A 30-amino acid truncation of the microsomal triglyceride transfer protein large subunit disrupts its interaction with protein disulfide-isomerase and causes abetalipoproteinemia

A novel p.Gly417Valfs*12 mutation in the MTTP gene causing abetalipoproteinemia: Presentation of the first patient in Mexico and analysis of the previously reported cases

Background

Our aims were to describe the first Mexican patient with abetalipoproteinemia and to perform a comparative analysis of biochemical, clinical, and genetic characteristics of 100 cases reported in the literature.

Methods

We performed biochemical and molecular screenings in a Mexican girl with extremely low lipid levels and in her family. Further, we integrated and evaluated the characteristics of the cases with abetalipoproteinemia described in the literature.

Results

Our patient is a six‐year‐old girl who presented vomiting, chronic diarrhea, failure to thrive, malabsorption, acanthocytosis, anemia, transaminases elevation, and extremely low lipid levels. MTTP gene sequencing revealed homozygosity for a novel mutation p.Gly417Valfs*12 (G deletion c.1250). With the analysis of the reported cases, 60 clinical features (14 classical and 46 non‐classical) were observed, being the most common acanthocytosis (57.5%), malabsorption (43.7%), and diarrhea (42.5%); 48.8% of the patients presented only classic clinical features, while the remaining 51.2% developed secondary effects due to a fat‐soluble vitamin deficiency. An odds ratio analysis disclosed that patients diagnosed after 10 years of age have an increased risk for presenting clinical complications (OR = 18.0; 95% CI 6.0‐54.1, p < 0.0001). A great diversity of mutations in MTTP has been observed (n = 76, being the most common p.G865X and p.N139_E140) and some of them with possible residual activity.

Conclusion

The first Mexican patient with abetalipoproteinemia presents a novel MTTP mutation p.Gly417Valfs*12. Three factors that could modulate the phenotype in abetalipoproteinemia were identified: age at diagnosis, treatment, and the causal mutation.

A point mutation decouples the lipid transfer activities of microsomal triglyceride transfer protein

Apolipoprotein B-containing lipoproteins (B-lps) are essential for the transport of hydrophobic dietary and endogenous lipids through the circulation in vertebrates. Zebrafish embryos produce large numbers of B-lps in the yolk syncytial layer (YSL) to move lipids from yolk to growing tissues. Disruptions in B-lp production perturb yolk morphology, readily allowing for visual identification of mutants with altered B-lp metabolism. Here we report the discovery of a missense mutation in microsomal triglyceride transfer protein (Mtp), a protein that is essential for B-lp production. This mutation of a conserved glycine residue to valine (zebrafish G863V, human G865V) reduces B-lp production and results in yolk opacity due to aberrant accumulation of cytoplasmic lipid droplets in the YSL. However, this phenotype is milder than that of the previously reported L475P stalactite (stl) mutation. MTP transfers lipids, including triglycerides and phospholipids, to apolipoprotein B in the ER for B-lp assembly. In vitro lipid transfer assays reveal that while both MTP mutations eliminate triglyceride transfer activity, the G863V mutant protein unexpectedly retains ~80% of phospholipid transfer activity. This residual phospholipid transfer activity of the G863V mttp mutant protein is sufficient to support the secretion of small B-lps, which prevents intestinal fat malabsorption and growth defects observed in the mttpstl/stl mutant zebrafish. Modeling based on the recent crystal structure of the heterodimeric human MTP complex suggests the G865V mutation may block triglyceride entry into the lipid-binding cavity. Together, these data argue that selective inhibition of MTP triglyceride transfer activity may be a feasible therapeutic approach to treat dyslipidemia and provide structural insight for drug design. These data also highlight the power of yolk transport studies to identify proteins critical for B-lp biology.

Regulation of intestinal lipid metabolism: current concepts and relevance to disease

Lipids entering the gastrointestinal tract include dietary lipids (triacylglycerols, cholesteryl esters and phospholipids) and endogenous lipids from bile (phospholipids and cholesterol) and from shed intestinal epithelial cells (enterocytes). Here, we comprehensively review the digestion, uptake and intracellular re-synthesis of intestinal lipids as well as their packaging into pre-chylomicrons in the endoplasmic reticulum, their modification in the Golgi apparatus and the exocytosis of the chylomicrons into the lamina propria and subsequently to lymph. We also discuss other fates of intestinal lipids, including intestinal HDL and VLDL secretion, cytosolic lipid droplets and fatty acid oxidation. In addition, we highlight the applicability of these findings to human disease and the development of therapeutics targeting lipid metabolism. Finally, we explore the emerging role of the gut microbiota in modulating intestinal lipid metabolism and outline key questions for future research.

The crystal structure of human microsomal triglyceride transfer protein

This study provides a structure for microsomal triglyceride transfer protein, a key protein in lipid metabolism and transport. Microsomal triglyceride transfer protein is linked to a human disease state, abetalipoproteinemia. The structure helps us to understand how this protein functions and gives a rationale for how previously reported mutations result in loss of function of the protein and hence, cause disease. The structure also provides a means for rational drug design to treat cardiovascular disease, hypercholesterolemia, and obesity. Microsomal triglyceride transfer protein is composed of 2 subunits. The β-subunit, protein disulfide isomerase (PDI), also acts independently as a protein folding catalyst. The structure that we present here gives insights into how PDI functions in protein folding.

Microsomal triglyceride transfer protein (MTP) plays an essential role in lipid metabolism, especially in the biogenesis of very low-density lipoproteins and chylomicrons via the transfer of neutral lipids and the assembly of apoB-containing lipoproteins. Our understanding of the molecular mechanisms of MTP has been hindered by a lack of structural information of this heterodimeric complex comprising an MTPα subunit and a protein disulfide isomerase (PDI) β-subunit. The structure of MTP presented here gives important insights into the potential mechanisms of action of this essential lipid transfer molecule, structure-based rationale for previously reported disease-causing mutations, and a means for rational drug design against cardiovascular disease and obesity. In contrast to the previously reported structure of lipovitellin, which has a funnel-like lipid-binding cavity, the lipid-binding site is encompassed in a β-sandwich formed by 2 β-sheets from the C-terminal domain of MTPα. The lipid-binding cavity of MTPα is large enough to accommodate a single lipid. PDI independently has a major role in oxidative protein folding in the endoplasmic reticulum. Comparison of the mechanism of MTPα binding by PDI with previously published structures gives insights into large protein substrate binding by PDI and suggests that the previous structures of human PDI represent the “substrate-bound” and “free” states rather than differences arising from redox state.

Targeting microsomal triglyceride transfer protein and lipoprotein assembly to treat homozygous familial hypercholesterolemia

Homozygous familial hypercholesterolemia (HoFH) is a polygenic disease arising from defects in the clearance of plasma low-density lipoprotein (LDL), which results in extremely elevated plasma LDL cholesterol (LDL-C) and increased risk of atherosclerosis, coronary heart disease, and premature death. Conventional lipid-lowering therapies, such as statins and ezetimibe, are ineffective at lowering plasma cholesterol to safe levels in these patients. Other therapeutic options, such as LDL apheresis and liver transplantation, are inconvenient, costly, and not readily available. Recently, lomitapide was approved by the Federal Drug Administration as an adjunct therapy for the treatment of HoFH. Lomitapide inhibits microsomal triglyceride transfer protein (MTP), reduces lipoprotein assembly and secretion, and lowers plasma cholesterol levels by over 50%. Here, we explain the steps involved in lipoprotein assembly, summarize the role of MTP in lipoprotein assembly, explore the clinical and molecular basis of HoFH, and review pre-clinical studies and clinical trials with lomitapide and other MTP inhibitors for the treatment of HoFH. In addition, ongoing research and new approaches underway for better treatment modalities are discussed.

Clinical features and molecular genetics of two Tunisian families with abetalipoproteinemia

Abetalipoproteinemia (ABL) is a rare monogenic disease characterized by very low plasma levels of cholesterol and triglyceride and almost complete absence of apolipoprotein B (apoB)-containing lipoproteins. Typically, patients present with failure to thrive, acanthocytosis, pigmented retinopathy and neurological features. It has been shown that ABL results from mutations in the gene encoding the microsomal triglyceride transfer protein (MTTP). Sanger sequencing of MTTP was performed for two unrelated consanguineous Tunisian families with two affected individuals each, presenting a more severe ABL phenotype than previously reported in the literature. The patients were found to be homozygous for two novel mutations. In the first family, a nonsense mutation, c.2313T>A, leading to a truncated protein (p.Y771X) was identified. In the second family, a splice mutation, IVS 9+2T>G, was found. These mutations are believed to abolish the assembly and secretion of apoB-containing lipoproteins.

Molecular characterization of Tunisian families with abetalipoproteinemia and identification of a novel mutation in MTTP gene

BACKGROUND

Abetalipoproteinemia (ABL; OMIM 200100) is a rare monogenic disorder of lipid metabolism characterized by reduced plasma levels of total cholesterol (TC), low density lipoprotein-cholesterol (LDL-C) and almost complete absence of apolipoprotein B (apoB). ABL results from genetic deficiency in microsomal triglyceride transfer protein (MTP; OMIM 157147). In the present study we investigated two unrelated Tunisian patients, born from consanguineous marriages, with severe deficiency of plasma low-density lipoprotein (LDL) and apo B.

METHODS

Intestinal biopsies were performed and The MTTP gene was amplified by Polymerase chain reaction then directly sequenced in patients presenting chronic diarrhea and retarded growth.

RESULTS

First proband was homozygous for a novel nucleotide deletion (c. 2611delC) involving the exon 18 of MTTP gene predicted to cause a non functional protein of 898 amino acids (p.H871I fsX29). Second proband was homozygous for a nonsense mutation in exon 8 (c.923 G > A) predicted to cause a truncated protein of 307 amino acids (p.W308X), previously reported in ABL patients.

CONCLUSIONS

We discovered a novel mutation in MTTP gene and we confirmed the diagnosis of abetalipoproteinemia in new Tunisian families.

VIRTUAL SLIDES

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/8134027928652779.

Loss of both phospholipid and triglyceride transfer activities of microsomal triglyceride transfer protein in abetalipoproteinemia

Mutations in microsomal triglyceride transfer protein (MTP) cause abetalipoproteinemia (ABL), characterized by the absence of plasma apoB-containing lipoproteins. In this study, we characterized the effects of various MTP missense mutations found in ABL patients with respect to their expression, subcellular location, and interaction with protein disulfide isomerase (PDI). In addition, we characterized functional properties by analyzing phospholipid and triglyceride transfer activities and studied their ability to support apoB secretion. All the mutants colocalized with calnexin and interacted with PDI. We found that R540H and N780Y, known to be deficient in triglyceride transfer activity, also lacked phospholipid transfer activity. Novel mutants S590I and G746E did not transfer triglycerides and phospholipids and did not assist in apoB secretion. In contrast, D384A displayed both triglyceride and phospholipid transfer activities and supported apoB secretion. These studies point out that ABL is associated with the absence of both triglyceride and phospholipid transfer activities in MTP.

The protein disulfide isomerase family: key players in health and disease

SIGNIFICANCE

Protein disulfide isomerase (PDI) and its homologs have essential roles in the oxidative folding and chaperone-mediated quality control of proteins in the secretory pathway. In this review, the importance of PDI in health and disease will be examined, using examples from the fields of lipid homeostasis, hemostasis, infectious disease, cancer, neurodegeneration, and infertility.

RECENT ADVANCES

Recent structural studies, coupled with cell biological, biochemical, and clinical approaches, have demonstrated that PDI family proteins are involved in a wide range of physiological and disease processes.

CRITICAL ISSUES

Critical issues in the field include understanding how and why a PDI family member is involved in a given disease, and defining the physiological client specificity of the various PDI proteins when they are expressed in different tissues.

FUTURE DIRECTIONS

Future directions are likely to include the development of new and more specific reagents to study and manipulate PDI family function. The development of conditional mouse models in concert with clinical data will help us to understand the in vivo function of the different PDIs at the organism level. Taken together with advances in structural biology and biochemical studies, this should help us to further understand and modify PDIs' functional interactions.

Molecular and functional analysis of two new MTTP gene mutations in an atypical case of abetalipoproteinemia

Abetalipoproteinemia (ABL) is an inherited disease characterized by the defective assembly and secretion of apolipoprotein B-containing lipoproteins caused by mutations in the microsomal triglyceride transfer protein large subunit (MTP) gene (MTTP). We report here a female patient with an unusual clinical and biochemical ABL phenotype. She presented with severe liver injury, low levels of LDL-cholesterol, and subnormal levels of vitamin E, but only mild fat malabsorption and no retinitis pigmentosa or acanthocytosis. Our objective was to search for MTTP mutations and to determine the relationship between the genotype and this particular phenotype. The subject exhibited compound heterozygosity for two novel MTTP mutations: one missense mutation (p.Leu435His) and an intronic deletion (c.619-5_619-2del). COS-1 cells expressing the missense mutant protein exhibited negligible levels of MTP activity. In contrast, the minigene splicing reporter assay showed an incomplete splicing defect of the intronic deletion, with 26% of the normal splicing being maintained in the transfected HeLa cells. The small amount of MTP activity resulting from the residual normal splicing in the patient explains the atypical phenotype observed. Our investigation provides an example of a functional analysis of unclassified variations, which is an absolute necessity for the molecular diagnosis of atypical ABL cases.

Monogenic Hypocholesterolaemic Lipid Disorders and Apolipoprotein B Metabolism

The study of apolipoprotein (apo) B metabolism is central to our understanding of human lipoprotein metabolism. Moreover, the assembly and secretion of apoB-containing lipoproteins is a complex process. Increased plasma concentrations of apoB-containing lipoproteins are an important risk factor for the development of atherosclerotic coronary heart disease. In contrast, decreased levels of, but not the absence of, these apoB-containing lipoproteins is associated with resistance to atherosclerosis and potential long life. The study of inherited monogenic dyslipidaemias has been an effective means to elucidate key metabolic steps and biologically relevant mechanisms. Naturally occurring gene mutations in affected families have been useful in identifying important domains of apoB and microsomal triglyceride transfer protein (MTP) governing the metabolism of apoB-containing lipoproteins. Truncation-causing mutations in the APOB gene cause familial hypobetalipoproteinaemia, whereas mutations in MTP result in abetalipoproteinaemia; both rare conditions are characterised by marked hypocholesterolaemia. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the key biochemical, clinical, metabolic and genetic features of the monogenic hypocholesterolaemic lipid disorders affecting apoB metabolism.

Abetalipoproteinemia in Israel: evidence for a founder mutation in the Ashkenazi Jewish population and a contiguous gene deletion in an Arab patient

Abetalipoproteinemia (ABL) is a rare autosomal recessive metabolic disorder, characterized by the absence of plasma apolipoprotein B-containing lipoproteins and very low levels of plasma triglycerides and cholesterol. ABL is caused by mutations of the MTP gene. We investigated the genetic basis for ABL in a cohort of Israeli families. In Ashkenazi Jewish patients we identified a conserved haplotype and a common MTP mutation, p.G865X, with a carrier frequency of 1:131 in this population. We also report the first case of ABL and additional abnormalities in a Muslim Arab patient, due to a homozygous contiguous gene deletion of approximately 481 kb, including MTP and eight other genes.

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

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

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

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

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

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

Mutations in MTP gene in abeta- and hypobeta-lipoproteinemia

Familial hypobetalipoproteinemia (FHBL) and abetalipoproteinemia (ABL) are inherited disorders of apolipoprotein B (apo B)-containing lipoproteins that result from mutations in apo B and microsomal triglyceride transfer protein (MTP) genes, respectively. Here we report three patients with severe deficiency of plasma low-density lipoprotein (LDL) and apo B. Two of them (probands F.A. and P.E.) had clinical and biochemical phenotype consistent with ABL. Proband F.A. was homozygous for a minute deletion/insertion (c.1228delCCCinsT) in exon 9 of MTP gene predicted to cause a truncated MTP protein of 412 amino acids. Proband P. E. was heterozygous for a mutation in intron 9 (IVS9-1G>A), previously reported in an ABL patient. We failed to find the second pathogenic mutation in MTP gene of this patient. No mutations were found in apo B gene. The third proband (D.F.) had a less severe lipoprotein phenotype which was similar to that of heterozygous FHBL and appeared to be inherited as a co-dominant trait. However, he had no mutations in apo B gene. He was found to be a compound heterozygote for two missense mutations (D384A and G661A), involving highly conserved regions of MTP. Since this proband was also homozygous for varepsilon2 allele of apolipoprotein E (apo E), it is likely that his hypobetalipoproteinemia derives from a combined effect of a mild MTP deficiency and homozygosity for apo E2 isoform.

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.

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.

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.

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.

Traffic jam: a compendium of human diseases that affect intracellular transport processes

As sequencing of the human genome nears completion, the genes that cause many human diseases are being identified and functionally described. This has revealed that many human diseases are due to defects of intracellular trafficking. This 'Toolbox' catalogs and briefly describes these diseases.

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.

Protein disulfide isomerase: the multifunctional redox chaperone of the endoplasmic reticulum

Protein disulfide isomerase (PDI) is a protein-thiol oxidoreductase that catalyzes the oxidation, reduction and isomerization of protein disulfides. In the endoplasmic reticulum PDI catalyzes both the oxidation and isomerization of disulfides on nascent polypeptides. Under the reducing condition of the cytoplasm, endosomes and cell surface. PDI catalyzes the reduction of protein disulfides. At those locations, PDI has been demonstrated to participate in the regulation of reception function, cell-cell interaction, gene expression, and actin filament polymerization. These activities of PDI will be discussed, as well as its activity as a chaperone and subunit of prolyl 4-hydroxylase and microsomal triglyceride transfer protein.

Abetalipoproteinemia caused by maternal isodisomy of chromosome 4q containing an intron 9 splice acceptor mutation in the microsomal triglyceride transfer protein gene

Uniparental disomy (UPD), a rare inheritance of 2 copies of a single chromosome homolog or a region of a chromosome from one parent, can result in various autosomal recessive diseases. Abetalipoproteinemia (ABL) is a rare autosomal recessive deficiency of apoB-containing lipoproteins caused by a microsomal triglyceride transfer protein (MTP) deficiency. In this study, we describe a patient with ABL inherited as a homozygous intron 9 splice acceptor G(-1)-to-A mutation of the transfer protein gene. This mutation alters the splicing of the mRNA, resulting in a 36 amino acids, in-frame deletion of sequence encoded by exon 10. We analyzed chromosome 4, including MTP gene (4q22-24), using short tandem repeat markers. The proband has only his mother's genes in chromosome 4q spanning a 150-centimorgan region; ie, segmental maternal isodisomy 4q21-35, probably due to mitotic recombination. Nonpaternity between the proband and his father was excluded using 6 polymorphic markers from different chromosomes (paternity probability, 0.999). Maternal isodisomy (maternal UPD 4q) was the basis for homozygosity of the MTP gene mutation in this patient.

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

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

The structure of vitellogenin provides a molecular model for the assembly and secretion of atherogenic lipoproteins

The assembly of atherogenic lipoproteins requires the formation in the endoplasmic reticulum of a complex between apolipoprotein (apo)B, a microsomal triglyceride transfer protein (MTP) and protein disulphide isomerase (PDI). Here we show by molecular modelling and mutagenesis that the globular amino-terminal regions of apoB and MTP are closely related in structure to the ancient egg yolk storage protein, vitellogenin (VTG). In the MTP complex, conserved structural motifs that form the reciprocal homodimerization interfaces in VTG are re-utilized by MTP to form a stable heterodimer with PDI, which anchors MTP at the site of apoB translocation, and to associate with apoB and initiate lipid transfer. The structural and functional evolution of the VTGs provides a unifying scheme for the invertebrate origins of the major vertebrate lipid transport system.

The acidic C-terminal domain of protein disulfide isomerase is not critical for the enzyme subunit function or for the chaperone or disulfide isomerase activities of the polypeptide

Protein disulfide isomerase (PDI) is a multifunctional polypeptide that acts as a subunit in the animal prolyl 4-hydroxylases and the microsomal triglyceride transfer protein, and as a chaperone that binds various peptides and assists their folding. We report here that deletion of PDI sequences corresponding to the entire C-terminal domain c, previously thought to be critical for chaperone activity, had no inhibitory effect on the assembly of recombinant prolyl 4-hydroxylase in insect cells or on the in vitro chaperone activity or disulfide isomerase activity of purified PDI. However, partially overlapping critical regions for all these functions were identified at the C-terminal end of the preceding thioredoxin-like domain a'. Point mutations introduced into this region identified several residues as critical for prolyl 4-hydroxylase assembly. Circular dichroism spectra of three mutants suggested that two of these mutations may have caused only local alterations, whereas one of them may have led to more extensive structural changes. The critical region identified here corresponds to the C-terminal alpha helix of domain a', but this is not the only critical region for any of these functions.

An MTP inhibitor that normalizes atherogenic lipoprotein levels in WHHL rabbits

Patients with abetalipoproteinemia, a disease caused by defects in the microsomal triglyceride transfer protein (MTP), do not produce apolipoprotein B-containing lipoproteins. It was hypothesized that small molecule inhibitors of MTP would prevent the assembly and secretion of these atherogenic lipoproteins. To test this hypothesis, two compounds identified in a high-throughput screen for MTP inhibitors were used to direct the synthesis of a highly potent MTP inhibitor. This molecule (compound 9) inhibited the production of lipoprotein particles in rodent models and normalized plasma lipoprotein levels in Watanabe-heritable hyperlipidemic (WHHL) rabbits, which are a model for human homozygous familial hypercholesterolemia. These results suggest that compound 9, or derivatives thereof, has potential applications for the therapeutic lowering of atherogenic lipoprotein levels in humans.

Collagen hydroxylases and the protein disulfide isomerase subunit of prolyl 4-hydroxylases

Prolyl 4-hydroxylases catalyze the formation of 4-hydroxyproline in collagens and other proteins with an appropriate collagen-like stretch of amino acid residues. The enzyme requires Fe(II), 2-oxoglutarate, molecular oxygen, and ascorbate. This review concentrates on recent progress toward understanding the detailed mechanism of 4-hydroxylase action, including: (a) occurrence and function of the enzyme in animals; (b) general molecular properties; (c) intracellular sites of hydroxylation; (d) peptide substrates and mechanistic roles of the cosubstrates; (e) insights into the development of antifibrotic drugs; (f) studies of the enzyme's subunits and their catalytic function; and (g) mutations that lead to Ehlers-Danlos Syndrome. An account of the regulation of collagen hydroxylase activities is also provided.

The use of a highly informative CA repeat polymorphism within the abetalipoproteinaemia locus (4q22-24)

Abetalipoproteinaemia is a rare autosomal-recessive disorder caused by a defect in the large subunit of the microsomal triglyceride transfer protein (MTP) which is required for the assembly and secretion of apolipoprotein B-containing lipoproteins. We report here the use of a polymorphic CA dinucleotide repeat in intron 10, MTPIVS10, of the large subunit of the human MTP protein in the analysis of a pregnancy in a consanguineous family, in which abetalipoproteinaemia was suspected, although prenatal diagnosis was subsequently refused. The mutation in the family has been identified as a novel four-nucleotide insertion/duplication of exon 17 between nucleotides 2349 and 2350 of the cDNA sequence of the MTP gene. However, the marker, MTPIVS10, can be used as an alternative to the time-consuming mutation detection techniques.

The enzymatic and non-enzymatic roles of protein-disulfide isomerase in apolipoprotein B secretion

Secretion of apolipoprotein B (apoB) from mammalian cells requires the presence of functional microsomal triglyceride transfer protein (MTP). We previously reported that co-expressing the human intestinal form of apoB, B48, with both subunits of human MTP in oleate-treated Sf21 cells led to a dramatic induction of B48 secretion. Deletion mutagenesis studies showed that the cysteine-enriched amino terminus of apoB was necessary for the MTP responsiveness (Gretch, D. G., Sturley, S. L., Wang, L., Dunning, A., Grunwald, K. A. A., Wetterau, J. R., Yao, Z., Talmud, P., and Attie, A. D. (1996) J. Biol. Chem. 271, 8682-8691). We therefore hypothesized that the small subunit of MTP, protein-disulfide isomerase (PDI), plays a role in apoB secretion by facilitating correct disulfide bond formation. To determine whether the enzymatic activities of PDI are important for MTP-stimulated apoB secretion, the wild type PDI subunit was replaced with an active site mutant, mPDI (Cys36 --> Ser/Cys380 --> Ser), lacking both disulfide shuffling and redox activities. MTP containing mPDI was fully functional in promoting apoB and triglyceride secretion. Therefore, the shufflase and redox activities of PDI are not necessary for the function of MTP. Since PDI exists in large molar excess over the other subunit of MTP, the role of free PDI (independent of the MTP complex) was investigated. PDI or mPDI was co-expressed with B48 and B17, a fragment encompassing the amino-terminal 17% of apoB. Mutant PDI significantly and specifically reduced the accumulation of the B17 and B48 both intracellularly and in the culture medium. The reduction was partially eliminated by the protease inhibitor N-acetyl-leucyl-leucyl-norleucinal, consistent with rapid co- or post-translational degradation of apoB in the presence of mPDI. Treating the cells with oleate reversed the effect of mPDI on B48 secretion in a dose-dependent manner, but had no effect on B17.

IN CONCLUSION

1) the role of PDI in the MTP complex involves functions other than its known enzymatic activities; 2) one or both of the enzymatic activities of free PDI is/are important for the MTP-independent steps of apoB secretion; 3) oleate can affect apoB secretion at high physiological concentrations and compensate for the insufficiency of PDI activities.

Apolipoprotein B binding to microsomal triglyceride transfer protein decreases with increases in length and lipidation: implications in lipoprotein biosynthesis

Microsomal triglyceride transfer protein (MTP), a heterodimer of 97 kDa and protein disulfide isomerase, is required for the assembly of apolipoprotein B (apoB)-containing triglyceride-rich lipoproteins. These proteins have been shown to interact with each other during early stages of lipoprotein biosynthesis. Our studies indicated that binding between apoB and heterodimeric MTP was of high affinity (Kd 10-30 nM) due to ionic interactions. In contrast to MTP, protein disulfide isomerase alone interacted very poorly with lipoproteins, indicating the importance of the heterodimer in these bindings. Preincubation of lipoproteins with detergents enhanced their interaction with MTP. Native VLDL bound poorly to MTP, but its preincubation with Tween-20 resulted in significantly increased binding to MTP. Furthermore, binding of LDL was enhanced by preincubation with taurocholate, indicating that partial delipidation of apoB-containing lipoproteins results in increased binding to MTP. Subsequently, attempts were made to study interactions between C-terminally truncated apoB polypeptides and MTP. Binding of all the polypeptides to MTP was enhanced in the presence of taurocholate. Comparisons revealed that the binding of different apoB polypeptides to MTP was in the order of apoB18 > apoB28 > apoB42 > apoB100. These studies indicated that optimum interactions occur between apoB18 and MTP, and that the increase in apoB length beyond apoB18 has a negative effect on these interactions. Since apoB18 does not assemble triglyceride-rich lipoproteins, these studies suggest that apoB may interact with MTP before its lipidation. It is proposed that steps in lipoprotein biosynthesis may be dictated by the sequential display of different functional domains on the apoB polypeptide.

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

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

Organ loci of catabolism of short truncations of apoB

Truncations of apolipoprotein (apo) B shorter than 3200 amino acids (3200/4536 = apoB-70) do not possess the LDL receptor-recognition domain and are not recognized by altered cells with normally functioning LDL receptors. To ascertain which organs remove such truncated apoB-containing particles, we isolated apoB-31-, apoB-38.9-, and apoB-43.7-containing particles from plasmas of familial hypobetalipoproteinemia heterozygous humans by a combination of sequential ultracentrifugation and preparative electrophoresis. Particles with labeled 125I- or 131I-dilactitol tyramine (I-DLT), were injected into New Zealand White rabbits, along with I-DLT-apoB-100-containing LDLs, and the decay of 125I- and 131I-TCA-precipitated counts was followed over 24 hours. At the end of 24 hours, rabbits were anesthetized and their bodies perfused. Organs were removed and homogenized, and TCA-precipitable counts determined. Fractional catabolic rates of apoB truncation particles were two to five times greater than those of apoB-100 LDLs. ApoB truncations accumulated in adrenals at one fifth the rates of apoB-100 LDL, compatible with the functional absences of LDL receptor-recognition domains in truncated apoBs. The major organ of uptake for apoB-100-LDLs was the liver, whereas truncation particles were readily removed by the kidney (kidney: liver uptake ratios were 0.10 to 0.30 for apoB-100 LDLs and 1.03 to 3.77 for truncations). Spleens accumulated little of either apoB-100 or truncation particles, suggesting particles were not "damaged" or aggregated. Thus, the absence of > 56% of the carboxyl end of apoB-100 increases the plasma clearance and redirects the organ uptake of the apoB truncation-containing lipoproteins from liver to kidney.

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.

A novel abetalipoproteinemia genotype. Identification of a missense mutation in the 97-kDa subunit of the microsomal triglyceride transfer protein that prevents complex formation with protein disulfide isomerase

The microsomal triglyceride transfer protein (MTP) is a heterodimer composed of the ubiquitous multifunctional protein, protein disulfide isomerase, and a unique 97-kDa subunit. Mutations that lead to the absence of a functional 97-kDa subunit cause abetalipoproteinemia, an autosomal recessive disease characterized by a defect in the assembly and secretion of apolipoprotein B (apoB) containing lipoproteins. Previous studies of abetalipoproteinemic patient, C.L., showed that the 97-kDa subunit was undetectable. In this report, [35S]methionine labeling showed that this tissue was capable of synthesizing the 97-kDa MTP subunit. Electrophoretic analysis showed two bands, one with a molecular mass of the wild type 97-kDa subunit and the other with a slightly lower molecular weight. Sequence analysis of cDNAs from additional intestinal biopsies showed this patient to be a compound heterozygote. One allele contained a perfect in-frame deletion of exon 10, explaining the lower molecular weight band. cDNAs of the second allele were found to contain 3 missense mutations: His297 --> Gln, Asp384 --> Ala, and Arg540 --> His. Transient expression of each mutant showed that only the Arg540 --> His mutant was non-functional based upon its inability to reconstitute apoB secretion in a cell culture system. The other amino acid changes are silent polymorphisms. High level coexpression in a baculovirus system of the wild type 97-kDa subunit or the Arg540 --> His mutant along with human protein disulfide isomerase showed that the wild type was capable of forming an active MTP complex while the mutant was not. Biochemical analysis of lysates from these cells showed that the Arg to His conversion interrupted the interaction between the 97-kDa subunit and protein disulfide isomerase. Replacement of Arg540 with a lysine residue maintained the ability of the 97-kDa subunit to complex with protein disulfide isomerase and form the active MTP holoprotein. These results indicate that a positively charged amino acid at position 540 in the 97-kDa subunit is critical for the productive association with protein disulfide isomerase. Of the 13 mutant MTP 97-kDa subunit alleles described to date, this is the first encoding a missense mutation.

An inhibitor of the microsomal triglyceride transfer protein inhibits apoB secretion from HepG2 cells

The microsomal triglyceride (TG) transfer protein (MTP) is a heterodimeric lipid transfer protein that catalyzes the transport of triglyceride, cholesteryl ester, and phosphatidylcholine between membranes. Previous studies showing that the proximal cause of abetalipoproteinemia is an absence of MTP indicate that MTP function is required for the assembly of the apolipoprotein B (apoB) containing plasma lipoproteins, i.e., very low density lipoproteins and chylomicrons. However, the precise role of MTP in lipoprotein assembly is not known. In this study, the role of MTP in lipoprotein assembly is investigated using an inhibitor of MTP-mediated lipid transport, 2-[1-(3, 3-diphenylpropyl)-4-piperidinyl]-2,3-dihydro-1H-isoindol-1-o ne (BMS-200150). The similarity of the IC50 for inhibition of bovine MTP-mediated TG transfer (0.6 microM) to the Kd for binding of BMS-200150 to bovine MTP (1.3 microM) strongly supports that the inhibition of TG transfer is the result of a direct effect of the compound on MTP. BMS-200150 also inhibits the transfer of phosphatidylcholine, however to a lesser extent (30% at a concentration that almost completely inhibits TG and cholesteryl ester transfer). When BMS-200150 is added to cultured HepG2 cells, a human liver-derived cell line that secretes apoB containing lipoproteins, it inhibits apoB secretion in a concentration dependent manner. These results support the hypothesis that transport of lipid, and in particular, the transport of neutral lipid by MTP, plays a critical role in the assembly of apoB containing lipoproteins.

Microsomal triacylglycerol transfer protein prevents presecretory degradation of apolipoprotein B-100. A dithiothreitol-sensitive protease is involved

The role of microsomal triacylglycerol transfer protein (MTP) in the secretion of apolipoprotein B-100 (apoB-100) has been studied using an inhibitor of MTP: 4'-bromo-3'-methylmetaqualone. In vitro, this compound inhibits trioleoylglycerol transfer between lipid vesicles mediated by MTP with an IC50 of 0.9 microM whereas it does not inhibit the lipid transfer mediated by the cholesteryl ester transfer protein. In HepG2 cells, 4'-bromo-3'-methylmetaqualone inhibits the secretion of apoB-100 with an IC50 of 0.3 microM, without affecting the secretion of several other proteins like apoA-I or albumin. Moreover, there is no accumulation of apoB-100 in treated cells. Oleic acid, which increases apoB-100 secretion, only slightly modifies the IC50 of 4'-bromo-3'-methylmetaqualone (0.5 microM). The latter has no effect on the synthesis of major lipids within the cell, but decreases the secretion of triacylglycerol into apoB-100-containing lipoproteins. Pulse/chase experiments reveal that 4'-bromo-3'-methylmetaqualone acts on apoB-100 production either at the co-translational or post-translational level. The cysteine protease inhibitor N-acetyl-leucyl-leucyl-norleucinal does not protect apoB-100 from the 4'-bromo-3'-methylmetaqualone effect but seems to be involved in a later step of apoB-100 intracellular degradation. By contrast, dithiothreitol can totally reverse the effect of the MTP inhibitor on apoB-100 production. The mechanism of MTP-mediated lipid assembly with apoB-100 is discussed.

The role of protein disulphide isomerase in the microsomal triacylglycerol transfer protein does not reside in its isomerase activity

The microsomal triacylglycerol transfer protein (MTP), an alpha beta dimer, is obligatory for the assembly of apoB-containing lipoproteins in liver and intestinal cells. The beta subunit is identical with protein disulphide isomerase, a 58 kDa endoplasmic reticulum luminal protein involved in ensuring correct disulphide bond formation of newly synthesized proteins. We report here the expression of the human MTP subunits in Spodoptera frugiperda cells. When the alpha subunit was expressed alone, the polypeptide formed insoluble aggregates that were devoid of triacylglycerol transfer activity. In contrast, when the alpha and beta subunits were co-expressed, soluble alpha beta dimers were formed with significant triacylglycerol transfer activity. Expression of the alpha subunit with a mutant protein disulphide isomerase polypeptide in which both -CGHC- catalytic sites had been inactivated also yielded alpha beta dimers that had comparable levels of lipid transfer activity relative to wild-type dimers. The results indicate that the role of the beta subunit in MTP seems to be to keep the alpha subunit in a catalytically active, non-aggregated conformation and that disulphide isomerase activity of the beta subunit is not required for this function.

The amino terminus of apolipoprotein B is necessary but not sufficient for microsomal triglyceride transfer protein responsiveness

Human apolipoprotein (apo) B mediates the formation of neutral lipid-containing lipoproteins in the liver and intestine. The association of apoB with lipid is thought to be promoted by the microsomal triglyceride transfer protein complex. We have reconstituted lipoprotein assembly in an insect cell line that normally does not support this process. Expression of human microsomal triglyceride transfer protein (MTP) and apolipoprotein B48 (apoB48) together enabled Sf-21 insect cells to secrete approximately 60-fold more lipoprotein-associated triacylglycerol than control cells. This dramatic effect demonstrates that effective partitioning of triacylglycerol into the secretory pathway requires an endoplasmic reticulum-associated neutral lipid transporter (provided by MTP) and an apolipoprotein to shuttle the lipid through the pathway. Expression of the human apoB48 gene in insect cells resulted in secretion of the protein product. Including both MTP subunits with apoB48 and oleic acid specifically increased apoB48 secretion 8-fold over individual subunits alone. To assess whether specific regions of apoB are necessary for MTP responsiveness, nine apoB segments were expressed. These included NH2-terminal segments as well as internal and COOH-terminal regions of apoB fused with a heterologous signal sequence. ApoB segments containing the NH2-terminal 17% of the protein were secreted and responded to MTP activity; however, a segment containing only the NH2-terminal 17% of the protein was not significantly responsive to MTP. Segments lacking the NH2 terminus were not MTP-responsive, and five of six of these proteins were trapped intracellularly but, in certain cases, could be rescued by fusion to apoB17. These results suggest that the NH2 terminus of apoB is necessary but not sufficient for MTP responsiveness.