The apo E-system: genetic control of plasma lipoprotein concentration.
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Alleles↗
Biomedical subjects
G Utermann
Publications and source records attributed to G Utermann.
Apolipoprotein E3-Leiden. A new variant of human apolipoprotein E associated with familial type III hyperlipoproteinemia.
A variant of apolipoprotein E, denoted apo E3-Leiden, has been identified in a 41-year-old male suffering from type III hyperlipoproteinemia with xanthomatosis. Apo E3-Leiden focus in the E3 position. In contrast with normal apo E3, apo E3-Leiden is defective in binding to the low density lipoprotein (LDL) receptor and does not contain cysteine as evaluated by cysteamine treatment of very low density lipoprotein followed by isoelectric focusing and conventional protein staining and by amino acid analysis. On sodium dodecyl sulfate polyacrylamide gel electrophoresis, apo E3-Leiden displays an electrophoretic mobility intermediate to that of normal apo E3 and apo E2 (Arg158----Cys). The mother and four siblings of the proband also have apo E3-Leiden and hyperlipoproteinemia type III; three of them with xanthomatosis. Two siblings do not show apo E3-Leiden in their VLDL fraction and do not have hyperlipoproteinemia type III. In the VLDL fractions of all affected family members only the presence of apo E3-Leiden could be detected after cysteamine treatment and isoelectric focusing followed by conventional protein staining. However, isoelectric focusing of cysteamine-treated sera followed by immunoblotting, using anti-apo E antiserum as first antiserum, demonstrates the presence of low amounts of normal apo E3 in addition to apo E3-Leiden in serum of the affected family members. These results indicate that all affected family members are heterozygotes E3/E3-Leiden and suggest that in this family type III hyperlipoproteinemia is transmitted as a dominant trait.
Adult↗
Schnyder's dystrophy. Progression and metabolism.
In the first long-term cohort study of Schnyder's corneal dystrophy the authors examined affected and unaffected members of two unrelated families in 1975 and 1976 respectively, and again in 1984. They identified carriers, catalogued changes in the diffuse and crystalline corneal opacities which characterize this dystrophy and analysed the patient's lipid metabolism. Corneal opacities never regressed. Progression was more frequent in diffuse than in crystalline opacities. Both crystalline and diffuse opacities reappeared and progressed following penetrating keratoplasty. Mean cholesterol levels in the carrier group were above normal and six had a moderate type IIa dyslipoproteinemia; conversely, two carriers had low apo B. The degree of corneal opacification showed no relationship to dyslipoproteinemia. Schnyder's corneal dystrophy appears to involve the corneal lipid metabolism only and not to be a systemic disease.
Adolescent↗
Apolipoprotein E polymorphism in the Finnish population: gene frequencies and relation to lipoprotein concentrations.
ApoE phenotypes were determined in 615 unrelated Finnish individuals. The apoE gene frequencies observed (epsilon 2, 0.041; epsilon 3, 0.733; epsilon 4, 0.227) differ significantly from those in other populations. The frequency of the allele epsilon 2 was lower and that of epsilon 4 higher than in all other studied populations. Plasma lipids and apolipoproteins A-I, A-II and B were recorded in 207 of the typed subjects. By comparison with the most frequent homozygous apoE 3/3 phenotype, it was found that total cholesterol, LDL-cholesterol, and apoB concentrations were all markedly higher in apoE 4/4 and to a lesser degree in apoE 4/3 phenotypic groups. On the other hand, these lipid and apolipoprotein levels tended to be lower in E-2 heterozygotes. These data confirm and extend, in a different ethnic group, previous results of an effect of apoE genes on plasma lipoprotein concentrations. The data suggest that the apoE gene locus may be one factor responsible for the high LDL cholesterol concentrations in the Finnish population.
Alleles↗
Abnormal interaction of the human apolipoprotein A-I variant [Lys107----0] with high density lipoproteins.
Several isoforms of apoprotein A-I [apoA-I], the major apoprotein of high density lipoproteins [HDL], have been described. We compared the in vivo and in vitro properties of normal human apoA-I with those of apoA-I [Lys107----0]. Fluorescence and circular dichroic spectra showed that deletion of Lys107 decreases apoprotein self-association. In vivo metabolic studies in the rat indicated that the interaction of apoA-I [Lys107----0] with HDL was lower than normal. We conclude that deletion of Lys107 results in a reorganization of the apoprotein structure that decreases its potential to form hydrophobic associations.
Animals↗
Activation of phosphatidylcholine-sterol acyltransferase by human apolipoprotein E isoforms.
The reaction catalysed by phosphatidylcholine-sterol acyltransferase (EC 2.3.1.43) is believed to be the major source of cholesteryl ester in human plasma; the enzyme requires a protein activator. Several human apolipoproteins were found to exhibit an activator function, the major one being apolipoprotein A-I. Human apolipoprotein E exists in the population mainly in three different genetic isoforms; apolipoprotein E-2, E-3 and E-4. These isopeptides were isolated from subjects homozygous for one of the isoforms, incorporated into phospholipid/cholesterol/[14C]cholesterol complexes by the cholate dialysis procedure and used to measure capacity to activate phosphatidylcholine-sterol acyltransferase in comparison to apolipoprotein A-I lipid substrate particles prepared by the same procedure. Acyltransferase activity was measured by the formation of [14C]cholesteryl ester from [14C]cholesterol using purified enzyme. With egg yolk phosphatidylcholine as acyl donor, apo E was 15-19% as efficient as apolipoprotein A-I for activation of the acyltransferase. Apo-E-stimulated cholesteryl ester formation by the enzyme was enhanced when 1-oleoyl-2-palmitoyl-glycerophosphocholine was used as a substrate phospholipid (45% of apo A-I/phosphatidylcholine control) and most pronounced with dimyristoylglycerophosphocholine (75% of apo A-I/phosphatidylcholine control). No significant difference in activation was found between apo E isoforms. It is concluded that apolipoprotein E activates phosphatidylcholine-sterol acyltransferase in vitro and that apolipoprotein E isoforms are similarly effective.
Apolipoproteins E↗
Activation of lecithin: cholesterol acyltransferase by human apolipoprotein A-IV.
Human plasma apoproteins (apo) A-I and A-IV both activate the enzyme lecithin:cholesterol acyltransferase (EC 2.3.1.43). Lecithin:cholesterol acyltransferase activity was measured by the conversion of [4-14C] cholesterol to [4-14C]cholesteryl ester using artificial phospholipid/cholesterol/[4-14C]cholesterol/apoprotein substrates. The substrate was prepared by the addition of apoprotein to a sonicated aqueous dispersion of phospholipid/cholesterol/[4-14C]cholesterol. The activation of lecithin:cholesterol acyltransferase by apo-A-I and -A-IV differed, depending upon the nature of the hydrocarbon chains of the sn-L-alpha-phosphatidylcholine acyl donor. Apo-A-I was a more potent activator than apo-A-IV with egg yolk lecithin, L-alpha-dioleoylphosphatidylcholine, and L-alpha-phosphatidylcholine substituted with one saturated and one unsaturated fatty acid regardless of the substitution position. When L-alpha-phosphatidylcholine esterified with two saturated fatty acids was used as acyl donor, apo-A-IV was more active than apo-A-I in stimulating the lecithin:cholesterol acyltransferase reaction. Complexes of phosphatidylcholines substituted with two saturated fatty acids served as substrate for lecithin:cholesterol acyltransferase even in the absence of any activator protein. Essentially the same results were obtained when substrate complexes (phospholipid-cholesterol-[4-14C]cholesterol-apoprotein) were prepared by a detergent dialysis procedure. Apo-A-IV-L-alpha-dimyristoylphosphatidylcholine complexes thus prepared were shown to be homogeneous particles by column chromatography and density gradient ultracentrifugation. It is concluded that apo-A-IV is able to facilitate the lecithin:cholesterol acyltransferase reaction in vitro.
Apolipoprotein A-I↗
Apolipoprotein E mutants, hyperlipidemia and arteriosclerosis.
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Amino Acid Sequence↗
Apolipoprotein A-IGiessen (Pro143----Arg). A mutant that is defective in activating lecithin:cholesterol acyltransferase.
Apolipoprotein A-IGiessen is a variant form of apo A-I that is displaced from the corresponding normal A-I isoforms on isoelectric focusing gels by a single charge unit towards the cathode [Utermann et al. (1982) J. Biol. Chem. 257, 501-507]. Three subjects heterozygous for the variant were detected in one family. The percentage of the total A-I in plasma represented by the A-IGiessen in these subjects ranged over 25-30%. The variant and normal major A-I isoforms from the proband (Y.J.) were purified by preparative isoelectric focusing and cleaved with CNBr. Analytical focusing of CNBr fragments demonstrated a charge difference between CB3Giessen and normal CB3. Sequence analysis of CB3Giessen revealed that a proline existing in normal A-I was replaced by an arginine in the variant A-I at residue 143. The ability of the mutant A-I to activate purified lecithin:cholesterol acyltransferase was determined in vitro. The cofactor activity of [Arg143]apolipoprotein A-I was about 60-70% of that demonstrated by control A-I. Residue 143 is in a putative beta-turn between two of the repeating amphiphilic helices in apolipoprotein A-I and may be a critical determinant of the protein's structure and function.
Amino Acid Sequence↗
Abnormal lecithin:cholesterol acyltransferase activation by a human apolipoprotein A-I variant in which a single lysine residue is deleted.
An apolipoprotein (apo) A-I variant that has a relative charge of -1 compared to normal apo-A-I on isoelectric focusing gels has been identified in five unrelated families as a result of screening a large number of individuals. The cause of the electrophoretic abnormality has been examined by analyzing the variant apo-A-I structure. The evidence suggests that a single amino acid, lysine 107, has been deleted in the variant apo-A-I of all affected individuals studied from these families, with the remainder of the variant apo-A-I sequence being unaffected. The deletion of this single basic amino acid residue is sufficient to account for the charge difference between the variant and normal apo-A-I as seen on isoelectric focusing gels. This variant, previously referred to as A-I-Marburg or A-I-Münster-2, can now be designated by the structural abnormality apo-A-I(Lys107----0). The evidence from extensive pedigree analysis suggests the likelihood that the deletion mutant gene is allelic to the normal apo-A-I gene. At the same time, the kindred analyses have failed to yield a lipid abnormality that can be unequivocally related to the presence of this deletion mutant of apo-A-I. However, all subjects expressing apo-A-I(Lys107----0) also express normal apo-A-I, so that any abnormality caused by the variant apo-A-I might be adequately compensated for by the normal apo-A-I. To examine directly the functional consequence of the lysine deletion, the isolated variant was tested in vitro for its ability to activate lecithin:cholesterol acyltransferase, the principal cholesterol-esterifying enzyme in plasma. It was found that apo-A-I(Lys107----0) is deficient in its ability to activate lecithin:cholesterol acyltransferase, having only 40-60% of the cofactor activity of normal apo-A-I. The cofactor activity of the pro-apo-A-I component of the variant was also reduced to about 60% of either normal A-I or normal pro-apo-A-I. The functional defect is probably related to a disruption in the secondary and/or tertiary structure of the protein caused by the deletion of lysine 107 in the primary structure.
Adult↗
Mutations of apolipoprotein AI can affect cofactor function for cholesteryl ester formation by lecithin: cholesterol acyltransferase.
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Apolipoprotein A-I↗
Apolipoprotein E phenotypes and hyperlipidemia.
Apolipoprotein E phenotypes were determined in 361 patients with hyperlipidemia and in controls. The E2 isoform was significantly more frequent in the group of hyperlipidemics (P less than 0.0005). This was not due to a higher frequency of E-2/2 homozygotes with type III hyperlipoproteinemia, but rather to a significantly higher frequency of E2 heterozygotes (P less than 0.0005). Subgrouping of hyperlipidemics into patients with a) hypertriglyceridemia, b) hypercholesterolemia and c) mixed hyperlipidemia revealed i) that isoform E2 was significantly more frequent in patients with hypertriglyceridemia (0.001 greater than P greater than 0.005), ii) that isoform E4 was significantly more frequent in patients with hypercholesterolemia (0.01 greater than P greater than 0.005) and iii) that isoforms E2 (P less than 0.005) and E4 (0.05 greater than P greater than 0.025) were both more frequent in patients with mixed hyperlipidemia. Roughly 20% of patients with mixed hyperlipidemia had one of the rare phenotypes E-4/4, -4/2 or -2/2. We conclude that alleles epsilon 2 and epsilon 4 both contribute to the susceptibility for, and/or phenotypic expression of hyperlipidemia. Whereas the gene epsilon 2 seems to exert its influence on plasma lipoproteins by an abnormal gene product (E2) that has reduced binding activity to lipoprotein receptors, the mechanism underlying the association of the epsilon 4 gene with hyperlipidemia is presently unclear.
Apolipoproteins↗
Apolipoprotein E phenotypes in patients with myocardial infarction.
The frequencies of genetic apo E isoforms E2, E3 and E4 were determined in 523 patients with myocardial infarction and compared to those in a control group (1031 blood donors). A significant difference in the frequency of apo E4 was noted between patients and controls (0.05 greater than P greater than 0.025). No differences in the frequencies of isoforms E3 and E2 were observed. In particular, there was no significant difference between the two groups in the frequency of apo E2 homozygosity, a condition that is associated with type III hyperlipoproteinemia. However, all E2 homozygote survivors of myocardial infarction had hyperlipoproteinemia type III (cholesterol 269 +/- 29 mg/dl; triglyceride 419 +/- 150 mg/dl; age 54 +/- 14 years; N = 5). On the contrary, E2 homozygote controls (all apo E-2/2 blood donors and their apo E-2/2 relatives who were from the same age range as the patients) had primary dysbetalipoproteinemia but normal or subnormal plasma cholesterol concentrations (cholesterol 184 +/- 28 mg/dl; triglyceride 151 +/- 52 mg/dl; age 56 +/- 13 years; N = 11). This indicates that E2 homozygotes with hyperlipoproteinemia type III who occur rarely in the population but comprise about 1% of myocardial infarction patients have a markedly increased risk for coronary atherosclerosis, whereas the risk for E2 homozygotes with normal or subnormal plasma cholesterol (= primary dysbetalipoproteinemia) may be considerably lower than for the general population. The data illustrate the complex relationship between apo E genes, lipid levels, and risk for atherosclerosis.
Adult↗
The conformation of apolipoprotein E isoforms in phospholipid complexes and their interaction with human Hep G2 cells.
The human hepatoma cell line, Hep G2, has been used to compare the metabolism by isolated liver cells of purified isoforms of human apolipoprotein E (apo E). Complexes of [125I]apo E-3/3, 2/2, 3/2 and 4/3 with dimyristoyl phosphatidylcholine (DMPC) were prepared by a detergent-dialysis method: discoidal, bilayer complexes with a stoichiometry of 125 +/- 15 mol DMPC/mol apo E resulted. The predominant phenotype apo E-3/3, and the phenotype apo E-2/2 characteristic of patients with Type III hyperlipoproteinemia, interact similarly with DMPC and adopt the same conformation with 60-70% alpha-helix, as monitored by circular dichroism spectroscopy. The uptake and degradation at 37 degrees C, and binding at 4 degrees C by Hep G2 cells, of [125I]apo E-3/3/DMPC and [125I]apo E-2/2/DMPC complexes were compared. Apo E-3/3 was degraded more rapidly than apo E-2/2 suggesting that the diminished catabolism of the latter phenotype by intact livers is due to lack of recognition by the hepatocytes. The observed degradation of apo E was 3-4 times greater than that which could be attributed to fluid phase endocytosis and low-affinity adsorptive endocytosis. The degradation of [125I]apo A-I by Hep G2 cells can be accounted for by the above endocytotic mechanisms. The distinction between apo E-3/3 and apo E-2/2 isoforms is attributed to the presence of a cell-surface receptor on Hep G2 cells which binds apo E-3/3 with a higher affinity than apo E-2/2.
Apolipoproteins↗
Genetic polymorphism of apolipoprotein E: a variant form of apolipoprotein E2 distinguished by sodium dodecyl sulfate--polyacrylamide gel electrophoresis.
The apolipoprotein E2 ( apoE2 ) variant that possesses a cysteine substituted for an arginine at residue 158 in the amino acid sequence E2( Arg158 ----Cys) can be distinguished by sodium dodecyl sulfate-polyacrylamide gel electrophoresis from other forms of apoE, including E3 (the parent form), E4( Cys112 ----Arg), E2( Arg145 ----Cys), and E2( Lys146 ----Gln). The E2( Arg158 ----Cys) migrates as a distinctly separable band with a higher apparent molecular weight than the other forms. Chemical modification of apoE2 ( Arg158 ----Cys) with sulfhydryl reagents (2-bromoethyl)-trimethylammonium bromide or cysteamine, which convert cysteine to arginyl or lysyl analogues, respectively, abolishes the difference in apparent molecular weight and results in the co-electrophoresis of E2( Arg158 ----Cys) with other apoE forms. The mobilities of the other apoE variants are not affected by these modifications. These results suggest that the substitution site at residue 158 is a key location, important in modifying the behavior of apoE and in modulating its apparent molecular weight on sodium dodecyl sulfate-polyacrylamide gels. Furthermore, the technique used in this study may be very helpful in distinguishing specific mutant forms of apoE2 .
Amino Acid Sequence↗
Binding and partial inactivation of Staphylococcus aureus alpha-toxin by human plasma low density lipoprotein.
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Bacterial Toxins↗
Protein composition of Lp(a) lipoprotein from human plasma.
The apolipoprotein composition of purified human Lp(a) lipoprotein was investigated by SDS--polyacrylamide gel electrophoresis and immunochemically. The lipoprotein contains two different polypeptides. One is identical by its app. Mr of approximately 250 000 and immunologically with apolipoprotein B of LDL (B-100). The other polypeptide has a higher app. Mr (approximately 350 000) and stains strongly with the periodate-Schiff's reagent. This high-Mr glycoprotein contains the specific Lp(a) immunoreactivity but does not react with antibodies against apo B. Apo B and Lp(a)-protein seem to be linked by disulfide bonds in the native lipoprotein. The unreduced detergent delipidized protein moiety from Lp(a) lipoprotein shows a single band of Mr approximately 700 000 in SDS--polyacrylamide gel electrophoresis and the immunoprecipitates formed against anti-Lp(a) and anti-apo B by the unreduced protein show a reaction of immunological identity.
Apolipoproteins↗
Myelomatosis with type III hyperlipoproteinemia: clinical and metabolic studies.
We investigated the metabolism of intermediate-density lipoproteins (IDL [1.006 to 1.019 g per milliliter]) and low-density lipoproteins (LDL [1.019 to 1.063 g per milliliter]) in two men with Type III hyperlipoproteinemia associated with myelomatosis. In vivo kinetic studies using radiolabeled autologous lipoproteins demonstrated a greatly reduced fractional catabolic rate of IDL, relative to control values (patients vs. normal, 0.006 and 0.025 per hour vs. 0.20 +/- 0.08 per hour [mean +/- S.E.M]) and a greatly prolonged IDL-to-LDL conversion time (45 and 17 hours vs. 5.4 +/- 1.6 hours). In studies in vitro, LDL from both patients failed to bind to the LDL receptor of normal blood lymphocytes, whereas LDL from subjects with familial Type III hyperlipoproteinemia bound normally to the receptor. In one patient immunoglobulin was shown to be associated with IDL and LDL. Thus, hyperlipoproteinemia reflected an impaired metabolism of IDL, probably secondary to the binding of immunoglobulin to the lipoproteins. A similar impairment of receptor-mediated LDL catabolism did not elevate the plasma LDL concentration because of the low IDL-to-LDL conversion rate.
Apoproteins↗