and PL were extracted from cells and media and the association of preformed and nascent lipids with lipoproteins was determined. All the lipoproteins contained higher amounts of preformed PL compared with nascent PL. VLDL contained equal amounts of nascent and preformed TG, whereas CM contained higher amounts of nascent TG even when nascent TG constituted a small fraction of the total cellular pool. These studies indicate that nascent TG and preformed PL are preferentially used for CM assembly and provide a molecular explanation for the in vivo observations that the fatty acid composition of TG, but not PL, of secreted CM reflects the composition of dietary fat. It is proposed that in the intestinal cells the preformed PL from the endoplasmic reticulum bud off with apoB as primordial particles and the assembly of larger lipoproteins is dependent on the synthesis and delivery of nascent TG to these particles.

Analysis of the role of microsomal triglyceride transfer protein in the liver of tissue-specific knockout mice

A deficiency in microsomal triglyceride transfer protein (MTP) causes the human lipoprotein deficiency syndrome abetalipoproteinemia. However, the role of MTP in the assembly and secretion of VLDL in the liver is not precisely understood. It is not clear, for instance, whether MTP is required to move the bulk of triglycerides into the lumen of the endoplasmic reticulum (ER) during the assembly of VLDL particles. To define MTP's role in hepatic lipoprotein assembly, we recently knocked out the mouse MTP gene (Mttp). Unfortunately, achieving our objective was thwarted by a lethal embryonic phenotype. In this study, we produced mice harboring a "floxed" Mttp allele and then used Cre-mediated recombination to generate liver-specific Mttp knockout mice. Inactivating the Mttp gene in the liver caused a striking reduction in VLDL triglycerides and large reductions in both VLDL/LDL and HDL cholesterol levels. The Mttp inactivation lowered apo B-100 levels in the plasma by >95% but reduced plasma apo B-48 levels by only approximately 20%. Histologic studies in liver-specific knockout mice revealed moderate hepatic steatosis. Ultrastructural studies of wild-type mouse livers revealed numerous VLDL-sized lipid-staining particles within membrane-bound compartments of the secretory pathway (ER and Golgi apparatus) and few cytosolic lipid droplets. In contrast, VLDL-sized lipid-staining particles were not observed in MTP-deficient hepatocytes, either in the ER or in the Golgi apparatus, and there were numerous cytosolic fat droplets. We conclude that MTP is essential for transferring the bulk of triglycerides into the lumen of the ER for VLDL assembly and is required for the secretion of apo B-100 from the liver.

Chinese hamster ovary cells require the coexpression of microsomal triglyceride transfer protein and cholesterol 7alpha-hydroxylase for the assembly and secretion of apolipoprotein B-containing lipoproteins

Due to the absence of microsomal triglyceride transfer protein (MTP), Chinese hamster ovary (CHO) cells lack the ability to translocate apoB into the lumen of the endoplasmic reticulum, causing apoB to be rapidly degraded by an N-acetyl-leucyl-leucyl-norleucinal-inhibitable process. The goal of this study was to examine if expression of MTP, whose genetic deletion is responsible for the human recessive disorder abetalipoproteinemia, would recapitulate the lipoprotein assembly pathway in CHO cells. Unexpectedly, expression of MTP mRNA and protein in CHO cells did not allow apoB-containing lipoproteins to be assembled and secreted by CHO cells expressing apoB53. Although expression of MTP in cells allowed apoB to completely enter the endoplasmic reticulum, it was degraded by a proteolytic process that was inhibited by dithiothreitol (1 mM) and chloroquine (100 microM), but resistant to N-acetyl-leucyl-leucyl-norleucinal. In marked contrast, coexpression of the liver-specific gene product cholesterol 7alpha-hydroxylase with MTP resulted in levels of MTP lipid transfer activity that were similar to those in mouse liver and allowed intact apoB53 to be secreted as a lipoprotein particle. These data suggest that, although MTP-facilitated lipid transport is not required for apoB translocation, it is required for the secretion of apoB-containing lipoproteins. We propose that, in CHO cells, MTP plays two roles in the assembly and secretion of apoB-containing lipoproteins: 1) it acts as a chaperone that facilitates apoB53 translocation, and 2) its lipid transfer activity allows apoB-containing lipoproteins to be assembled and secreted. Our results suggest that the phenotype of the cell (e.g. expression of cholesterol 7alpha-hydroxylase by the liver) may profoundly influence the metabolic relationships determining how apoB is processed into lipoproteins and/or degraded.

Multiple molecular chaperones interact with apolipoprotein B during its maturation. The network of endoplasmic reticulum-resident chaperones (ERp72, GRP94, calreticulin, and BiP) interacts with apolipoprotein b regardless of its lipidation state

The present study was undertaken to identify and characterize molecular chaperones that assist in the folding of apolipoprotein (apo) B, a secretory protein that requires assembly with lipids (lipidation) for its secretion. Both HepG2 cells, normally secreting full-length apoB (apoB-100), and C127 cells transfected to secrete truncated forms of apoB, apoB-41, apoB-29, and apoB-17, respectively, were employed. C127 cells were used to determine whether chaperone binding is dependent on apoB lipidation as they secrete both unlipidated and lipidated apoB forms despite their lack of microsomal triglyceride transfer protein (MTP), which mediates lipidation of apoB in HepG2 cells. The endoplasmic reticulum (ER)-resident molecular chaperones GRP94, calreticulin, and ERp72 were co-immunoprecipitated with apoB-100 from HepG2 cell lysates following cross-linking of proteins in living cells. The same chaperones including BiP/GRP78 were also associated with all truncated forms of apoB. Sequential immunoprecipitation with antibodies to MTP and apoB revealed the presence of ternary complexes containing apoB-100, MTP, and ERp72. However, MTP is not obligatory for the binding of ERp72 as it was associated with all truncated forms of apoB in C127 cells that lack MTP. The interactions between apoB-100 and ERp72 or GRP94 persisted for at least 2 h following a 30-min pulse. Thus, BiP/GRP78, calreticulin, ERp72, and GRP94 may participate in critical steps in the folding of apoB before any substantial lipidation occurs. ERp72 and GRP94 may also mediate the folding of more advanced folding intermediates and/or target the misfolded underlipidated pool of apoB for degradation.

Apo B100-containing lipoproteins are secreted by the heart

The apo B gene is expressed in the human heart and in the hearts of human apo B transgenic mice generated with large genomic clones spanning the human apo B gene. [35S]Methionine metabolic labeling experiments demonstrated that apo B100-containing lipoproteins are secreted by human heart tissue and by human apo B transgenic and nontransgenic mouse heart tissue. Density gradient analysis revealed that most of the secreted heart lipoproteins were LDLs, even when the labeling experiments were performed in the presence of tetrahydrolipstatin, an inhibitor of lipoprotein lipase. Western blots with a microsomal triglyceride transfer protein) (MTP)-specific antiserum demonstrated that the microsomes of the heart contain the 97-kD subunit of MTP (the subunit involved in the transfer of lipids and assembly of lipoproteins). Metabolic labeling of mouse heart tissue in the presence of BMS-192951, an MTP inhibitor, abolished lipoprotein secretion by the heart but resulted in the secretion of two apo B proteolytic fragments (80 and 120 kD), which were found in the bottom fraction of the density gradient. These studies reveal that the heart, and not just the liver and intestine, secretes apo B-containing lipoproteins. We speculate that lipoprotein secretion by the heart represents a mechanism for removing excess lipids from the heart.

Assembly and secretion of VLDL in nondifferentiated Caco-2 cells stably transfected with human recombinant ApoB48 cDNA

Intestinal cells secrete apoB48-containing very low density lipoproteins (VLDLs) and chylomicrons for the transport of biliary and dietary lipids. The molecular mechanisms regulating the assembly of intestinal lipoproteins are not known due to a lack of reliable and specific cell culture models. Caco-2 (a human colon carcinoma) cells have been used to study intestinal lipid metabolism. These cells have been shown to secrete both apoB100- and apoB48-containing triglyceride (TG)-rich lipoproteins only after differentiation into enterocyte-like cells. To study lipoprotein assembly in nondifferentiated Caco-2 cells, we stably expressed human recombinant apoB48 cDNA under the control of a constitutive cytomegalovirus promoter. Pulse-chase analysis revealed that the majority (> 50%) of apoB48 synthesized was degraded intracellularly in the presence or absence of oleic acid. Transfected nondifferentiated cells secreted lipoproteins with flotation densities similar to those of plasma HDL or LDL when cultured in serum-free or serum-containing media, respectively. Incubation of cells with media containing serum and oleic acid resulted in the secretion of VLDL-like particles. Secretion of VLDL was inhibited (> 80%) by triacsin C due to > 60% inhibition of oleate-induced TG synthesis. However, inhibition of cholesteryl ester synthesis by 70% with an acyl coenzyme A:cholesterol acyltransferase inhibitor did not affect VLDL secretion. Efficient assembly of lipoproteins usually requires the microsomal TG transfer protein (MTP). The presence of MTP in transfected Caco-2 cells was investigated by measuring TG transfer activity in microsomal fractions. Microsomal fractions had 0.2% TG transfer activity per hour per microgram of protein, which corresponds to 30% to 60% of the MTP activity present in liver-derived cells. To determine whether MTP activity was required for lipoprotein assembly, transfected cells were incubated in the presence of the MTP inhibitor CP-10,447. This compound completely abolished the secretion of apoB. These data show that the transfected cell lines secrete lipoproteins of different densities under different culture conditions and that the assembly of larger VLDL particles requires active TG synthesis and MTP activity. Thus, in nondifferentiated Caco-2 cells, the amount of apoB secreted and not the MTP activity is the limiting factor for lipoprotein assembly.

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.

Folding of the amino-terminal domain of apolipoprotein B initiates microsomal triglyceride transfer protein-dependent lipid transfer to nascent very low density lipoprotein

The initial assembly of apolipoprotein B100 (apoB) into lipoprotein particles occurs cotranslationally. To examine steps required to initiate this process, the intracellular folding and assembly of the amino-terminal 28% of apoB (apoB28) was examined using several criteria including nonreducing gel electrophoresis, sensitivity to dithiothreitol (DTT)-mediated reduction, and buoyant density gradient centrifugation. In hepatoma cells, after a 1-min pulse with radiolabeled amino acids, labeled apoB28 migrated during gel electrophoresis in the folded position and was resistant to reduction in vivo with 2 mM DTT. A similar rate and extent of folding was observed in Chinese hamster ovary cells, a microsomal triglyceride transfer protein (MTP)-negative cell line that can neither lipidate nor efficiently secrete apoB28. Amino-terminal folding of apoB28 was essential for its subsequent intracellular lipidation as apoB28 synthesized in hepatoma cells under reducing conditions remained lipid poor (d > 1.25 g/ml) and was retained intracellularly. Upon DTT removal, reduced apoB28 underwent a process of rapid (t1/2 approximately 2 min) post-translational folding followed by a slower process of MTP-dependent lipidation. As with the cotranslational assembly pathway, post-translational lipidation of apoB28 displayed a strict dependence upon amino-terminal folding. We conclude that: 1) folding of the amino-terminal disulfide bonded domain of apoB is achieved prior to the completion of translation and is independent of MTP and events associated with buoyant lipoprotein formation and 2) domain-specific folding of apoBs amino-terminal region is required to initiate MTP-dependent lipid transfer to nascent apoB in the hepatic endoplasmic reticulum.

In vitro reconstitution of assembly of apolipoprotein B48-containing lipoproteins

Human apolipoprotein B48 (apoB48) and apoB15 (the NH2-terminal 48 and 15% of apoB100, respectively) were translated in vitro from their respective mRNAs using a rabbit reticulocyte lysate and microsomes derived from rat liver or dog pancreas. Synthesis of phosphatidylcholine and triacylglycerols was reconstituted in freshly isolated microsomes by the addition of precursors of these glycerolipids (acylcoenzyme A, glycerol 3-phosphate, and CDP-choline) before, during, or after translation. Assembly of apoB15 and apoB48 with newly synthesized phospholipids and triacylglycerols was favored by active, co-translational lipid synthesis. Moreover, translocation of apoB48 but not B15 into the microsomal lumen was increased in the presence of co-translational lipid synthesis. When apoB48 was translated in vitro, approximately 50% of apoB48 was buoyant at a density of <1.10 g/ml in the lumen of liver microsomes only when lipid synthesis was reconstituted during translation. Microsomal triacylglycerol transfer protein has been proposed to be essential for lipidation and/or translocation of apoB48. However, apoB48 was translocated into the lumen of dog pancreas microsomes in which the activity of the microsomal triacylglycerol transfer protein was not detectable. These data indicate that (i) apoB15 and apoB48 bind newly synthesized phosphatidylcholine during translocation; (ii) apoB48 but not apoB15 associates co-translationally with triacylglycerols; (iii) translocation of apoB48 but not apoB15 is stimulated by lipid synthesis; (iv) assembly of buoyant apoB48-containing lipoproteins can be reconstituted in vitro in the presence of active lipid synthesis; and (v) even in microsomes lacking microsomal triacylglycerol transfer protein activity, apoB48 is translocated into the lumen.