Translational and post-translational processing in apolipoprotein metabolism.
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Apolipoproteins↗
Biomedical subjects
R E Gregg
Publications and source records attributed to R E Gregg.
Identification of an apoC-II variant (apoC-IIBethesda) in a kindred with apoC-II deficiency and type I hyperlipoproteinemia.
Apolipoprotein (apo) C-II deficiency is characterized by elevated plasma triglycerides, chylomicrons, and very low density lipoproteins, as well as reduced levels of low density and high density lipoproteins. A subject with apoC-II deficiency has been identified with an apoC-II plasma level of less than 0.05 mg/dl. The plasma apoC-II in the proband was immunochemically similar to apoC-II in normal subjects when analyzed by Ouchterlony immunodiffusion, however the apoC-II had an apparently lower molecular weight and higher pI when analyzed by two-dimensional gel electrophoresis. This apoC-II variant, designated apoC-IIBethesda, was not affected by neuraminidase treatment or reduction. Two-dimensional gel electrophoresis of the plasma of the mother of the proband revealed both normal apoC-II and apoC-IIBethesda, whereas analysis of the father and two siblings revealed apoC-II of normal electrophoretic mobility. These results were interpreted as indicating that the proband was a compound heterozygote with one allele for apoC-IIBethesda inherited from the mother and an allele coding for an abnormality which results in the virtual or complete absence of plasma apoC-II from the father. This proband represents the first example of a compound heterozygote for an apolipoprotein defect associated with a dyslipoproteinemia.
Apolipoprotein C-II↗
Studies on the mechanism of hypertriglyceridemia in Tangier disease. Determination of plasma lipolytic activities, k1 values and apolipoprotein composition of the major lipoprotein density classes.
Mechanisms responsible for hypertriglyceridemia in Tangier disease were elucidated by an analysis of the plasma post-heparin lipolytic activities and the structural and metabolic properties of very low (VLDL) and low (LDL) density lipoproteins. The levels of lipoprotein lipase activity in six Tangier patients were significantly lower (P less than 0.001) than in 40 control subjects (8.1 +/- 3.3 (+/- S.D.) vs. 14.1 +/- 3.7 units/ml). In contrast, the levels of hepatic triacylglycerol lipase were higher (P less than 0.01) than in normal controls (14.4 +/- 3.9 vs. 9.3 +/- 4.0 units/ml). Because kinetic parameters such as Km or Vmax cannot be obtained with naturally occurring triacylglycerol-rich lipoproteins, the pseudo-first-order rate constant (k1) of triacylglycerol hydrolysis was used to assess the effectiveness of triacylglycerol-rich lipoproteins as substrates for lipoprotein lipase. The k1 values for Tangier VLDL (k1 = 0.017 +/- 0.002 min-1) were significantly lower (P less than 0.001) than the k1 values (0.036 +/- 0.008 min-1) for control VLDL. Both the Tangier and control LDL2 are similar in their resistance to the action of lipoprotein lipase, as shown by their low k1 values (0.002 +/- 0.001 and 0.001 +/- 0.001 min-1, respectively). The major compositional difference between the lipoproteins of Tangier disease and normal subjects was a significant increase in the percent content of apolipoprotein A-II in all lipoprotein particles with d less than 1.063 g/ml, with the greatest increase occurring in VLDL and the lowest in LDL2. These results were interpreted as indicating that, in Tangier disease, there is a lower reactivity of VLDL with lipoprotein lipase which may in part be attributed to the abnormal apolipoprotein composition. This finding, in conjunction with the reduced levels of lipoprotein lipase activity, may explain the hypertriglyceridemia in Tangier disease.
Adult↗
Semi-automated enzyme-linked immunosorbent assay (ELISA) for the quantification of apolipoprotein B using monoclonal antibodies.
A semi-automated competitive, double-antibody, solid-phase enzyme-linked immunosorbent assay for apolipoprotein B (Apo B) has been developed which utilizes microtiter plates with commercially available monoclonal antibodies and alkaline phosphatase-conjugated second antibody. The working range of the assay is 20-200 ng. The concentration of plasma Apo B was 0.88 +/- 0.20 g/l (n = 40) for a random sample of normal adults. The correlation coefficient for this assay, compared to a radial immunodiffusion assay, was 0.95 (slope = 1.13, intercept = -15). The quantification of the samples was not influenced by freezing and thawing, storage at -20 degrees C for up to 9 mth, or the lipoprotein particle on which the Apo B was present. The method is suitable for measurement of apolipoprotein B in either normal or pathological plasma, lipoprotein density classes, and is sensitive enough to quantify Apo B in cell biological and molecular biological investigations.
1-Propanol↗
In vivo metabolism of proapolipoprotein A-I in Tangier disease.
Tangier disease is a rare familial disorder characterized by extremely low levels of apolipoprotein A-I (apoA-I) and high density lipoproteins (HDL). In normal subjects, proapoA-I is secreted into plasma and converted to mature apoA-I by the cleavage of the amino-terminal six amino acids with the major isoprotein in plasma being mature apoA-I. In contrast, in Tangier disease there is a marked relative increase of proapoA-I as compared with mature apoA-I. ProapoA-I and mature apoA-I were isolated from normal and Tangier disease subjects, radio-labeled, and autologous apoA-I isoproteins injected into normal and Tangier subjects. The in vivo catabolism and conversion of proapoA-I and mature apoA-I in normal and Tangier disease subjects were quantitated. A comparison of the rate of catabolism of apoA-I isoproteins from plasma revealed a significantly faster rate of catabolism of both isoproteins of apoA-I in Tangier subjects when compared with normal subjects. The fractional conversion rate of proapoA-I to mature apoA-I was 3.9 d-1 in normal subjects and 3.6 d-1 in Tangier subjects. The results indicate that (a) apoA-I enters plasma as the pro isoprotein in both normal and Tangier subjects, (b) Tangier disease subjects have a normal fractional rate of conversion of proapoA-I to mature apoA-I, (c) proapoA-I is catabolized at the same rate as mature apoA-I in Tangier subjects, and (d) Tangier subjects catabolize both pro and mature apoA-I at a much greater rate than do normal subjects. Therefore, the relative increase in proapoA-I in Tangier disease is due to a marked decrease in mature apoA-I resulting from rapid catabolism of both pro- and mature apoA-I and not to defective conversion of proapoA-I to mature apoA-I.
Adult↗
Metabolism of low-density lipoproteins by cultured hepatocytes from normal and homozygous familial hypercholesterolemic subjects.
The profoundly elevated concentrations of low-density lipoproteins (LDL) present in homozygous familial hypercholesterolemia lead to symptomatic cardiovascular disease and death by early adulthood. Studies conducted in nonhepatic tissues demonstrated defective cellular recognition and metabolism of LDL in these patients. Since mammalian liver removes at least half of the LDL in the circulation, the metabolism of LDL by cultured hepatocytes isolated from familial hypercholesterolemic homozygotes was compared to hepatocytes from normal individuals. Fibroblast studies demonstrated that the familial hypercholesterolemic subjects studied were LDL receptor-negative (less than 1% normal receptor activity) and LDL receptor-defective (18% normal receptor activity). Cholesterol-depleted hepatocytes from normal subjects bound and internalized 125I-labeled LDL (Bmax = 2.2 micrograms LDL/mg cell protein). Preincubation of normal hepatocytes with 200 micrograms/ml LDL reduced binding and internalization by approx. 40%. In contrast, 125I-labeled LDL binding and internalization by receptor-negative familial hypercholesterolemic hepatocytes was unaffected by cholesterol loading and considerably lower than normal. This residual LDL uptake could not be ascribed to fluid phase endocytosis as determined by [14C]sucrose uptake. The residual LDL binding by familial hypercholesterolemia hepatocytes led to a small increase in hepatocyte cholesterol content which was relatively ineffective in reducing hepatocyte 3-hydroxy-3-methylglutaryl-CoA reductase activity. Receptor-defective familial hypercholesterolemia hepatocytes retained some degree of regulatable 125I-labeled LDL uptake, but LDL uptake did not lead to normal hepatocyte cholesterol content or 3-hydroxy-3-methylglutaryl-CoA reductase activity. These combined results indicate that the LDL receptor abnormality present in familial hypercholesterolemia fibroblasts reflects deranged hepatocyte LDL recognition and metabolism. In addition, a low-affinity, nonsaturable uptake process for LDL is present in human liver which does not efficiently modulate hepatocyte cholesterol content or synthesis.
Albumins↗
Effectiveness of mevinolin on plasma lipoprotein concentrations in type II hyperlipoproteinemia.
Patients with low-density lipoprotein (LDL) concentrations in the top 10th percentile of the population (type II hyperlipoproteinemia [HLP]) are at increased risk for premature cardiovascular disease; however, the incidence of myocardial infarction and death can be decreased by LDL cholesterol reduction. Mevinolin, an inhibitor of endogenous cholesterol synthesis, has been shown to reduce LDL cholesterol concentrations in a subset of type II patients with heterozygous familial hypercholesterolemia (FH). Using a double-blind, randomized, crossover, placebo-controlled trial, the safety and efficacy of mevinolin were compared in 24 patients with type II HLP with heterozygous FH (n = 6) or without FH type II HLP (n = 18). Compared with placebo treatment, both apolipoprotein B and LDL cholesterol levels were reduced (p less than 0.01) in both FH and non-FH patients by 28 to 34% with mevinolin treatment. In addition, high-density lipoprotein cholesterol levels were significantly increased (p less than 0.001) in both patients with FH (16%) and those with non-FH type II HLP (14%). Patients had no serious or clinically significant adverse effects. Thus, mevinolin is a useful drug for treatment of most patients with elevated plasma LDL cholesterol concentrations.
Adult↗
ApoE deficiency: markedly decreased levels of cellular ApoE mRNA.
Apolipoprotein (apo) E deficiency is a rare genetic disease characterized by palmar and tuberoeruptive xanthomas, type III hyperlipoproteinemia, and premature atherosclerotic vascular disease. The plasma level of apoE in apoE deficiency is less than 0.05 mg/dl by radioimmunoassay, and no structural variants of apoE were detected by immunoblot of plasma or VLDL separated by two-dimensional gel electrophoresis. The apoE gene is present in the apoE deficient patient, and there are no major insertions or deletions in the gene by Southern blot analysis. Blood monocyte-macrophages isolated from a patient with apoE deficiency contain 1-3% of the level of apoE mRNA present in monocyte-macrophages isolated from normal subjects. The apoE mRNA in the monocyte-macrophages of the apoE deficient patient is similar in size to normal apoE mRNA. The deficiency of plasma apoE in the patient with apoE deficiency is due to a markedly decreased level of apoE mRNA and decreased production of the E apolipoprotein. The decreased apoE mRNA may be due to a defect in transcription or processing of the primary transcript of the apoE gene or to instability of the apoE mRNA. The decreased plasma level of apoE results in delayed clearance of remnants of triglyceride rich lipoproteins, hyperlipidemia, and a type III phenotype.
Apolipoproteins E↗
The role of apolipoprotein E in modulating the metabolism of apolipoprotein B-48 and apolipoprotein B-100 containing lipoproteins in humans.
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Apolipoprotein B-100↗
In vivo metabolism of apolipoprotein E in humans.
A method for the investigation of the in vivo metabolism of apoE in humans has been described. In this method, isolated apoE is radioiodinated by the iodine monochloride method and reassociated with lipoproteins. Detailed studies have established that the radiolabeled apoE prepared by this procedure uniformly labels the different plasma lipoprotein pools. The study subjects are studied in steady state on a dietary regimen with multiple feedings. The kinetic results are analyzed by either multiexponential curve fitting or by computer-assisted multicompartmental modeling techniques. Examples of the types of results that may be obtained utilizing these methods are described.
Apolipoproteins E↗
Abnormal metabolism of shellfish sterols in a patient with sitosterolemia and xanthomatosis.
Sitosterolemia and xanthomatosis together are a disease characterized by premature cardiovascular disease, and by elevated plasma concentrations of total sterols and of plant sterols, especially sitosterol which is hyperabsorbed. In order to determine whether this abnormal metabolism also involved other sterols, a patient with sitosterolemia was fed a diet high in shellfish that contain significant quantities of noncholesterol sterols, some of which are less well absorbed than cholesterol in humans. Compared with control subjects (n = 8), the sitosterolemic subject had an increased absorption of 22-dehydrocholesterol (71.5% vs. 43.8 +/- 11.4%, mean +/- SD), C-26 sterol (80.6% vs. 49.3 +/- 11.4%), brassicasterol (51.8% vs. 4.8 +/- 4.2%), and 24-methylene cholesterol (60.5% vs. 16.0 +/- 8.3%). This enhanced absorption was associated with an increased plasma total shellfish sterol level (13.1 mg/dl vs. 1.9 +/- 0.7 mg/dl in normals). In the sitosterolemic subject, as in normals, the shellfish sterols were not preferentially concentrated in any lipoprotein class, and 50-65% of these sterols were in the esterified form in plasma. Bile acids and neutral sterols were quantitated in bile obtained by duodenal aspiration. The bile acid composition did not differ significantly in the sitosterolemic subject compared with the normal controls. The sitosterolemic subject, though, was unable to concentrate normally the neutral shellfish sterols in bile. The normal controls concentrated the shellfish sterols in bile 6.3 +/- 1.7-fold relative to the plasma shellfish sterol concentration whereas the study subject was only able to concentrate them 2.1-fold. We propose that sitosterolemia and xanthomatosis occur from a generalized abnormality in the usual ability of the gut mucosa and other tissues of the body to discriminate among many different sterols. This has important implications for the understanding of the pathophysiology of this disease and for therapeutic recommendations.
Adult↗
Abnormal in vivo metabolism of apolipoprotein E4 in humans.
Apolipoprotein E (apoE) is important in modulating the catabolism of remnants of triglyceride-rich lipoprotein particles. It is a polymorphic protein with the three common alleles coding for apoE2, apoE3, and apoE4. ApoE3 is considered the normal isoform, while apoE4 is associated both with hypercholesterolemia and type V hyperlipoproteinemia. We quantitated the kinetics of metabolism of apoE4 in 19 normolipidemic apoE3 homozygotes and 1 normolipidemic apoE4 homozygote, and compared this with the metabolism of apoE3 in 12 normolipidemic apoE3 homozygotes. In the apoE3 homozygous subjects, apoE4 was catabolized twice as fast as apoE3, with a mean plasma residence time of 0.37 +/- 0.01 d (+/- SEM) and 0.73 +/- 0.05 (P less than 0.001), respectively. When plasma was fractionated into the lipoprotein subclasses, the greatest amount of labeled apoE4 was present on very low density lipoproteins, while the largest fraction of labeled apoE3 was associated with high density lipoproteins. The plasma apoE concentration was decreased in an apoE4 homozygote compared with the apoE3 homozygotes (3.11 mg/dl vs. 4.83 +/- 0.35 mg/dl). The reduced apoE4 concentration was entirely due to a decreased apoE4 residence time in the apoE4 homozygote (0.36 d vs. 0.73 +/- 0.05 d for apoE3 in apoE3 homozygotes). These results indicate that apoE4 is kinetically different than apoE3, and suggest that the presence of apoE4 in hypercholesterolemic and type V hyperlipoproteinemic individuals may play an important pathophysiological role in the development of these dyslipoproteinemias.
Adolescent↗
Familial apolipoprotein E deficiency.
A unique kindred with premature cardiovascular disease, tubo-eruptive xanthomas, and type III hyperlipoproteinemia (HLP) associated with familial apolipoprotein (apo) E deficiency was examined. Homozygotes (n = 4) had marked increases in cholesterol-rich very low density lipoproteins (VLDL) and intermediate density lipoproteins (IDL), which could be effectively lowered with diet and medication (niacin, clofibrate). Homozygotes had only trace amounts of plasma apoE, and accumulations of apoB-48 and apoA-IV in VLDL, IDL, and low density lipoproteins. Radioiodinated VLDL apoB and apoE kinetic studies revealed that the homozygous proband had markedly retarded fractional catabolism of VLDL apoB-100, apoB-48 and plasma apoE, as well as an extremely low apoE synthesis rate as compared to normals. Obligate heterozygotes (n = 10) generally had normal plasma lipids and mean plasma apoE concentrations that were 42% of normal. The data indicate that homozygous familial apoE deficiency is a cause of type III HLP, is associated with markedly decreased apoE production, and that apoE is essential for the normal catabolism of triglyceride-rich lipoprotein constituents.
Adolescent↗
Analysis of the apolipoprotein B gene and messenger ribonucleic acid in abetalipoproteinemia.
The apolipoprotein B-100 (apoB-100) gene in leukocytes and the apoB-100 messenger RNA (mRNA) and translated apolipoprotein in the livers from normal and abetalipoproteinemic individuals were evaluated. Four complementary DNA probes for apoB-100 covering the 5', middle, and 3' regions of the apoB-100 mRNA were utilized and Southern blot analysis indicated that the apoB-100 gene is present in abetalipoproteinemia without major insertions or deletions. Polyadenylated hepatic apoB-100 mRNA from two abetalipoproteinemic patients was normal in size, and the concentration of apoB-100 mRNA was increased sixfold compared with control hepatic apoB-100 mRNA levels. ApoB-100 was detected in hepatocytes of abetalipoproteinemic patients by immunohistochemical techniques. These results indicate that the biochemical defect in abetalipoproteinemic patients studied is most consistent with a posttranslational defect in apoB-100 processing or secretion with an up-regulation of the apoB-100 mRNA.
Abetalipoproteinemia↗
Separation of apolipoprotein B species by agarose-acrylamide gel electrophoresis.
Human apolipoprotein (apo) B has been recognized to exist in two different forms designated apoB-100 and apoB-48. The two apoB forms are usually separated by NaDodSO4 gel electrophoresis with a low percentage polyacrylamide gel in a tube gel apparatus. However, the matrix of this low percentage gel is relatively weak, and one can separate the two forms of apoB in a slab gel apparatus only if one utilizes a gradient polyacrylamide gel or a higher percentage polyacrylamide gel which results in a poorer separation of the protein bands. We have developed an agarose-acrylamide gel electrophoretic method to separate the two major apoB forms. The gel is a mixture of 0.5% agarose and 2% acrylamide. The agarose-acrylamide method is fast, has the advantage of being able to be used on an analytical or preparative scale in a vertical slab gel apparatus, and the gel is of sufficient strength to be used in immunoblotting and/or radioautography.
Acrylamide↗
Abnormal low density lipoprotein metabolism in apolipoprotein E deficiency.
Apolipoprotein(apo) E deficiency is an inherited disease characterized by type III hyperlipoproteinemia and less than 1% normal plasma apoE concentration. The role of apoE in LDL metabolism was investigated by quantitating the metabolism of radiolabeled normal and apoE-deficient LDL in both normal and apoE-deficient subjects. ApoE deficiency resulted in an accumulation of plasma IDL, and a decreased synthesis of LDL consistent with a block in the conversion of IDL to LDL. The LDL isolated from the apoE-deficient patient was similar to normal LDL in hydrated density, size, and composition. However, the apoE-deficient LDL was kinetically abnormal with delayed catabolism in both normal subjects and the apoE-deficient patient. In addition, the catabolism of normal LDL in the apoE-deficient subject was increased. These results were interpreted as indicating that apoE is necessary for the conversion of IDL to LDL and the formation of kinetically normal LDL. The rapid catabolism of normal LDL in the apoE-deficient patient suggests an up-regulation of the hepatic LDL receptor pathway. Based on these results, apoE is proposed to play an important role in the conversion of IDL to LDL, the formation of kinetically normal LDL, and the regulation of LDL receptor function.
Adult↗
Clinical disorders of lipoprotein metabolism.
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Apolipoproteins↗
Estimation of VLDL cholesterol in hyperlipidemia.
Lipoprotein data from 10947 fasting blood samples drawn between 1968 and 1982 in the Molecular Disease Branch at the National Institutes of Health were used to test the generalizability of estimating very low density lipoprotein cholesterol (VLDL-C) from plasma triglyceride (TG). Patient samples with total cholesterol levels over 500 mg/l and triglyceride values in the 0-100 000 mg/1 range were included in this study. A previously defined linear relationship VLDL-C = 0.20 (TG) was observed in the past by Friedewald and collaborators, allowing estimation of low density lipoprotein cholesterol (LDL-C) without ultracentrifugation for TG values up to 4 000 mg/1. The results from this report extend the use of the Friedewald relationship to higher TG levels, and to various dyslipidemic states. As the VLDL-C estimates become increasingly imprecise for TG values greater than 10 000 mg/l, caution should be exercised using the estimate in the higher TG ranges. Comparisons with an alternative equation VLDL-C = 0.166 (TG) showed equal or improved accuracy with this estimation procedure, particularly at high TG levels.
Adolescent↗