Secretion of apolipoprotein B-containing lipoproteins from HeLa cells is dependent on expression of the microsomal triglyceride transfer protein and is regulated by lipid availability

Microsomal triglyceride transfer protein promotes the secretion of Xenopus laevis vitellogenin A1

Vitellogenins (Vtg) are ancient lipid transport and storage proteins and members of the large lipid transfer protein (LLTP) gene family, which includes insect apolipophorin II/I, apolipoprotein B (apoB), and the microsomal triglyceride transfer protein (MTP). Lipidation of Vtg occurs at its site of synthesis in vertebrate liver, insect fat body, and nematode intestine; however, the mechanism of Vtg lipid acquisition is unknown. To explore whether Vtg biogenesis requires the apoB cofactor and LLTP family member, MTP, Vtg was expressed in COS cells with and without coexpression of the 97-kDa subunit of human MTP. Expression of Vtg alone gave rise to a approximately 220-kDa apoprotein, which was predominantly confined to an intracellular location. Coexpression of Vtg with human MTP enhanced Vtg secretion by 5-fold, without dramatically affecting its intracellular stability. A comparison of wild type and a triglyceride transfer-defective form of MTP revealed that both were capable of promoting Vtg secretion, whereas only wild type MTP could promote the secretion of apoB41 (amino-terminal 41% of apoB). These studies demonstrate that the biogenesis of Vtg is MTP-dependent and that MTP is the likely ancestral member of the LLTP gene family.

Retina expresses microsomal triglyceride transfer protein: implications for age-related maculopathy

The principal extracellular lesions of age-related maculopathy (ARM), the leading cause of vision loss in the elderly, involve Bruch's membrane (BrM), a thin vascular intima between the retinal pigment epithelium (RPE) and its blood supply. With age, 80-100 nm solid particles containing esterified cholesterol (EC) accumulate in normal BrM, and apolipoprotein B (apoB) immunoreactivity is detectable in BrM- and ARM-associated lesions. Yet little evidence indicates that increased plasma cholesterol is a risk factor for ARM. To determine if RPE is capable of assembling its own apoB-containing lipoprotein, we examined RPE for the expression of microsomal triglyceride transfer protein (MTP), which is required for this process. Consistent with previous evidence for apoB expression, MTP is expressed in RPE, the ARPE-19 cell line, and, unexpectedly, retinal ganglion cells, which are neurons of the central nervous system. De novo synthesis and secretion of neutral lipid by ARPE-19 was supported by high levels of radiolabeled EC and triglyceride in medium after supplementation with oleate. Lipoprotein assembly and secretion is implicated as a constitutive retinal function and a plausible candidate mechanism involved in forming extracellular cholesterol-containing lesions in ARM. The pigmentary retinopathy and neuropathy of abetalipoproteinemia (Mendelian Inheritance of Man 200100; Bassen-Kornzwieg disease), which is caused by mutations in the MTP gene, may involve loss of function at the retina.

Identification of microsomal triglyceride transfer protein in intestinal brush-border membrane

Microsomal triglyceride transfer protein (MTP) is a heterodimeric complex consisting of a unique large 97-kDa protein and the multifunctional 58-kDa protein disulfide isomerase (PDI). It plays an essential role in the assembly of lipoproteins by shuttling lipids between phospholipid membranes. Based on cell fractionation, early studies have suggested the endoplasmic reticulum (ER) as the exclusive site of MTP. Focusing on the plasma membrane in this study, our attempts with immunoelectron microscopy and specific antibodies surprisingly revealed that labeling was not exclusively confined to the microsomes of rat absorptive cells. Immunogold labeling was also detected over the microvillus membrane of enterocytes. Western blot analysis and biochemical activity measurement confirmed MTP protein expression in brush-border membrane vesicles (BBMV) isolated from the intestinal epithelial cells of various species. Furthermore, MTP was coexpressed in microvilli membrane with PDI that is crucial to maintain the structure and activity of the MTP complex. The treatment of Caco-2 cells with nocodazole and colchicine blocked the appearance of MTP in the apical membrane. Similarly, the addition of BMS-197636, a known inhibitor of MTP transfer activity, suppressed the latter. In conclusion, the present studies suggest that MTP is present in the brush-border membrane of the enterocyte. Understanding the possible physiological role of MTP in this location may reveal additional functions.

The microsomal triglyceride transfer protein gene-493T variant lowers cholesterol but increases the risk of coronary heart disease

BACKGROUND

The microsomal triglyceride transfer protein (MTP) transfers lipids into apolipoprotein B-containing lipoproteins for secretion from liver, intestine, and heart. The T-variant of a functional polymorphism in the MTP promoter, MTP-493G/T, has been associated with reduced low-density lipoprotein cholesterol concentrations. We hypothesize that this polymorphism impacts on coronary heart disease (CHD) risk.

METHODS AND RESULTS

The effect of the polymorphism was therefore tested in the West of Scotland Coronary Prevention Study biobank (580 cases and 1160 controls). MTP-493T carrier status was associated with significantly increased risk of CHD despite a small reduction in total cholesterol. Compared with the genotypic group with the lowest event rate (MTP-493GG, pravastatin treatment), the respective odds ratios (95% confidence interval) in the placebo group for CHD events were: GG, 1.23 (0.92 to 1.63); GT, 1.53 (1.12 to 2.08); and TT, 2.78 (1.53 to 5.05), suggestive of a gene-dose effect. The excess risk for CHD of the MTP-493T-variant was eliminated by pravastatin treatment. The Uppsala Longitudinal Study of Adult Men (ULSAM), which is a 20-year follow-up study of CHD, was used as an independent confirmatory database. These unexpected findings prompted the investigation of non-plasma lipid factors that could associate the MTP gene with CHD risk. In a limited number of subjects (n=18), heart muscle biopsies showed a MTP-493T genotype-specific depression of MTP mRNA expression.

CONCLUSIONS

The MTP-493T variant confers an increased risk of CHD that is unrelated to plasma lipids and lipoproteins, but eliminated by pravastatin treatment. A direct effect of the MTP polymorphism on myocardial lipid metabolism and vulnerability upon ischemic damage cannot be excluded.

A simple, rapid, and sensitive fluorescence assay for microsomal triglyceride transfer protein

Microsomal triglyceride transfer protein (MTP) is critical for the assembly and secretion of apolipoprotein B (apoB) lipoproteins. Its activity is classically measured by incubating purified MTP or cellular homogenates with donor vesicles containing radiolabeled lipids, precipitating the donor vesicles, and measuring the radioactivity transferred to acceptor vesicles. Here, we describe a simple, rapid, and sensitive fluorescence assay for MTP. In this assay, purified MTP or cellular homogenates are incubated with small unilamellar donor vesicles containing quenched fluorescent lipids (triacylglycerols, cholesteryl esters, and phospholipids) and different types of acceptor vesicles made up of phosphatidylcholine or phosphatidylcholine and triacylglycerols. Increases in fluorescence attributable to MTP-mediated lipid transfer are measured after 30 min. MTP's lipid transfer activity could be assayed using apoB lipoproteins but not with high density lipoproteins as acceptors. The assay was used to measure MTP activity in cell and tissue homogenates. Furthermore, the assay was useful in studying the inhibition of the cellular as well as purified MTP by its antagonists. This new method is amenable to automation and can be easily adopted for large-scale, high-throughput screening.

A role for palmitoylation in the quality control, assembly and secretion of apolipoprotein B

ApoB (apolipoprotein B)-containing lipoprotein particles, such as chylomicrons, very-low-density and low-density lipoprotein particles, transport triacylglycerol and cholesteryl esters in the bloodstream. A palmitoylation site was previously mapped to Cys-1085 in a functional truncated apoB variant (apoB-29) and abolished by mutagenesis. This Cys-1085Ser mutation resulted in secretion of smaller and denser lipoprotein particles containing 80% less cholesteryl ester and triacylglycerol than wild-type controls. We show that palmitoylation of apoB-29 occurs in the ER (endoplasmic reticulum), stimulates the ER-Golgi transport rate of apoB-29 almost 2-fold, doubles the secretion efficiency of wild-type apoB-29 in comparison with (Cys-1085Ser)apoB-29 and reduces significantly the association of wild-type apoB-29 with calnexin in comparison with (Cys-1085Ser)apoB-29. While non-palmitoylated apoB-29 co-localized extensively with constitutively secreted transferrin, wild-type apoB-29 did so only partially and was enriched in ER extensions. Our results suggest that palmitoylation of apoB regulates the biogenesis of nascent apoB-containing lipoprotein particles by concentrating apoB in a specialized ER compartment and by stimulating dissociation from constituents of the ER quality-control machinery. This reduced interaction would lead to a faster ER-Golgi transit time and a higher secretion efficiency of wild-type apoB-29. Palmitoylation could regulate the amount of apoB available for secretion of neutral lipids.

The low density lipoprotein receptor prevents secretion of dense apoB100-containing lipoproteins from the liver

The assembly and secretion of very low density lipoproteins (VLDL) require microsomal triglyceride transfer protein (MTP). Recent evidence also suggests a role for the low density lipoprotein (LDL) receptor in this process. However, the relative importance of MTP in the two steps of VLDL assembly and the specific role of the LDL receptor still remain unclear. To further investigate the role of MTP and the LDL receptor in VLDL assembly, we bred mice harboring "floxed" Mttp alleles (Mttpflox/flox) and a Cre transgene on a low-density lipoprotein receptor-deficient background to generate mice with double deficiency in the liver (Ldlr-/- MttpDelta/Delta). In contrast to the plasma of Ldlr+/+ MttpDelta/Delta mice, the plasma of Ldlr-/- MttpDelta/Delta mice contained apoB100. Accordingly, Ldlr-/- MttpDelta/Delta but not Ldlr+/+ MttpDelta/Delta hepatocytes secreted apoB100-containing lipoprotein particles. The secreted lipoproteins were of LDL and HDL sizes but no VLDL-sized lipoproteins could be detected. These findings indicate that hepatic LDL receptors function as "gatekeepers" targeting dense apoB100-containing lipoproteins for degradation. In addition, these results suggest that very low levels of MTP are insufficient to mediate the second step but sufficient for the first step of VLDL assembly.

Subcellular localization of microsomal triglyceride transfer protein

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

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

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

Ileal adenocarcinoma in a mild phenotype of abetalipoproteinemia

Abetalipoproteinemia (ABL) is a rare autosomal recessive disorder that is characterized by defective assembly and secretion of plasma apolipoprotein (apo) B-containing lipoproteins. This disorder results from mutations in the MTP gene encoding the microsomal triglyceride transfer protein. We report a 58-year-old male homozygote for a missense mutation, S590I, in MTP. The patient had a lifelong history of fat malabsorption, but was only diagnosed with ABL at age 52, based upon such classic features as absence of apo B-containing lipoproteins, acanthocytosis, atypical retinitis pigmentosa and markedly depressed serum beta-carotene concentration. However, his presentation was notable not only by survival to the sixth decade of life without specific treatment, but also by the absence of neurological involvement and by normal serum vitamin E concentration. He subsequently developed adenocarcinoma of the ileum, which required ileal resection. Therefore, this missense mutation appears to be associated with a late-presenting and relatively mild ABL phenotype that lacks some classical features, particularly neuropathy, but appears to be associated with other atypical features, specifically small intestinal cancer.

Microsomal triglyceride transfer protein and its role in apoB-lipoprotein assembly

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) are necessary for lipoprotein assembly. ApoB consists of five structural domains, betaalpha(1)-beta(1)-alpha(2)-beta(2)-alpha(3). We propose that MTP contains three structural motifs (N-terminal beta-barrel, central alpha-helix, and C-terminal lipid cavity) and three functional domains (lipid transfer, membrane associating, and apoB binding). MTP's lipid transfer activity is required for the assembly of lipoproteins. This activity renders nascent apoB secretion-competent and may be involved in the import of triglycerides into the lumen of endoplasmic reticulum. In addition, MTP binds to apoB with high affinity involving ionic interactions. MTP interacts at multiple sites in the N-terminal betaalpha(1) structural domain of apoB. A novel antagonist that inhibits apoB-MTP binding decreases apoB secretion. Furthermore, site-directed mutagenesis and deletion analyses that inhibit apoB-MTP binding decrease apoB secretion. Lipids modulate protein-protein interactions between apoB and MTP. Lipids associated with MTP increase apoB-MTP binding whereas lipids associated with apoB decrease this binding. Thus, specific antagonist, site-directed mutagenesis, deletion analyses, and modulation studies support the notion that apoB-MTP binding plays a role in lipoprotein biogenesis. However, specific steps in lipoprotein assembly that require apoB-MTP binding have not been identified. ApoB-MTP binding may be important for the prevention of degradation and lipidation of nascent apoB.

A proteomic approach identifies proteins in hepatocytes that bind nascent apolipoprotein B

The biogenesis of apolipoprotein B is quite complex in view of its huge size, hydrophobicity, obligate association with lipids such as cholesterol and triglycerides prior to secretion, and intracellular degradation of a substantial proportion of newly synthesized molecules. Multiple proteins likely serve roles as molecular chaperones to assist in folding, assembly with lipids, and regulation of the secretion of apolipoprotein B. In these studies, we developed a strategy to isolate proteins associated with apolipoprotein B in rat livers. The purification consisted of two stages: first, microsomes were prepared from rat liver and treated with chemical cross-linkers, and second, the solubilized proteins were co-immunoprecipitated with antibody against apolipoprotein B. We found that several proteins were cross-linked to apolipoprotein B. The proteins were digested with trypsin, and the released peptides were sequenced by tandem mass spectrometry. The sequences precisely matched 377 peptides in 99 unique proteins. We show that at least two of the identified proteins, ferritin heavy and light chains, can directly bind apolipoprotein B. These and possibly other proteins identified by this proteomic approach are novel candidates for proteins that affect apolipoprotein B during its biogenesis.

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

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

Apolipoprotein B is synthesized in selected human non-hepatic cell lines but not processed into mature lipoprotein

We studied apolipoprotein B100 (apoB) metabolism in a series of non-hepatic cell lines (HT29 colon adenocarcinoma, HeLa cervical epithelioid carcinoma, and 1321N1J astrocytoma human cell lines) and in the human hepatoma cell line HepG2. ApoB mRNA was detected by reverse transcription polymerase chain reaction in each non-hepatic cell line. ApoB was detected in HepG2 cells by immunoprecipitation, Western blotting, and immunocytochemistry using a polyclonal anti-human low-density lipoprotein (LDL) antibody, an anti-human apoB peptide antibody, and several monoclonal anti-apoB antibodies. ApoB was identified in the three non-hepatic cell lines by each method using the anti-apoB peptide and monoclonal antibodies, but not with the anti-LDL antibody. Immunocytochemistry indicated that epitopes of apoB were evident throughout the endoplasmic reticulum, and gel mobility of newly labeled apoB and immunoblot with anti-ubiquitin showed that apoB was highly ubiquinated in non-hepatic cells. The observations that apoB is synthesized in non-hepatic cell lines but never recognized by the anti-LDL antibody suggests that apoB is not processed into a nascent lipoprotein in these cells. Immunocytochemical localization of apoB epitopes at many locations throughout non-hepatic cells raises the exciting possibility that apoB can be used for other purposes in these cells.

Disordered fat storage and mobilization in the pathogenesis of insulin resistance and type 2 diabetes

The primary genetic, environmental, and metabolic factors responsible for causing insulin resistance and pancreatic beta-cell failure and the precise sequence of events leading to the development of type 2 diabetes are not yet fully understood. Abnormalities of triglyceride storage and lipolysis in insulin-sensitive tissues are an early manifestation of conditions characterized by insulin resistance and are detectable before the development of postprandial or fasting hyperglycemia. Increased free fatty acid (FFA) flux from adipose tissue to nonadipose tissue, resulting from abnormalities of fat metabolism, participates in and amplifies many of the fundamental metabolic derangements that are characteristic of the insulin resistance syndrome and type 2 diabetes. It is also likely to play an important role in the progression from normal glucose tolerance to fasting hyperglycemia and conversion to frank type 2 diabetes in insulin resistant individuals. Adverse metabolic consequences of increased FFA flux, to be discussed in this review, are extremely wide ranging and include, but are not limited to: 1) dyslipidemia and hepatic steatosis, 2) impaired glucose metabolism and insulin sensitivity in muscle and liver, 3) diminished insulin clearance, aggravating peripheral tissue hyperinsulinemia, and 4) impaired pancreatic beta-cell function. The precise biochemical mechanisms whereby fatty acids and cytosolic triglycerides exert their effects remain poorly understood. Recent studies, however, suggest that the sequence of events may be the following: in states of positive net energy balance, triglyceride accumulation in "fat-buffering" adipose tissue is limited by the development of adipose tissue insulin resistance. This results in diversion of energy substrates to nonadipose tissue, which in turn leads to a complex array of metabolic abnormalities characteristic of insulin-resistant states and type 2 diabetes. Recent evidence suggests that some of the biochemical mechanisms whereby glucose and fat exert adverse effects in insulin-sensitive and insulin-producing tissues are shared, thus implicating a diabetogenic role for energy excess as a whole. Although there is now evidence that weight loss through reduction of caloric intake and increase in physical activity can prevent the development of diabetes, it remains an open question as to whether specific modulation of fat metabolism will result in improvement in some or all of the above metabolic derangements or will prevent progression from insulin resistance syndrome to type 2 diabetes.

The late addition of core lipids to nascent apolipoprotein B100, resulting in the assembly and secretion of triglyceride-rich lipoproteins, is independent of both microsomal triglyceride transfer protein activity and new triglyceride synthesis

Although microsomal triglyceride transfer protein (MTP) and newly synthesized triglyceride (TG) are critical for co-translational targeting of apolipoprotein B (apoB100) to lipoprotein assembly in hepatoma cell lines, their roles in the later stages of lipoprotein assembly remain unclear. Using N-acetyl-Leu-Leu-norleucinal to prevent proteasomal degradation, HepG2 cells were radiolabeled and chased for 0-90 min (chase I). The medium was changed and cells chased for another 150 min (chase II) in the absence (control) or presence of Pfizer MTP inhibitor CP-10447 (CP). As chase I was extended, inhibition of apoB100 secretion by CP during chase II decreased from 75.9% to only 15% of control (no CP during chase II). Additional studies were conducted in which chase I was either 0 or 90 min, and chase II was in the presence of [(3)H]glycerol and either BSA (control), CP (inhibits both MTP activity and TG synthesis),BMS-1976360-1) (BMS) (inhibits only MTP activity), or triacsin C (TC) (inhibits only TG synthesis). When chase I was 0 min, CP, BMS, and TC reduced apoB100 secretion during chase II by 75.3, 73.9, and 53.9%. However, when chase I was 90 min, those agents reduced apoB100 secretion during chase II by only 16.0, 19.2, and 13.9%. Of note, all three inhibited secretion of newly synthesized TG during chase II by 80, 80, and 40%, whether chase I was 0 or 90 min. In both HepG2 cells and McA-RH7777 cells, if chase I was at least 60 min, inhibition of TG synthesis and/or MTP activity did not affect the density of secreted apoB100-lipoproteins under basal conditions. Oleic acid increased secretion of TG-enriched apoB100-lipoproteins similarly in the absence or presence of either of CP, BMS, or TC. We conclude that neither MTP nor newly synthesized TG is necessary for the later stages of apoB100-lipoprotein assembly and secretion in either HepG2 or McA-RH7777 cells.

Decreased microsomal triglyceride transfer protein activity contributes to initiation of alcoholic liver steatosis in rats

BACKGROUND/AIMS

To elucidate the role of microsomal triglyceride transfer protein (MTP) in the pathogenesis of alcoholic fatty liver, the effects of ethanol on MTP activity and gene expression were investigated.

METHODS AND RESULTS

Male Sprague-Dawley rats fed an ethanol-containing liquid diet for 37 days, respectively, showed 2.9- and 4.9-fold increases in hepatic cholesterol and triglyceride content in comparison with rats fed an isocaloric ethanol-free diet (P<0.01). Furthermore, a significant decrease in MTP activity and mRNA expression (by 27 and 58%, respectively) was observed after ethanol administration. Intravenous injection of human recombinant hepatocyte growth factor (hrHGF) on each of the last 7 days markedly suppressed ethanol-induced lipid accumulation in the liver. This inhibition of fatty change by hrHGF was accompanied by recovery of MTP activity and gene expression. No inhibitory effect of hrHGF on ethanol-induced acyl-CoA synthetase activation was observed. Experiments using human hepatoma-derived HepG2 cells indicated a direct positive effect of hrHGF on MTP gene expression as well as apolipoprotein B secretion.

CONCLUSIONS

These results suggest that reduced MTP activity is crucial to development of alcoholic fatty liver, while promotion of MTP activity by HGF might serve as a therapeutic measure against alcoholic liver steatosis.

Visceral obesity and hyperinsulinemia modulate the impact of the microsomal triglyceride transfer protein -493G/T polymorphism on plasma lipoprotein levels in men

The dyslipidemic state of visceral obesity is characterized by increased plasma triglyceride levels, low high-density lipoprotein-cholesterol concentration and alterations in low-density lipoprotein (LDL) composition and concentration. A functional, non-coding microsomal triglyceride transfer protein (MTP) -493G/T polymorphism of the microsomal triglyceride transfer protein gene has been related to variations in LDL-cholesterol levels. To study the effect of the MTP -493G/T polymorphism on lipoprotein levels in visceral obesity and hyperinsulinemia, a total of 227 men were assigned into two groups on the basis of their MTP -493G/T polymorphism, including 121 GG homozygotes and 105 carriers of the T allele (92 GT and 13 TT). The two genotypic groups did not differ for their physiological characteristics nor for lipoprotein--lipid profiles, before and after adjustment for age. However, GG homozygotes were characterized by higher fasting insulin levels than carriers of the T allele (P<0.05). When the two genotypic groups were further divided on the basis of their visceral adipose tissue (AT) accumulation, assessed by computed tomography, we observed that T allele carriers with low levels of visceral AT (<130 cm(2)) had decreased plasma total cholesterol and LDL-apolipoprotein B (LDL-apoB) levels compared to viscerally obese men (P=0.035 and P=0.0001, respectively). Among GG homozygotes, no significant difference were observed. Although not significant, T allele carriers characterized by visceral obesity tended to have smaller, denser LDL particles than T allele carriers characterized by a low accumulation of visceral AT. When subjects were divided on the basis of their fasting insulin levels, it appears that hyperinsulinemic men were characterized by a deteriorated lipoprotein--lipid profile when they were carriers of the T allele compared to normoinsulinemic men. In summary, visceral obesity and hyperinsulinemia modulate the impact of the MTP -493G/T polymorphism on plasma total cholesterol and LDL-apoB levels, as well as on LDL peak particle diameter.

Diaminoindanes as microsomal triglyceride transfer protein inhibitors

The synthesis and biological activities of biarylamide-substituted diaminoindanes as microsomal triglyceride transfer protein (MTP) inhibitors are described. One of the more potent compounds, 8aR, inhibited both the secretion of apoB from Hep G2 cells and the MTP-mediated transfer of triglycerides between synthetic acceptor and donor liposomes with IC(50) values of 0.7 and 70 nM, respectively. In normolipidemic rats and dogs, oral administration of 8aR dose-dependently reduced both plasma triglycerides and total cholesterol. Moreover, in rats and dogs, 8aR also prevented the postprandial rise in plasma triglycerides following a bolus administration of a fat load. Because MTP inhibitors decrease very low density lipoprotein assembly in the liver, the potential for hepatic lipid accumulation was evaluated. In normolipidemic rats, hepatic cholesterol and triglyceride contents were dose-dependently increased by 8aR. However, hepatic lipid accumulation resulted in negligible change in total liver weight and was reversible after withdrawal of the compound.

Binding of microsomal triglyceride transfer protein to lipids results in increased affinity for apolipoprotein B: evidence for stable microsomal MTP-lipid complexes

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) are known to interact with each other. We evaluated the effect of different lipids on the protein-protein interactions between MTP and apoB100 or its C-terminally truncated forms. Negatively charged lipids decreased protein-protein interactions between apoB and MTP. In contrast, zwitterionic phospholipids enhanced (2-4-fold) the binding of apoB100 to MTP by increasing affinity (1.5-3-fold) between these proteins without affecting the number of binding sites. Similarly, phospholipids augmented (1.5-4-fold) the binding of various C-terminally truncated apoB peptides to MTP. The increased binding was greater for apoB peptides containing lipid-binding domains, such as apoB28 and apoB42. Surprisingly, preincubation of apoB28 with lipid vesicles had no effect on MTP binding. In contrast, incubation of MTP with lipid vesicles resulted in a stable association of MTP with vesicles, and MTP-lipid vesicles bound better (5-fold increase) to LDL than did lipid-free MTP. To determine whether MTP exists stably associated with lipids in cells, microsomal contents from COS cells expressing MTP, HepG2 cells, and mouse liver were ultracentrifuged, and MTP was visualized in different density fractions. MTP was found associated and unassociated with lipids. In contrast, apoB17 and apoB:270-570 were present unassociated with lipids in COS cells. These studies show that the binding of MTP to lipids results in increased affinity for apoB and that stable MTP-lipid complexes exist in the lumen of the endoplasmic reticulum. Protein-protein interactions between apoB and MTP may juxtapose lipids associated with MTP to lipid-binding domains of apoB and facilitate hydrophobic interactions leading to enhance affinity. We speculate that MTP-lipid complexes may serve as nuclei to form "primordial lipoproteins" and may also play a role in the bulk addition of lipids during the "core expansion" of these lipoproteins.

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.

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.

Immunolocalization, ontogeny, and regulation of microsomal triglyceride transfer protein in human fetal intestine

To examine the multiple stages of lipoprotein packaging during development, we studied localization, ontogeny, and regulation of microsomal transfer protein (MTP), a crucial protein for lipid transport. With the use of immunofluorescence, MTP was identified in villus and crypt epithelial cells in different regions of human fetal intestine, including colon. Staining was detected as early as the 13th wk of gestation in all gut segments and was almost entirely confined to the columnar epithelial cells of the jejunum and colon. Unlike immunofluorescence, which provides qualitative but not quantitative information on MTP signal, enzymatic assays revealed a decreasing gradient from proximal small intestine to distal, as confirmed by immunoblot. Activity of MTP in small intestinal explants cultured for different incubation periods (0, 4, 8, and 24 h) peaked at 4 h but remained insensitive to different concentrations of oleic acid. Also, a trend toward increasing MTP activity was observed at 20-22 wk of gestation. Finally, in strong contrast to jejunal efficiency, colonic explants displayed impaired lipid production, apolipoprotein biogenesis, and lipoprotein assembly, in association with poor expression of MTP. These findings provide the first evidence that human fetal gut is able to express MTP and emphasize the distinct regional distribution, regulation by oleic acid, and ontogeny of MTP.

The role of the microsomal triglygeride transfer protein in abetalipoproteinemia

The microsomal triglyceride transfer protein (MTP) is a dimeric lipid transfer protein consisting of protein disulfide isomerase and a unique 97-kDa subunit. In vitro, MTP accelerates the transport of triglyceride, cholesteryl ester, and phospholipid between membranes. It was recently demonstrated that abetalipoproteinemia, a hereditary disease characterized as an inability to produce chylomicrons and very low-density lipoproteins in the intestine and liver, respectively, results from mutations in the gene encoding the 97-kDa subunit of the microsomal triglyceride transfer protein. Downstream effects resulting from this defect include malnutrition, very low plasma cholesterol and triglyceride levels, altered lipid and protein compositions of membranes and lipoprotein particles, and vitamin deficiencies. Unless treated, abetalipoproteinemic subjects develop gastrointestinal, neurological, ophthalmological, and hematological abnormalities.

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.

Etiology of fatty liver in dairy cattle: effects of nutritional and hormonal status on hepatic microsomal triglyceride transfer protein

We conducted three experiments to determine the effects of nutritional and hormonal status on microsomal triglyceride transfer protein (MTP) activity and mass. In experiment 1, 18 nonlactating Holstein cows, 75 d before expected calving date, in their second gestation or greater were monitored from d 75 to 55 prepartum. Cows were fed a control diet from d 75 to 62 prepartum for covariable measurements. From d 61 to 55 prepartum, six cows continued to receive the control diet, six cows were restricted to 2.3 kg of grass hay/d, and six cows were fed the control diet plus 1.8 kg of concentrate/d and 500 ml of propylene glycol given 2 times/d as an oral drench. Plasma glucose and serum insulin concentrations were highest in cows that received propylene glycol and lowest in feed restricted cows. Plasma nonesterified fatty acids (NEFA) and liver triglyceride (TG) concentrations were highest in feed restricted cows and not different between cows that received the control diet and cows that received propylene glycol. Hepatic MTP activity and mass were not affected by treatment in experiment 1. In experiment 2, bovine hepatocytes isolated from the caudate process of five preruminating Holstein bull calves were incubated with either 0, 0.5, 1.0, or 2.0 mM NEFA for 48 h. Intracellular TG increased linearly as NEFA concentration in the media increased. Concentration of NEFA in the incubation media had no effect on MTP activity or mass. There was a quadratic effect of concentration of NEFA in the incubation media on MTP mRNA. In experiment 3, bovine hepatocytes isolated from the caudate process of five preruminating Holstein bull calves were incubated with 2 mM [1-14C]oleate for 24 h to accumulate TG, followed by a 36-h period of TG depletion, during which hepatocytes were incubated with no hormone, 10 nM insulin, or 10 nM glucagon. There was no effect of insulin or glucagon on intracellular TG, MTP activity or mass. Cells incubated with no hormone had higher levels of MTP mRNA compared to cells incubated with insulin or glucagon during the depletion period. Results suggest that hepatic MTP mRNA may be affected by TG accumulation, insulin, and glucagon in vitro. However, hepatic MTP activity and mass are not affected by nutritional status of nonlactating dairy cows, TG accumulation in vitro, or insulin and glucagon in vitro.

Changes in hepatic microsomal triglyceride transfer protein and triglyceride in periparturient dairy cattle

We determined the relationship between microsomal triglyceride transfer protein (MTP) (activity, mass, and mRNA) and liver triglyceride concentration in 16 dairy cows (13 multiparous and three primiparous) from 27 d before expected calving (d -27) to 35 d postpartum (d 35), the time period when fatty liver is most likely to develop. In addition, dry matter intake, plasma nonesterified fatty acids (NEFA), and plasma glucose were monitored. There were no significant parity x time interactions. Dry matter intake, plasma NEFA, plasma glucose, and liver triglyceride were significantly affected by day of sampling. Dry matter intake was 10.7, 8.0, and 19.5 kg/d on d -27, 2, and 35, respectively. Plasma NEFA concentration was higher on d 2 (1113 microEq/L) compared with d -27 (201 microEq/L) and 35 (358 microEq/L). Plasma glucose concentration was 63.3, 54.3, and 57.8 mg/dl on d -27, 2, and 35, respectively. Hepatic triglyceride (TG) concentration increased from 1.8 to 11.8% liver TG (DM basis) on d -27 and 2, respectively. There was no difference between hepatic triglyceride concentration on d 2 and 35. There was a significant effect of day of sampling on hepatic MTP activity and mRNA. Hepatic MTP activity decreased from 2.08 to 1.79 nmole triolein transferred/ h per mg of microsomal protein on d -27 and 2, respectively, and increased from 1.79 to 2.17 nmole triolein transferred/h per mg of microsomal protein on d 2 and 35, respectively. Hepatic MTP mRNA increased from d -27 to 2 and remained elevated from d 2 to 35. There was no effect of day of sampling on MTP mass. There were no significant correlations between hepatic MTP activity, mass, or mRNA with either liver TG or plasma NEFA on any of the sampling days. The cause of a decrease in hepatic MTP activity and increase in mRNA on d 2 is unknown. However, the lack of correlation between MTP activity, mass, or mRNA with either liver TG or plasma NEFA on d 2 postpartum suggests that MTP probably does not play a role in the etiology of fatty liver that occurs in dairy cows at calving.

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.

Mechanisms of hepatic very low density lipoprotein overproduction in insulin resistance. Evidence for enhanced lipoprotein assembly, reduced intracellular ApoB degradation, and increased microsomal triglyceride transfer protein in a fructose-fed hamster model

A novel animal model of insulin resistance, the fructose-fed Syrian golden hamster, was employed to investigate the mechanisms mediating the overproduction of very low density lipoprotein (VLDL) in the insulin resistant state. Fructose feeding for a 2-week period induced significant hypertriglyceridemia and hyperinsulinemia, and the development of whole body insulin resistance was documented using the euglycemic-hyperinsulinemic clamp technique. In vivo Triton WR-1339 studies showed evidence of VLDL-apoB overproduction in the fructose-fed hamster. Fructose feeding induced a significant increase in cellular synthesis and secretion of total triglyceride (TG) as well as VLDL-TG by primary hamster hepatocytes. Increased TG secretion was accompanied by a 4.6-fold increase in VLDL-apoB secretion. Enhanced stability of nascent apoB in fructose-fed hepatocytes was evident in intact cells as well as in a permeabilized cell system. Analysis of newly formed lipoprotein particles in hepatic microsomes revealed significant differences in the pattern and density of lipoproteins, with hepatocytes derived from fructose-fed hamsters having higher levels of luminal lipoproteins at a density of VLDL versus controls. Immunoblot analysis of the intracellular mass of microsomal triglyceride transfer protein, a key enzyme involved in VLDL assembly, showed a striking 2.1-fold elevation in hepatocytes derived from fructose-fed versus control hamsters. Direct incubation of hamster hepatocytes with various concentrations of fructose failed to show any direct stimulation of its intracellular stability or extracellular secretion, further supporting the notion that the apoB overproduction in the fructose-fed hamster may be related to the fructose-induced insulin resistance in this animal model. In summary, hepatic VLDL-apoB overproduction in fructose-fed hamsters appears to result from increased intracellular stability of nascent apoB and an enhanced expression of MTP, which act to facilitate the assembly and secretion of apoB-containing lipoprotein particles.

Absence of association between genetic variation in the promoter of the microsomal triglyceride transfer protein gene and plasma lipoproteins in the Framingham Offspring Study

Microsomal triglyceride transfer protein (MTP) is a lipid transfer protein that is required for the assembly and secretion of very low density lipoproteins (VLDL) by the liver and chylomicrons by the intestine. The common G-493T polymorphism of the MTP promoter has been shown to be associated with decreased plasma LDL-cholesterol and ApoB content of VLDL. The purpose of the present study was, therefore, to investigate the association of this mutation with variations in lipid and apoprotein levels, lipoprotein subclass profiles and coronary heart disease (CHD) risk in a population-based sample of 1226 male and 1284 female Framingham Offspring participants. In men and women, no significant association was found between the G-493T MTP polymorphism and variations of plasma levels of total cholesterol, LDL-cholesterol, apoprotein B, HDL-cholesterol, apoprotein AI and triglycerides. In order to further investigate potential relationships with variations of lipoprotein phenotypes, lipoprotein subclass profiles were measured using automated nuclear magnetic resonance (NMR) spectroscopy. Each NMR profile yielded information on lipid mass of VLDL, LDL, and HDL subclasses. In both genders, there was no significant association between the G-493T polymorphism and variability of lipoprotein subclass distributions or lipoprotein particle size. Furthermore, no significant association was found between the polymorphism of the MTP promoter and prevalence or the age of onset of CHD. Thus, our results suggest that the G-493T mutation in the MTP promoter is unlikely to have significant implications for cardiovascular disease in men and women.

A proposed model for the assembly of chylomicrons

The intestine synthesizes very low density lipoproteins (VLDL) and chylomicrons (CM) to transport fat and fat-soluble vitamins into the blood. VLDL assembly occurs constitutively whereas CM assembly is a characteristic property of the enterocytes during the postprandial state. The secretion of CM is specifically inhibited by Pluronic L81. CM are very heterogeneously-sized particles that consist of a core of triglycerides (TG) and cholesterol esters and a monolayer of phospholipids (PL), cholesterol and proteins. The fatty acid composition of TG, but not PL, in CM mirrors the fatty acid composition of fat in the diet. CM assembly is deficient in abetalipoproteinemia and CM retention disease. Abetalipoproteinemia results due to mutation in the mttp gene and is characterized by the virtual absence of apoB-containing lipoproteins in the plasma. Patients suffer from neurologic disorders, visual impairment, and exhibit acanthocytosis. CM retention disease, an inherited recessive disorder, is characterized by chronic diarrhea with steatorrhea in infancy, abdominal distention and failure to thrive. It is caused by a specific defect in the secretion of intestinal lipoproteins; secretion of lipoproteins by the liver is not affected. Besides human disorders, mice that do not assemble intestinal lipoproteins have been developed. These mice are normal at birth, but defective in fat and fat-soluble vitamin absorption, and fail to thrive. Thus, fat and fat-soluble vitamin transport by the intestinal lipoproteins is essential for proper growth and development of neonates. Recently, differentiated Caco-2 cells and rabbit primary enterocytes have been described that synthesize and secrete CM. These cells can be valuable in distinguishing between the two different models proposed for the assembly of CM. In the first model, the assembly of VLDL and CM is proposed to occur by two 'independent' pathways. Second, CM assembly is proposed to be a product of 'core expansion' that results in the synthesis of lipoproteins of different sizes. According to this model, intestinal lipoprotein assembly begins with the synthesis of 'primordial' lipoprotein particles and involves release of the nascent apoB with PL derived from the endoplasmic reticulum (ER) membrane. In addition, TG-rich 'lipid droplets' of different sizes are formed independent of apoB synthesis. The fusion of lipid droplets and primordial lipoproteins results in the formation of different size lipoproteins due to the 'core expansion' of the primordial lipoproteins.

Linkage of a candidate gene locus to familial combined hyperlipidemia: lecithin:cholesterol acyltransferase on 16q

Familial combined hyperlipidemia (FCHL) is a common lipid disorder characterized by elevated levels of plasma cholesterol and triglycerides that is present in 10% to 20% of patients with premature coronary artery disease. To study the pathophysiological basis and genetics of FCHL, we previously reported recruitment of 18 large families. We now report linkage studies of 14 candidate genes selected for their potential involvement in the aspects of lipid and lipoprotein metabolism that are altered in FCHL. We used highly polymorphic markers linked to the candidate genes, and these markers were analyzed using several complementary, nonparametric statistical allele-sharing linkage methodologies. This current sample has been extended over the one in which we identified an association with the apolipoprotein (apo) AI-CIII-AIV gene cluster. We observed evidence for linkage of this region and FCHL (P<0.001), providing additional support for its involvement in FCHL. We also identified a new locus showing significant evidence of linkage to the disorder: the lecithin:cholesterol acyltransferase (LCAT) locus (P<0.0006) on chromosome 16. In addition, analysis of the manganese superoxide dismutase locus on chromosome 6 revealed a suggestive linkage result in this sample (P<0.006). Quantitative traits related to FCHL also provided some evidence of linkage to these regions. No evidence of linkage to the lipoprotein lipase gene, the microsomal triglyceride transfer protein gene, or several other genes involved in lipid metabolism was observed. The data suggest that the lecithin:cholesterol acyltransferase and apolipoprotein AI-CIII-AIV loci may act as modifying genes contributing to the expression of FCHL.

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.

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.

Liver-specific inactivation of the abetalipoproteinemia gene completely abrogates very low density lipoprotein/low density lipoprotein production in a viable conditional knockout mouse

Conventional knockout of the microsomal triglyceride transfer protein large subunit (lMTP) gene is embryonic lethal in the homozygous state in mice. We have produced a conditional lMTP knockout mouse by inserting loxP sequences flanking exons 5 and 6 by gene targeting. Homozygous floxed mice were born live with normal plasma lipids. Intravenous injection of an adenovirus harboring Cre recombinase (AdCre1) produced deletion of exons 5 and 6 and disappearance of lMTP mRNA and immunoreactive protein in a liver-specific manner. There was also disappearance of plasma apolipoprotein (apo) B-100 and marked reduction in apoB-48 levels. Wild-type mice showed no response, and heterozygous mice, an intermediate response, to AdCre1. Wild-type mice doubled their plasma cholesterol level following a high cholesterol diet. This hypercholesterolemia was abolished in AdCre1-treated lMTP-/- mice, the result of a complete absence of very low/intermediate/low density lipoproteins and a slight reduction in high density lipoprotein. Heterozygous mice showed an intermediate lipoprotein phenotype. The rate of accumulation of plasma triglyceride following Triton WR1339 treatment in lMTP-/- mice was <10% that in wild-type animals, indicating a failure of triglyceride-rich lipoprotein production. Pulse-chase experiments using hepatocytes isolated from wild-type and lMTP-/- mice revealed a failure of apoB secretion in lMTP-/- animals. Therefore, the liver-specific inactivation of the lMTP gene completely abrogates apoB-100 and very low/intermediate/low density lipoprotein production. These conditional knockout mice are a useful in vivo model for studying the role of MTP in apoB biosynthesis and the biogenesis of apoB-containing lipoproteins.

Baculovirus expression and biochemical characterization of the human microsomal triglyceride transfer protein

The microsomal triglyceride transfer protein (MTP) complexed to protein disulphide isomerase (PDI) is obligatory for the assembly of chylomicrons and very-low-density lipoproteins. The determination of the atomic structure of the MTP-PDI heterodimer has important implications for the treatment of those forms of hyperlipidaemia associated with the overproduction of very-low-density lipoproteins, which predispose to premature coronary heart disease. To perform structural studies of the human MTP-PDI complex it was necessary to produce milligram quantities of pure protein. We chose the baculovirus expression system for this purpose. Insects cells were co-infected with recombinant viruses encoding FLAG-tagged MTP and His-tagged PDI; the resulting heterodimer was purified by affinity chromatography. From 5 litres of insect cells, 4-6 mg of more than 95% pure recombinant protein was obtained. CD and attenuated total reflection Fourier-transform infrared spectroscopy indicate that the purified protein has around 34% alpha-helical and 33% beta-structure content. The recombinant protein had a comparable triglyceride transfer activity to that of bovine MTP-PDI. The production of polyclonal antibodies raised against the MTP and PDI subunits of the purified protein is described. The present study demonstrates the feasibility of expressing two proteins at high levels in insect cells and describes a transferable methodology for the purification of the resulting protein complex.

Insights into apolipoprotein B biology from transgenic and gene-targeted mice

Over the past five years, several laboratories have used transgenic and gene-targeted mice to study apolipoprotein (apo) B biology. Genetically modified mice have proven useful for investigating the genetic and environmental factors affecting atherogenesis, for defining apoB structure/function relationships, for understanding the regulation of the apoB gene expression in the intestine, for defining the "physiologic rationale" for the existence of the two different forms of apoB (apoB48 and apoB100) in mammalian metabolism and for providing mechanistic insights into the human apoB deficiency syndrome, familial hypobetalipoproteinemia. This review will provide several examples of how genetically modified mice have contributed to our understanding of apoB biology, including our new discovery that human heart myocytes secrete nascent apoB-containing lipoproteins.