Radioimmunoassay of human arginine-rich apolipoprotein, apoprotein E. Concentration in blood plasma and lipoproteins as affected by apoprotein E-3 deficiency

Genetic polymorphism in human apolipoprotein E

This chapter provides the methodologies employed to study the polymorphism of human apoE. These and other related studies have advanced our understanding of the structure and function of this protein as follows: The complex array of human apoE observed by two-dimensional gel electrophoresis results from genetic variation and posttranslational modification. The genetic polymorphism of apoE is explained by the existence of three common alleles (epsilon 4, epsilon 3, epsilon 2) at a single structural gene locus. Combinations of above alleles can generate three homozygous (E4/4, E3/3, E2/2) and three heterozygous (E4/3, E3/2, E4/2) apoE phenotypes. The apoE phenotype E2/2 is found in 91% of patients with type III hyperlipoproteinemia and can be used as a molecular marker for the diagnosis of this disease. However, other rare or common apoE phenotypes have been observed in patients with type III HLP. ApoE originating from E2/2 phenotype (Arg 158 to Cys 158 substitution) has reduced affinity for the LDL receptor. This property of apoE2 can account partially for the accumulation of apoE-rich lipoprotein remnants in the plasma of patients with type III HLP. However, other genetic or environmental factors are necessary for the phenotypic expression of the disease.

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.

Human apolipoprotein E expression in Escherichia coli: structural and functional identity of the bacterially produced protein with plasma apolipoprotein E

Human apolipoprotein E (apoE) was produced in Escherichia coli by transforming cells with an expression vector containing a reconstructed apoE cDNA, a lambda PL promoter regulated by the thermolabile cI repressor, and a ribosomal binding site derived from the lambda cII or the E. coli beta-lactamase gene. Transformed cells induced at 42 degrees C for short periods of time (less than 20 min) produced apoE, which accumulated in the cells at levels of approximately equal to 1% of the total soluble cellular protein. Longer induction periods resulted in cell lysis and the proteolytic destruction of apoE. The bacterially produced apoE was purified by heparin-Sepharose affinity chromatography, Sephacryl S-300 gel filtration, and preparative Immobiline isoelectric focusing. The final yield was approximately equal to 20% of the initial apoE present in the cells. Except for an additional methionine at the amino terminus, the bacterially produced apoE was indistinguishable from authentic human plasma apoE as determined by NaDodSO4 and isoelectric focusing gel electrophoresis, amino acid composition of the total protein as well as its cyanogen bromide fragments, and partial amino acid sequence analysis (residues 1-17 and 109-164). Both the bacterially produced and authentic plasma apoE bound similarly to apolipoprotein B,E(low density lipoprotein) receptors of human fibroblasts and to hepatic apoE receptors. Intravenous injection resulted in similar rates of clearance for both the bacterially produced and authentic apoE from rabbit and rat plasma (approximately equal to 50% removed in 20 min). The ability to synthesize a bacterially produced human apolipoprotein with biological properties indistinguishable from those of the native protein will allow the production of large quantities of apoE for use in further investigations of the biological and physiological properties of this apolipoprotein.

Serum and interstitial fluid apolipoprotein E levels in the healthy and in hyperlipoproteinemia type III as studied by radioimmunoassay

In order to study the relationships between serum lipoprotein lipid concentrations and the concentrations of apo E in serum and interstitial fluid, we have developed a specific, sensitive and rapid radioimmunoassay for this apolipoprotein. Studies of the interstitial fluid lipoproteins and of the gradient between the lipoprotein concentrations in interstitial fluid and serum may add to our understanding of the development of atherosclerosis and xanthomatosis. Serum, interstitial fluid, lipoproteins or standards were incubated with 125I-labelled apo E and rabbit antiserum against apo E for 90-120 min at room temperature. The immune complexes were harvested with the use of formalin-treated staphylococci. The displacement curves produced by standard and samples of serum, interstitial fluid and isolated lipoproteins were linear in logit-log plots and had identical slopes. Delipidation did not change the results and the recovery of added apo E to a serum sample was 96 +/- 5% (n = 5). Apo E was found in all major lipoprotein classes and the concentrations of apo E in serum and in interstitial fluid were 36 +/- 19 mg/l and 8 +/- 4 mg/l, respectively, in normals (n = 21) and 305 +/- 125 mg/ml and 20 +/- 9 mg/l, respectively, in patients with HLP type III (n = 11). Highly significant positive correlations were found in HLP type III between the interstitial fluid level of apo E and the corresponding concentrations of cholesterol and triglyceride. Interstitial fluid apo E concentrations were significantly correlated to apo E but not to the lipid levels in serum, indicating that only some subclasses of the serum lipoproteins are transported to the interstitial compartment.

Impaired catabolism of very low-density lipoprotein-triglyceride in a family with primary hypertriglyceridemia

In this report, kinetic studies of plasma very low-density lipoprotein-triglyceride (VLDL-TG) were examined in five brothers (three affected and two unaffected) from a family with primary hypertriglyceridemia. Synthesis and catabolism of VLDL-TG were studied by in vivo labelling of plasma TG with 3H-glycerol, and multicompartmental analysis of the plasma die-away curves. Results of the kinetic studies revealed the following information: (1) one brother, who had the highest plasma TG level and was obese, had both overproduction and a reduced fractional catabolic rate (FCR) of VLDL-TG; (2) second brother, who had moderate hypertriglyceridemia, had a low FCR and high-normal synthesis of VLDL-TG; (3) a third, who had only mildly elevated TG, had a low FCR and normal synthesis of VLDL-TG; and (4) the two normolipidemic brothers had neither overproduction nor decreased FCR of VLDL-TG. The composition of the soluble apoproteins of VLDL was normal. The apoprotein E phenotypes were E4/3 in four brothers, and E3/2 in the fifth. We have reached the following conclusions regarding this family: (1) the common kinetic abnormality of VLDL-TG metabolism in the hypertriglyceridemic brothers was a low clearance of VLDL-TG; (2) impaired catabolism of VLDL could not be explained by the apoprotein C or E patterns; and (3) the most severe hypertriglyceridemia occurred when the decreased FCR was present in conjunction with VLDL-TG overproduction due to obesity. Thus, a moderate defect in catabolism of plasma TG appears to be responsible for one familial form of primary hypertriglyceridemia.

Effect of heparin-induced lipolysis on the distribution of apolipoprotein e among lipoprotein subclasses. Studies with patients deficient in hepatic triglyceride lipase and lipoprotein lipase

In normal subjects, apolipoprotein E (apo E) is present on very low density lipoproteins (VLDL) (fraction I) and on particles of a size intermediate between VLDL and low density lipoproteins (LDL) (fraction II). The major portion of apo E is, however, on particles smaller than LDL but larger than the average high density lipoproteins (HDL) (fraction III). To investigate the possible role of the vascular lipases in determining this distribution of apo E among the plasma lipoproteins, we studied subjects with primary deficiency of either hepatic lipase or of lipoprotein lipase and compared them with normal subjects. Subjects with familial hepatic triglyceride lipase deficiency (n = 2) differ markedly from normal in that fraction II is the dominant apo E-containing group of lipoproteins. When lipolysis of VLDL was enhanced in these subjects upon release of lipoprotein lipase by intravenous heparin, a shift of the apo E from VLDL into fractions II and III was observed. In contrast, apolipoproteins CII and CIII (apo CII and CIII, respectively) did not accumulate in intermediate-sized particles but were shifted markedly from triglyceride rich lipoproteins to HDL after treatment with heparin. In subjects with primary lipoprotein lipase deficiency (n = 4), apo E was confined to fractions I and III. Release of hepatic triglyceride lipase by heparin injection in these subjects produced a shift of apo E from fraction I to III with no significant increase in fraction II. This movement of apo E from large VLDL and chylomicron-sized particles occurred with little hydrolysis of triglyceride and no significant shift of apo CII or CIII into HDL from triglyceride rich lipoproteins. When both lipoprotein lipase and hepatic triglyceride lipase were released by intravenous heparin injection into normal subjects (n = 3), fraction I declined and the apo E content of fraction III increased by an equivalent amount. Either moderate or no change was noted in the intermediate sized particles (fraction II). These data strongly support the hypothesis that fraction II is the product of the action of lipoprotein lipase upon triglyceride rich lipoproteins and is highly dependent on hepatic triglyceride lipase for its further catabolism. In addition, the hydrolysis by hepatic triglyceride lipase of triglyceride rich lipoproteins in general results in a preferential loss of apo E and its transfer to a specific group of large HDL.

Total plasma apo E and high density lipoprotein apo E in survivors of myocardial infarction

Total apo E in plasma and the amount of apo E-HDL were measured in 40 normolipidemic male survivors of myocardial infarction and in 40 controls. LDL-C, Lp(a) and apo B were significantly higher and HDL-C and apo A-I were significantly lower in survivors than in controls. Total plasma apo E did not differ between patients and controls, but HDL-E and the ratio HDL-E/apo A-I were lower in survivors. The data support the view that atherosclerotic patients are often characterized by abnormalities in the concentration and distribution of lipoproteins as well as of apoproteins, even in the presence of normal total plasma lipids.

Clinical, biochemical, and genetic features in familial disorders of high density lipoprotein deficiency

This review assesses current knowledge of the clinical, genetic, and biochemical features of familial high density lipoprotein (HDL) deficiency syndromes. The focus is on HDL deficiency states occurring in the absence of severe hypertriglyceridemia or lecithin/cholesterol acyltransferase deficiency. Specific entities falling within this category include Tangier disease, familial HDL deficiency with planar xanthomas, familial apolipoprotein A-I and C-III deficiency (formerly known as apolipoprotein A-I absence), familial deficiency of apolipoprotein A-I and C-III, fish-eye disease, familial hypoalphalipoproteinemia, and apolipoprotein A-I variants (apo A-I Milano, apo A-I Marburg, apo A-I Giessen, and apo A-I Munster 1-3). Diffuse corneal opacification and premature coronary artery disease are common features in many of these kindreds. No striking clinical abnormalities have been noted in patients with currently known apolipoprotein A-I variants, possibly because these subjects are heterozygotes for their respective defects. The HDL deficiency in many of these disorders has been associated with abnormalities or deficiencies of apolipoprotein A-I. Further research will undoubtedly define the defects in all the disorders that have been described, uncover new mutations, as well as provide additional insights into the precise relationship between HDL deficiency and atherosclerosis.

Serum lipids, lipoproteins and apolipoprotein E phenotypes in relatives of patients with type III hyperlipoproteinaemia

Eighty-six relatives of nineteen probands with type III hyperlipoproteinaemia were studied to determine the occurrence of hyperlipidaemia and to investigate the relation between apo E phenotypes, the occurrence of hyperlipidaemia, and the composition of the very low density lipoprotein (VLDL) fraction. Thirty-nine relatives were hyperlipidaemic: four type IIa or IIb, nine type III and twenty-six type IV. The predisposition for hyperlipidaemia was independent of the apo E phenotype. Hyperlipidaemic relatives with apo E phenotype E2/2 had a significantly (P less than 0.01) higher VLDL-cholesterol/VLDL-triglycerides ratio (1.26 +/- 0.35, n = 9) than those heterozygous for apo E allele epsilon 2 (0.66 +/- 0.12, n = 23) or without apo E allele epsilon 2 (0.69 +/- 0.11, n = 7). Normolipidaemic homozygous apo E-2 relatives had also a significantly (P less than 0.05) higher ratio (0.97 +/- 0.19, n = 6) than those heterozygous for (0.77 +/- 0.19, n = 31) or without the apo E allele epsilon 2 (0.74 +/- 0.13, n = 10). Thus, both hyper- and normolipidaemic apo E2 homozygotes have higher concentrations of VLDL remnants than the subjects heterozygous or without allele epsilon 2.

Homozygous familial hypercholesterolemia occurring with apoprotein E3 deficiency. Report of two cases

This is the first report of homozygous familial hypercholesterolemia (FH) occurring together with dysbetalipoproteinemia. The former was demonstrated by deficiency of specific receptors for apoprotein B of low density lipoproteins and the latter by isoelectric focusing of the E isoapoproteins and the presence of a broad-beta band on electrophoresis. Two young boys of Lebanese extraction had extensive tuberous and tendinous xanthomata, serum cholesterol concentrations of 29.9 and 28.4 mmol/liter, respectively, and mildly raised serum triglycerides due to an accumulation of lipoprotein remnant particles. Homozygosity for FH was demonstrated in both boys by the deficiency of specific binding of low density lipoprotein to cultured skin fibroblasts (less than 15% and less than 10% of normal, respectively). The E apoprotein phenotypes showed E3/E2 in one boy and E2/E2 in the other. The treatment of both boys with cholestyramine and probucol reduced the serum cholesterol concentration to between 15 and 18 mmol/liter and dramatically lessened the severity of xanthomatosis.

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.

Distribution and concentration of apolipoprotein E in high and low density lipoproteins

An electroimmunoassay of apolipoprotein E (Apo E) in total human plasma and in the supernatant following precipitation with sodium phosphotungstate in the presence of MgCl2 of the apolipoprotein B (Apo B)-containing lipoproteins is described. The assay is specific and sensitive enough to record also low levels of Apo E found in isolated lipoprotein fractions. In 30 normal male subjects, the method gave a total plasma Apo E concentration of 6.67 +/- 1.92 mg/dl and a HDL Apo E concentration of 2.58 +/- 0.91 mg/dl. Total Apo E is positively correlated with total plasma triglycerides. The values of Apo E in the lipoproteins obtained by ultracentrifugation were lower than the corresponding values obtained by a precipitation procedure, with a mean Apo E loss of 12.1% in the low density lipoproteins (VLDL + LDL) and of 34.3% in HDL. After ultracentrifugation, a substantial amount of Apo E was detected in the d greater than 1.210 g/ml fraction. The proposed method allows to study the Apo E concentration and distribution between low and high density lipoproteins in large samples, avoiding the problems connected with high salt concentrations and centrifugal forces.

Secretion of lipoproteins from the liver of normal and Watanabe heritable hyperlipidemic rabbits

We compared the rate of accumulation of lipoproteins in perfusates of isolated livers from normal New Zealand White rabbits and Watanabe heritable hyperlipidemic (WHHL) rabbits, in which a gene mutation has produced a virtually complete deficiency of low density lipoprotein (LDL) receptors. The rate of accumulation of apolipoprotein B-100 did not differ in perfusates of livers from normal and mutant animals and little or no apolipoprotein B-48 was detected. In both groups, virtually all apolipoprotein B accumulated in very low density lipoprotein (VLDL). Experiments in which [3H]lysine was added to the perfusates showed that the apolipoprotein B that accumulated in VLDL was newly synthesized by the liver whereas the small amount of apolipoprotein B found in lipoproteins of higher density appeared to be washed out of extravascular spaces during perfusion. Perfusate VLDL from both groups contained more triglycerides and less cholesteryl esters than their counterparts from blood plasma. As compared with perfusate VLDL from normal livers, those from livers of WHHL rabbits were enriched in cholesteryl esters. Experiments in which Triton WR-1339 was injected into the blood of intact rabbits confirmed the observations with perfused livers. Previous studies have shown that the extent to which VLDL is converted to LDL is increased several-fold in WHHL rabbits. Taken together with our present results, which fail to provide evidence for increased secretion of apolipoprotein B or de novo secretion of lipoproteins other than VLDL that contain apolipoprotein B, it can be concluded that overproduction of LDL in rabbits lacking LDL receptors is solely the result of altered metabolism of VLDL.

Atypical familial dysbetalipoproteinemia associated with apolipoprotein phenotype E3/3

Familial dysbetalipoproteinemia has been reported to be associated uniquely with an apolipoprotein E phenotype (E2/2) that occurs in approximately 1% of all persons. We have observed the typical clinical and biochemical characteristics of this disorder in five members of a family, in all of whom the apolipoprotein E phenotype, as determined by isoelectric focusing electrophoresis, is E3/3. The disorder is present in three generations of the family: the proband, her mother, and three of the proband's five children. The proband's husband, father of all five children, also has apolipoprotein E phenotype E3/3, as do his two unaffected children. As in normal persons with phenotype E3/3, the apolipoprotein E of affected members appears to have a single residue of cysteine. When incorporated with egg lecithin into discoidal complexes, the apolipoprotein E from affected members was taken up normally into perfused livers of estradiol-treated rats, in which a high level of LDL receptors is expressed. When isoelectric focusing electrophoresis was carried out over a narrow range of pH (5-7), each of the apolipoprotein E isoforms of affected members was observed as a doublet, even after reduction of dimers of the protein with 2-mercaptoethanol and treatment with neuraminidase to minimize the content of sialylated forms of the protein. Doublets were also observed in the apolipoprotein E-2 of patients with classical dysbetalipoproteinemia, but only in the affected members of the family with atypical dysbetalipoproteinemia were the components of the doublets equally prominent. As in classical dysbetalipoproteinemia, both apolipoprotein B-100 and B-48 were present in the very low density lipoprotein fraction of plasma obtained in the postabsorptive state, indicating that remnantlike lipoproteins of both hepatic and intestinal origin accumulate. This observation, together with available evidence on the structural and functional heterogeneity of human apolipoprotein E, lead us to suggest that the disorder in this family is caused by one or two structurally abnormal forms of apolipoprotein E that contain a single residue of cysteine.

Subpopulations of apolipoprotein A-I in human high-density lipoproteins. Their metabolic properties and response to drug therapy

In this study immunological procedures were used to detect and quantify high-density lipoprotein (HDL) particles of differing apolipoprotein A composition. In the plasma of eight healthy female subjects, 45% of the total apolipoprotein A-I existed in particles (called '(AI)HDL') devoid of apolipoprotein A-II. The remainder circulated in association with apolipoprotein A-II at a molar ratio of approximately 1:1. Nicotinic acid selectively raised the plasma apolipoprotein A-I/A-II ratio by increasing the proportion of (AI)HDL particles. Probucol produced the opposite effect, lowering the plasma concentration of these particles. The kinetic properties of apolipoprotein A-I in total HDL and in the (AI)HDL particle were the same despite the fact that apolipoprotein A-I equilibration between these two species was incomplete. Therefore, there appear to be at least two apolipoprotein A-containing particle populations in HDL which are immunochemically and metabolically distinct.

Cholesterol net transport, esterification, and transfer in human hyperlipidemic plasma

Cholesterol esterification, cholesteryl ester transfer between lipoproteins, and cholesterol transport between lipoproteins and cultured cells have been measured in the plasma of 22 patients with primary hyperlipidemia and 10 normolipidemic subjects. In hyperbetalipoproteinemia, increase in plasma low density lipoprotein levels was associated with a reduction of cholesteryl ester transfer rates, and with a reversal of the normal direction of sterol transport between fibroblasts and their plasma culture medium. Instead of net transport from cells to medium there was a net uptake of sterol from plasma by the cells, despite a level of plasma lecithin/cholesterol acyltransferase activity that was within the normal range. In dysbetalipoproteinemia, esterification rates were increased above normal levels, but cholesteryl ester transfer was reduced and the direction of sterol transport between the cells and plasma medium was reversed, as in the hyperbetalipoproteinemic group. In hypertriglyceridemia, those subjects with cardiovascular disease showed a metabolic pattern similar to the hyperbetalipoproteinemic group. The subjects in this group without symptoms of cardiovascular disease showed a normal direction of sterol transport, normal or raised rates of cholesteryl ester transfer between lipoproteins, and an increased rate of sterol esterification in plasma that decreased towards normal levels as plasma triglyceride levels decreased. Despite their quite distinct metabolic patterns there was no consistent difference between the two hypertriglyceridemic groups in triglyceride or cholesterol levels, very low density lipoprotein composition, or electrophoretic or isoelectric focussing patterns. All hypertriglyceridemic subjects with documented cardiovascular disease showed reversed cell-plasma sterol transport and all subjects without such disease showed a normal direction of cell-plasma sterol transport. The results of this study indicate major and reproducible abnormalities in plasma cholesterol metabolism in several groups of subjects with genetically distinct hyperlipidemias, who are at risk for atherosclerotic vascular disease. The possible predictive value of sterol metabolic measurements in the analysis of cardiovascular disease is discussed.

Effect of a high carbohydrate diet on the content of apolipoproteins C-II, C-III and E in human plasma high density lipoprotein subfractions

The effect of isocaloric high and low carbohydrate (Carb) diets on the structure and apoprotein composition of plasma high density lipoproteins (HDL) was assessed in four healthy men. The high Carb diet contained 65% calories as Carb and 15% as fat; the low Carb was 15% and 65%, respectively, with protein fixed at 20% of calories in each case. Cholesterol was 400 mg/day and the P/S ratio of the fat was 0.4. Each diet was sequentially consumed for periods of 3 weeks. At the end of each 3-week study period, plasma HDL2 and HDL3 were isolated by zonal ultracentrifugation and their apoprotein and lipid compositions were determined. Compared to the low Carb diet, the high Carb diet was associated with an increase in the size of HDL2 (116.0 +/- 1.8 vs. 109.1 +/- 1.8 A) and in the content (mean weight % +/- SEM) of apoE (2.81 +/- 0.71 vs. 1.79 +/- 0.49, P less than 0.01) and of apoC-II (1.73 +/- 0.09 vs. 1.11 +/- 0.12, P less than 0.01). HDL2 apoC-III content was not significantly different on the two diets (6.49 +/- 0.50 vs. 7.42 +/- 1.21). On the two diets, HDL3 size and HDL3 apoE content were not significantly changed. HDL3 apoC-II and apoC-III, however, were higher on the high Carb diet, P less than 0.05. The ratio (by weight) of HDL2 apoE/HDL2 apoC-II + C-III increased on the high Carb diet compared to the low Carb diet (0.344 +/- 0.058 vs. 0.228 +/- 0.053, P less than 0.01). We suggest that the increased amount of apolipoprotein E in HDL2 may influence its rate of catabolic clearance and may account for the well-known decrease in plasma HDL-cholesterol in subjects on high Carb diets.

Chromatographic depletion of lipoproteins from plasma and recovery of apolipoproteins

Selective adsorption of proteins from a complex mixture onto an affinity support presents a very powerful approach to protein purification. High density lipoprotein (HDL) and low density lipoprotein (LDL) have been removed from plasma by hydrophobic adsorption chromatography using phenyl-Sepharose. Plasma chromatographed on phenyl-Sepharose is depleted of beta-lipoprotein and apolipoproteins A-I, A-II and E. Less than 5% of the initial amounts of cholesterol, triacylglycerol, sphingomyelin, and phosphatidylcholine remain in the plasma. Column elution with propylene glycol permits recovery of apolipoproteins A-I, A-II and E. This procedure should provide a convenient alternative to ultracentrifugal removal of lipoproteins from plasma.

Normocholesterolemic tendon xanthomatosis with overproduction of apolipoprotein B

This report describes a 46-yr-old man with normocholesterolemic tendon xanthomatosis. He had severe bilateral xanthomas of Achilles tendons and small lesions on patellar tendons; biopsy of the latter revealed a fibroxanthoma of high cholesterol content. He did not have clinical evidence of atherosclerotic disease. The patient's total cholesterol (TC) and triglycerides (TG) were 245 and 258 mg/dl, respectively. LDL-TC was 168 mg/dl and HDL-TC was 32 mg/dl. VLDL consisted mainly of small particles (SfO 20-100) which were unusually rich in apolipoproteins B and E (and low in apo Cs). Plasma LDL-apo B was not increased (85-120 mg/dl), but VLDL-apo B was distinctly elevated (58 mg/dl). The synthesis rate of apoLDL (29.9 mg/kg/d) was increased markedly compared to a matched control (13.9 mg/kg/d) and to a patient with familial hypercholestrolemia (15.9 mg/kg/d). The concentration of apoLDL in our patient was not increased; this was because of an associated high FCR (0.484 day-1). His HDL was relatively low in TC but high in TG, which caused an increase in HDL2b. The patient's xanthomata may have been the result of an overproduction of apo B possibly combined with a defect in HDL metabolism.

Remnants of lipoproteins of intestinal and hepatic origin in familial dysbetalipoproteinemia

We used the low molecular weight form of apolipoprotein B (B-48) as a marker for the identification of remnant particles formed from chylomicrons in the plasma of patients with familial dysbetalipoproteinemia. In the serum of patients fasted 14 hours, the d less than 1.006 g/cm3 lipoproteins of prebeta mobility, separated by starch block electrophoresis, contained only the primary hepatogenous species of apolipoprotein B (B-100), and their lipid composition resembled that of normal prebeta very low density lipoproteins. In contrast, the fraction with beta mobility contained both the B-48 and B-100 proteins; the B-48 protein was found primarily among the largest particles. All fractions of beta mobility were greatly enriched with cholesteryl esters. The beta fraction thus contains remnant particles which appear to originate both from chylomicrons and hepatogenous very low density lipoproteins. It appears that these remnant particles share a common removal mechanism which is impaired in familial dysbetalipoproteinemia.

Concentration and composition of lipoproteins in blood plasma of the WHHL rabbit. An animal model of human familial hypercholesterolemia

Lipoproteins in blood plasma have been quantified and characterized in homozygous Watanabe-heritable hyperlipidemic (WHHL) rabbits, an animal model of human familial hypercholesterolemia. Like homozygous human hypercholesterolemics, WHHL rabbits have a severe deficiency of low density lipoprotein (LDL) receptors, a prolonged residence time for LDL, and an increased absolute rate of LDL catabolism. Although lipoproteins containing apolipoprotein B in WHHL rabbits are enriched in cholesteryl esters, their LDL as well as intermediate density lipoproteins (IDL) and very low density lipoproteins (VLDL) also contain a substantial amount of triglycerides and they consistently exhibit hypertriglyceridemia as well as hypercholesterolemia. The cholesteryl esters accumulating in lipoproteins of WHHL rabbits are rich in cholesteryl linoleate and appear to be produced almost exclusively by lecithin-cholesterol acyltransferase. Levels of apolipoprotein B-100 are elevated in VLDL and IDL as well as in LDL of WHHL rabbits and only trace amounts of apolipoprotein B-48 are present. Plasma levels of apolipoprotein E are also substantially increased, and VLDL and IDL are enriched in this protein. The accumulation of lipoproteins with the expected characteristics of remnants of hepatogenous triglyceride-rich lipoproteins contrasts with the efficient hepatic clearance of chylomicron remnants in WHHL rabbits.

Promotion of sterol efflux and net transport by apolipoprotein E in lecithin:cholesterol acyltransferase deficiency

In the plasma of 4 subjects homozygous for deficiency of lecithin:cholesterol acyltransferase, the level of many apolipoproteins (apo A-I, apo A-II, apo B, apo D) was greatly relative to normal, while that of apo E is increased 5-fold. The lipoprotein complex containing lecithin:cholesterol acyltransferase with apo A-I and apo D in normal plasma is completely absent. The major part of apo E is unassociated with other apolipoproteins. The apoprotein-dependence of sterol efflux and net transport from human skin fibroblasts into plasma was determined by immunoaffinity chromatography. In normal plasma the major component of efflux of sterol radioactivity from labeled fibroblasts was dependent upon unassociated apo A-I. In LCAT-deficient plasma, apoprotein-dependent efflux was largely a function of unassociated apo E. When fibroblasts were incubated with fibrinogen-free unfractionated LCAT-deficient plasma, there was no spontaneous net transport of sterol either into or from the cells, indicating that efflux and influx rates were in balance. When apo E was removed by affinity chromatography, there was net transport from plasma to cells. These findings suggest a novel metabolic role for apo E in the promotion of sterol transport uncoupled to LCAT-activity.

Human noninsulin-dependent diabetes: identification of a defect in plasma cholesterol transport normalized in vivo by insulin and in vitro by selective immunoadsorption of apolipoprotein E

Plasma cholesterol metabolism in patients with poorly controlled noninsulin-dependent diabetes was characterized by inhibition of cholesterol net transport between cultured cells (fibroblasts) and plasma, inhibition of cholesterol esterification, and inhibition of cholesteryl ester transfer to low and very low density lipoproteins, relative to a normal control group. Plasma from these patients also contained a 2-fold higher level of apolipoprotein E (apo E). Effective control of hyperglycemia with insulin normalized both the parameters of plasma cholesterol metabolism and plasma levels of apo E. Removal of apo E by immunoaffinity chromatography normalized cell-to-plasma cholesterol transport but was without effect on the rate of cholesterol esterification or of cholesteryl ester transfer. These findings suggest that an inhibition in the chain of reactions by which cellular cholesterol is transferred in esterified form to low and very low density lipoproteins is associated with the appearance of an apo E-dependent "shunt" pathway, returning cholesterol from plasma back to the cells and so nullifying the normal cell-to-plasma transport pathway.

Concentrations of apolipoproteins B, C-I, C-II, C-III and E in sera from normal men and their relation to serum lipoprotein levels

The concentrations of apolipoproteins B, C-I, C-II, C-III and E (by enzyme immunoassay), and cholesterol, triglycerides and phospholipids both in while serum and in serum very low (VLDL), low (LDL) and high (HDL) density lipoproteins, HDL2 and HDL3, were determined in sera from 29 randomly selected normolipidemic men, age 40-60 years, in Stockholm, Mean values, +/- SD, were for, apolipoprotein B, 720 +/- 162; C-I, 63 +/- 14; C-II, 27 +/- 11; C-III, 125 +/- 57; and for E, 25 +/- 6 mg/l. A skewness to the right of the distributions was found for apolipoproteins B and C-II and for serum triglycerides and VLDL lipids. The relations between the different variables were studied by linear correlation analysis. Several significant correlations existed between the lipoprotein levels. Apolipoprotein C-I, C-II and C-III were significantly correlated with each other, whereas neither apolipoprotein B nor apolipoprotein E was correlated with any other apolipoprotein. The following significant, positive correlations existed between the apolipoproteins and total serum lipids and/or lipids of lipoprotein density classes: apolipoprotein B with serum cholesterol and LDL lipids, apolipoprotein C-I with HDL3 cholesterol, apolipoprotein C-II with serum triglycerides and VLDL lipids, apolipoprotein C-III with serum cholesterol and phospholipids. Apolipoprotein E showed no correlation with either serum lipids or lipoproteins.

Isolation of human apolipoprotein E by chromatofocusing

Human prolipoprotein E is implicated in the transport of serum cholesterol and the binding of lipoproteins to cell receptors. Further investigations on this apolipoprotein would be facilitated by improved purification methods. We prepared human apo E by the combination of high performance gel filtration and chromatofocusing from serum very low density lipoproteins. Chromatofocusing was performed with a pH gradient from 7 to 4. Apo E contained all isoforms, but was homogeneous in SDS-polyacrylamide gel electrophoresis and in double immunodiffusion against a monospecific antiserum. The reported purification method allows a rapid and simple preparation of large amounts of apo E.

Apolipoprotein E levels in vegetarians

Vegetarians are known to have low lipoprotein lipid and apolipoprotein Al and B levels. Since dietary cholesterol has recently been shown to have important effects on apolipoprotein E (apo E) metabolism, we measured plasma apo E levels in three groups of vegetarians. Group I (n = 36) consumed less than 10 mg cholesterol daily and 42% of calories as fat (P:S ratio 2.6). Group II (n = 10) and Group III (n = 18) consumed 97 and 179 mg cholesterol daily, and 35% of calories as fat (P:S ratios 0.7 and 0.9) respectively. Compared to control values, vegetarian plasma cholesterol and triglyceride levels were decreased by 10%-30% and 30%-55%. Plasma apo E levels were decreased equally in all groups by 35% (2.4 +/- 0.1 mg/dl versus 3.6 +/0 0.1 mg/kl, P less than .001). Plasma apo E levels were increased in parallel with lipid levels in pregnant vegetarians but were not different from non-lactating vegetarians in postpartum lactating women. Decreased apo E levels did not correlate with relative body weight, P:S ratio or intake of fat, carbohydrates or protein. Since all vegetarian diets studied were low cholesterol diets, decreased cholesterol intake may contribute to the low apo E levels. The apparent modification of apo E metabolism by vegetarian diets may be important in mediating effects of lipid lowering diets on atherogenesis.

Increased lipoprotein-remnant formation in chronic renal failure

Since accelerated atherosclerosis may be induced by excess circulating remnants of triglyceride-rich lipoprotein catabolism, we looked for evidence of remnant particle accumulation in the lipoproteins of 11 patients on long-term dialysis. We found several abnormalities in lipoprotein protein and lipids: enrichment of intermediate-density lipoproteins (ILD) and low-density lipoproteins (LDL) with triglyceride; the presence of apoprotein B48 (a "marker" for intestinal lipoproteins) in very-low-density lipoproteins (VLDL); an increased concentration of apoprotein AIV (a protein related to chylomicron transport); the presence of AIV in VLDL, IDL, and LDL; and the presence in LDL of apoproteins C and E (proteins not normally found in LDL). These findings strongly suggest accumulation of remnants of triglyceride-rich lipoproteins in patients with chronic renal failure who are undergoing peritoneal dialysis or hemodialysis, and may explain in part the increased incidence of coronary deaths among these patients.

Increased prevalence of apolipoprotein E4 in type V hyperlipoproteinemia

Type V hyperlipoproteinemia (HLP) is characterized clinically by hepatosplenomegaly, occasional eruptive xanthomas, and an increased incidence of pancreatitis. These patients have striking hypertriglyceridemia due to increased plasma chylomicron and very low density lipoprotein concentrations in the fasting state, without a deficiency of lipoprotein lipase or its activator protein, apolipoprotein (apo) C-II. ApoE, a protein constituent of triglyceride-rich lipoproteins, has been implicated in the receptor-mediated hepatic uptake of these particles. ApoE has three major alleles: E2, E3, and E4, and the products of these alleles are apoE2, apoE3, and apoE4, respectively. ApoE phenotypes were determined in 30 type V HLP patients as well as in 37 normal volunteers. Among the type V patients, 33.3% were noted to be homozygous, and 40.0% heterozygous for E4 (normal, 2.7 and 21.6%, respectively). These data suggest that apoE4 may play a role in the etiology of the hyperlipidemia in a significant number of type V HLP patients.

Regulation of sterol transport in human microvascular endothelial cells

In cultured human dermal microvessel endothelial cells, the rate of efflux (about twofold greater than for fibroblasts under equivalent conditions) was coupled to an equivalent high rate of sterol net transport from the cells to the medium. This net transport was linked with esterification via lecithin:cholesterol acyltransferase. Since the use of free sterol by plasma transferase is constant, such increased net transport indicates that endothelial cells are highly efficient, in competition with plasma lipoproteins, in supplying free sterol for esterification. These results indicate the marked ability of endothelial cells to regulate and maintain their sterol balance in the face of high sterol levels to which these cells are uniquely exposed in human plasma.

[The apolipoproteins in man ]

Twelve different apolipoproteins have been described in human serum. Apo A-I and apo A-II are essential for the structure of the HDL particles and for the function of LCAT activity. Apo B is the main protein in LDL but does also occur in the triglyceride-rich particles. Apo B represents the binding protein for the LDL-receptor pathway. The C-apolipoproteins are located on the surface of VLDL. They are transferred to HDL throughout the catabolism of VLDL and affect lipoprotein lipase activity. This enzyme is also affected by the E-apolipoproteins which occur in the triglyceride-rich particles as well as in HDL. Apo E is the binding site for another specific cell receptor. The concentration and metabolism of apolipoproteins is affected by diet, drugs, hormones, body weight, alcohol, cigarettes, physical exercises, liver and renal diseases. There is a close relation between apolipoproteins and atherosclerosis.

Familial dysbetalipoproteinemia. New aspects of pathogenesis and diagnosis

New aspects in the pathogenesis and diagnosis of familial dysbetalipoproteinemia are discussed, including the clarification of the chemical basis of the polymorphism of apoprotein E, the allelic nature of the primary isoforms of the protein, the relationship of the abnormality of apoprotein E to the accumulation of remnant lipoproteins in dysbetalipoproteinemia and in persons carrying the trait for abnormal apoprotein E, and the pathogenesis of hyperlipidemia in this disorder. The related clinical features of dysbetalipoproteinemia are included in the discussion.

High-density lipoprotein cholesterol: methods and clinical significance

The known limitations and advantages of methods for determining serum high-density lipoprotein (HDL) cholesterol concentration are reviewed with special emphasis on the applicability of each method to clinical medicine. The evidence for and against the relevance of serum HDL cholesterol to the prediction of the likelihood of an individual man or woman developing clinically evident ischemic heart disease is discussed. The possibility that HDL subfractions may be more relevant to this issue is also discussed. Information about serum HDL cholesterol concentration in diseases other than ischemic heart disease is reviewed. The effect of diet, body-weight, exercise, cigarette-smoking, alcohol intake, and hyperlipoproteinemia and the effect of modification of these factors on serum HDL cholesterol levels is discussed. Finally, a practical approach to the patient with a low concentration of serum HDL cholesterol is suggested.