Abstract-Microsomal triglyceride transfer protein (MTP) is required for the assembly and cellular secretion of apolipoprotein B (apoB) -containing lipoproteins from the liver and intestine. The secretion pattern of apoB-containing lipoproteins is likely to influence the VLDL and LDL levels in plasma. By initial opportunistic screening for polymorphic sites in the regulatory region of the MTP gene by gene sequencing in 20 healthy male subjects, a common functional G/T polymorphism was detected 493 bp upstream from the transcriptional start point. There was differential binding of unique nuclear proteins at this site, as shown by electrophoretic mobility shift assay. The G variant seemed to bind two or three nuclear proteins that do not bind to the T variant. Expression studies with minimal promoter constructs linked to the chloramphenicol acetyltransferase reporter and transfected into HepG2 cells revealed marked enhancement of transcriptional activity with the T variant. The prevalence of the MTP promoter genotypes was investigated in a group of 184 healthy, middle-aged white men; the frequency of homozygosity for the MTP Ϫ493 T variant was .06 and the allele frequency of MTP Ϫ493T was .25 in the population. These homozygous subjects had a 22% lower LDL cholesterol concentration than did heterozygotes or subjects homozygous for the MTP Ϫ493 1 Functional MTP is an absolute requirement for the assembly and cellular secretion of apoB-containing lipoproteins. Cells that normally do not secrete apoB can acquire this competence if the genes encoding apoB and MTP are provided by gene transfection, as shown in HeLa cells and COS-1 cells.2,3 Conversely, if MTP activity is inhibited in cells that normally do secrete apoB-containing lipoproteins, the secretion of apoB is drastically reduced.4,5 A complete lack of MTP activity leads to abetalipoproteinemia, 6 a disease caused by mutations in the coding region of the MTP gene. 7-9The promoter region of the MTP gene is highly conserved between species and shows signs of both cell type-specific expression and response to metabolic regulators. The activity of the human MTP promoter is suppressed by insulin and enhanced by cholesterol. 10 The insulin response has been confirmed in HepG2 cells.11 It has also been shown that hamsters fed either a high-fat or a cholesterol-enriched diet have higher concentrations of MTP mRNA.Against this background we hypothesized that genetic variation in MTP expression might influence the plasma concentrations of apoB-containing lipoproteins in humans. We report herein a common functional polymorphism in the promoter region of the MTP gene, of which the rarer allele is linked to low plasma LDL cholesterol concentrations. Methods Human SubjectsA total of 184 healthy white men, aged 30 to 45 years, were recruited at random from a register containing all permanent residents of the Stockholm metropolitan area (response rate of 70%). Men with documented coronary heart disease or any other chronic disease were excluded. The mean age of the study group ...

We investigated the role the MTP Ϫ493G/T gene polymorphism in determining the apoB-100 secretion pattern and LDL heterogeneity in healthy human subjects. Groups of carriers of the T and the G variants (n ϭ 6 each) were recruited from a cohort of healthy 50-yr-old men. Kinetic studies were performed by endogenous [ 2 H3]leucine labeling of apoB and subsequent quantification of the stable isotope incorporation. apoB production rates, metabolic conversions, and eliminations were calculated by multicompartmental modeling (SAAM-II). LDL subfraction distribution was analyzed in the entire cohort (n ϭ 377). Carriers of the MTP Ϫ493T allele had lower plasma LDL apoB and lower concentration of large LDL particles [LDL-I: 136 Ϯ 57 (TT) vs. 175 Ϯ 55 (GG) mg/l, P Ͻ 0.01]. Kinetic modeling suggested that MTP Ϫ493T homozygotes had a 60% lower direct production rate of intermediate-density lipoprotein (IDL) plus LDL compared with homozygotes for the G allele (P Ͻ 0.05). No differences were seen in production rates of large and small VLDL, nor were there any differences in metabolic conversion or elimination rates of apoB between the genotype groups. This study shows that a polymorphism in the MTP gene affects the spectrum of endogenous apoB-containing lipoprotein particles produced in humans. Reduced direct production of LDL plus IDL appears to be related to lower plasma concentrations of large LDL particles.intermediate-density lipoprotein; low-density lipoprotein; apolipoprotein B; very-low-density lipoprotein secretion; lipidation; microsomal triglyceride transfer protein THE ASSEMBLY AND SECRETION of apolipoprotein B (apoB)-containing lipoproteins by the human liver is a complex process that is regulated at different levels by a variety of factors such as substrate availability, insulin concentration, and the activity or abundance of the proteins involved. It has been postulated that a major determinant of the lipoprotein heterogeneity observed in plasma is due to the heterogeneity in the production rate of apoB-containing lipoprotein (26). Increased influx of nonesterified fatty acids (NEFA) and uptake of triglycerides contained in remnant lipoproteins promotes the secretion of large VLDL particles by the hepatocyte. This process is rapidly and significantly inhibited by insulin, which also reduces the substrate availability of NEFA (2). As such, a direct inhibitory effect on the secretion process of VLDL has also been described in vitro and in vivo (12,25). Little is known of the mechanisms regulating the synthesis and secretion of the smaller apoB-containing lipoproteins small VLDL, intermediate-density lipoprotein (IDL), or even LDL. Neither substrate availability nor insulin appears to regulate their production (25). Rather, mechanisms intrinsic to the hepatocyte may determine the production of small apoB-containing lipoproteins, but very little is known of the genetic regulation of apoB secretion. The activity state of the microsomal triglyceride transfer protein (MTP) may provide such a mechanism, due to its strateg...

The microsomal triglyceride transfer protein (MTP) is required for the assembly and secretion of apolipoprotein B (apoB)-containing lipoproteins from liver and intestine. We set out to study the phenotypic modulation of all common genetic variants in the MTP gene. In addition, we aimed at characterizing the association between the various polymorphisms. A total of 564 healthy men were genotyped for the MTP ؊ 493 G/T, ؊ 400 A/T, and ؊ 164 T/C promoter polymorphisms, as well as the Q/H 95, I/T 128, Q/E 244, and H/Q 297 missense polymorphisms. The ؊ 493 G/T, ؊ 164 T/C, and I/T 128 polymorphisms showed to be in almost complete linkage disequilibrium. Subjects homozygous for the less common ؊ 493 T, ؊ 164 C, and T 128 alleles showed significantly lower plasma total and LDL cholesterol levels and plasma LDL apoB levels, and also significantly higher body mass index (BMI) and plasma insulin levels compared with carriers of the common alleles. The associations between plasma total cholesterol and MTP ؊ 493 genotype was verified in a cohort consisting of 1,117 disease-free control subjects of the West of Scotland Coronary Prevention Study (WOSCOPS). None of the other polymorphisms showed any significant change in either lipid and lipoprotein levels or anthropometric variables. In summary, two promoter polymorphisms and one missense polymorphism in the MTP gene alter plasma total and LDL cholesterol levels, plasma LDL apoB levels, BMI, and insulin levels. This may, in turn, have implications for genetic regulation of cardiovascular risk factors. -Ledmyr, H., F. Karpe, B. Lundahl, M. McKinnon, C. Skoglund-Andersson, and E. Ehrenborg. Variants of the microsomal triglyceride transfer protein gene are associated with plasma cholesterol levels and body mass index.

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