Apolipoprotein E2(Arg158----Cys) frequency in a hyperlipidemic French-Canadian population of apolipoprotein E2/2 subjects. Determination by synthetic oligonucleotide probes

An underlying cause of type III hyperlipoproteinemia is the presence of variant forms of apolipoprotein (apo) E that are defective in binding to apo B,E low density lipoprotein receptors. This disorder is associated almost exclusively with the apo E2/2 phenotype. However, structural and functional heterogeneity have been demonstrated within this phenotype. The apo E2(Arg158----Cys) variant, displaying 1% of normal apo E3 binding activity, is the most defective known form. In this study, we describe a method in which a pair of 19-mer synthetic oligonucleotide probes were used to distinguish between DNA coding for arginine or cysteine at position 158 in apo E. The specificity of the probes was demonstrated by using DNA from subjects whose apo E protein sequence or phenotype was known. The probes were used to screen a French-Canadian population of 34 apo E2/2 subjects to determine the frequency of the apo E2(Arg158----Cys) variant. All 34 subjects, most of whom displayed clinical or biochemical features of type III hyperlipoproteinemia, were found to be homozygous for apo E2(Arg158----Cys), strongly suggesting that this variant is the most common form of apo E2 within this ethnic and clinical population. In addition, the utility of this approach in detecting new apo E mutants was demonstrated when DNA from one of the apo E3/3 control subjects, whose family has a history of hyperlipidemia and coronary artery disease, reacted with both probes. This result suggests that this subject is heterozygous for normal apo E3 and a new apo E3 variant that is likely to be functionally equivalent to apo E2(Arg158----Cys).

Chylomicron remnant metabolism. Role of hepatic lipoprotein receptors in mediating uptake

Chylomicron remnants are catabolized by the liver, and their uptake within this organ is mediated by the presence of apolipoprotein (apo) E on the surface of these particles. In addition to a receptor for low density lipoproteins (LDL), several lines of evidence suggest that a unique receptor, referred to as the chylomicron remnant or apo E receptor, may be involved in the uptake of these lipoproteins. A possible candidate for the apo E receptor was previously isolated by affinity chromatography and was shown to possess high-affinity binding to apo E-containing lipoproteins, including chylomicron remnants, but not to possess high-affinity binding to apo B-containing LDL. However, it is now known that this fraction contains at least three proteins, all of which bind apo E with high affinity. An Mr congruent to 56,000 fraction contains two proteins that have been identified as the alpha- and beta-subunits of mitochondrial F1-adenosine 5'-triphosphatase (F1-ATPase). Furthermore, purified F1-ATPase binds apo E-containing lipoproteins with high affinity. It is very unlikely that these proteins, presumably isolated from mitochondrial membranes, are involved in chylomicron remnant metabolism. An Mr congruent to 59,000 fraction contains a unique apo E-binding protein that is not an ATPase. This protein appears to be localized to the endoplasmic reticulum of liver cells, but it is unclear whether this protein plays a role in chylomicron remnant catabolism. Furthermore, a cDNA clone coding for a protein that is apparently distinct from the ATPase and the Mr congruent to 59,000 protein has been obtained from a lambda gt11 library.(ABSTRACT TRUNCATED AT 250 WORDS)

Apolipoprotein E genotyping using the polymerase chain reaction and allele-specific oligonucleotide probes

A rapid procedure for determining apolipoprotein E genotype from genomic DNA has been developed. In this procedure, DNA is amplified by the polymerase chain reaction, and allele-specific oligonucleotide probes are used to detect the cysteine-arginine interchanges at residues 112 and 158 that distinguish the three common isoforms of apolipoprotein E. The method was tested with 68 subjects, representing the six common phenotypes, and yielded results consistent with the known phenotype.

Familial defective apolipoprotein B-100: enhanced binding of monoclonal antibody MB47 to abnormal low density lipoproteins

Familial defective apolipoprotein (apo) B-100 is a recently described genetic disorder that appears to result from a mutation in the apoB-100 gene. This disorder is characterized by hypercholesterolemia resulting from elevated plasma concentrations of low density lipoprotein LDL. The disorder was first detected in three members of one family. The LDL from affected subjects binds defectively (approximately 30% of normal) to LDL receptors, retarding the clearance of LDL from plasma. In the present study, two other members of the affected family were found to possess abnormal LDL. In addition, abnormal LDL with a similar binding defect were found in a second, unrelated family. In both families, the defect is transmitted over three generations as an autosomal codominant trait and all affected members are heterozygotes. Since there is only one apoB-100 molecule per LDL particle, the abnormal LDL in heterozygous subjects is made up of two populations of particles: one that has normal binding activity to receptors and one that binds defectively. To localize the mutation in apoB-100, the binding of five apoB-100-specific monoclonal antibodies to abnormal LDL was assessed in a solid-phase RIA. Only antibody MB47, whose epitope is between residues 3350 and 3506, distinguished abnormal LDL from normal LDL isolated from control subjects with normal lipid levels; MB47 bound with a higher affinity (by approximately 60%) to abnormal LDL. In every individual with abnormal LDL, the MB47 antibody bound with a higher affinity. The convenience of this assay will facilitate screening of large populations to determine the frequency of this disorder.

Expression of apolipoprotein E by cultured vascular smooth muscle cells is controlled by growth state

Rat vascular smooth muscle cells (SMC) in culture synthesize and secrete a approximately 38,000-Mr protein doublet or triplet that, as previously described (Majack and Bornstein. 1984. J. Cell Biol. 99:1688-1695), rapidly and reversibly accumulates in the SMC culture medium upon addition of heparin. In the present study, we show that this approximately 38,000-Mr heparin-regulated protein is electrophoretically and immunologically identical to apolipoprotein E (apo-E), a major plasma apolipoprotein involved in cholesterol transport. In addition, we show that expression of apo-E by cultured SMC varies according to growth state: while proliferating SMC produced little apo-E and expressed low levels of apo-E mRNA, quiescent SMC produced significantly more apo-E (relative to other proteins) and expressed markedly increased levels of apo-E mRNA. Northern analysis of RNA extracted from aortic tissue revealed that fully differentiated, quiescent SMC contain significant quantities of apo-E mRNA. These data establish aortic SMC as a vascular source for apo-E and suggest new functional roles for this apolipoprotein, possibly unrelated to traditional concepts of lipid metabolism.

Crystallization and preliminary X-ray diffraction studies on the amino-terminal (receptor-binding) domain of human apolipoprotein E3 from serum very low density lipoproteins

Human apolipoprotein E is a component of several classes of circulating plasma lipoproteins. In addition to binding lipids, this apolipoprotein, which is composed of two structural domains, mediates some lipoprotein-receptor interactions by binding to the low density lipoprotein receptor. The receptor-binding function, as well as some lipid-binding capability, is contained in the amino-terminal structural domain of apolipoprotein E. Thrombin-catalyzed hydrolysis of apolipoprotein E yields a fragment (residues 1 to 191) that has the same properties as, and seems to be a good model for, the amino-terminal domain. Crystals of this amino-terminal fragment suitable for high-resolution X-ray diffraction experiments have now been grown. The crystals belong to the orthorhombic space group P2(1)2(1)2(1) and have unit cell dimensions of a = 86.0 A, b = 40.9 A, and c = 53.3 A (1 A = 0.1 nm). This is the first human serum apolipoprotein to be crystallized.

Apolipoprotein E-binding proteins isolated from dog and human liver

Chylomicron remnant catabolism appears to be mediated by apolipoprotein (apo) E binding to hepatic lipoprotein receptors. Previously, the apo B,E(LDL) receptor and a unique apo E-binding protein (referred to as the apo E receptor) were isolated from solubilized canine and human livers. In the present study, the apo E-binding fraction was further characterized and found to contain at least three proteins, all of which bind apo E-containing lipoproteins with high affinity. The 56-kDa band was found to contain the alpha- and beta-subunits of F1-ATPase, presumably derived from mitochondrial membranes. In addition, an apo E-binding protein with an apparent Mr approximately equal to 59,000 was identified. The 59-kDa protein displays calcium-independent binding on ligand blots, but displays both calcium-dependent and -independent binding in assays performed with detergent-solubilized protein. The 59-kDa protein recognized lipid-free as well as lipid-bound apo E in ligand blots, and also bound apo E-2, apo E-3, and apo E-4 in a comparable way. Monoclonal antibodies produced against the 59-kDa protein did not react with the 56-kDa proteins. Normal human liver, as well as the liver of a patient lacking the apo B,E(LDL) receptor, possessed the 56-kDa and 59-kDa proteins. These data indicate that liver cells possess at least three proteins, in addition to the apo B,E(LDL) receptor, that bind apo E-containing lipoproteins with high affinity. The physiological role of these proteins in apo E metabolism remains to be determined.

Uptake of cholesterol-rich remnant lipoproteins by human monocyte-derived macrophages is mediated by low density lipoprotein receptors

The uptake and degradation of cholesterol-rich remnant lipoproteins, referred to as beta-VLDL, are shown in the present study to be mediated by LDL receptors (apoB,E(LDL) receptors), not by unique beta-VLDL receptors. Human blood monocytes cultured for 5-7 d bound apoB- and/or apoE-containing lipoproteins from different species with affinities equivalent to those demonstrated for the receptors on cultured human fibroblasts. Low density lipoproteins competed effectively and completely with 125I-beta-VLDL for binding to and degradation by monocyte-derived macrophages. Specific polyclonal antibodies to bovine apoB,E(LDL) receptors abolished both LDL and beta-VLDL uptake by normal human monocyte-macrophages. Immunoblots of monocyte-macrophage extracts with these antibodies revealed a single protein in human macrophages with an apparent molecular weight identical to that of the apoB,E(LDL) receptor found on human fibroblasts. Like receptors on cultured human fibroblasts, the apoB,E(LDL) receptors on monocyte-macrophages responsible for 125I-beta-VLDL and 125I-LDL uptake were efficiently down regulated by preincubation of the cells with beta-VLDL or LDL. Finally, monocyte-macrophages from seven homozygous familial hypercholesterolemia subjects were unable to metabolize beta-VLDL or LDL, but demonstrated normal uptake of acetoacetylated LDL. The classic apoB,E(LDL) receptors on human monocyte-macrophages thus mediate the uptake of beta-VLDL by these cells.

Apolipoprotein E: cholesterol transport protein with expanding role in cell biology

Apolipoprotein E is a plasma protein that serves as a ligand for low density lipoprotein receptors and, through its interaction with these receptors, participates in the transport of cholesterol and other lipids among various cells of the body. A mutant form of apolipoprotein E that is defective in binding to low density lipoprotein receptors is associated with familial type III hyperlipoproteinemia, a genetic disorder characterized by elevated plasma cholesterol levels and accelerated coronary artery disease. Apolipoprotein E is synthesized in various organs, including liver, brain, spleen, and kidney, and is present in high concentrations in interstitial fluid, where it appears to participate in cholesterol redistribution from cells with excess cholesterol to those requiring cholesterol. Apolipo-protein E also appears to be involved in the repair response to tissue injury; for example, markedly increased amounts of apolipoprotein E are found at sites of peripheral nerve injury and regeneration. Other functions of apolipoprotein E, unrelated to lipid transport, are becoming known, including immunoregulation and modulation of cell growth and differentiation.

Site-specific mutagenesis of human apolipoprotein E. Receptor binding activity of variants with single amino acid substitutions

Apolipoprotein (apo) E, an important protein involved in cholesterol transport in the plasma, binds with high specificity and high affinity to the apoB, E (low density lipoprotein) receptor. Several lines of evidence have indicated that key basic residues in the vicinity of residues 140-160 of apoE are important in mediating binding to the receptor. Furthermore, apoE variants exhibiting defective receptor binding are associated with the genetic lipid disorder type III hyperlipoproteinemia. To determine whether other basic amino acids in this region of apoE also affect receptor binding activity, site-specific mutagenesis of apoE in a bacterial expression system was undertaken. This system had been used successfully to produce apoE3 that was structurally and functionally equivalent to human plasma apoE3. Variants of apoE in which neutral amino acids were substituted for basic residues at positions 136, 140, 143, and 150 were produced. The variants all displayed defective binding; their activity ranged from 9 to 52% of normal (a range similar to that seen with naturally occurring variants of human apoE). In addition, to determine whether the conformation of this region is important for receptor binding, we designed variants in which proline was substituted for leucine 144 or alanine 152. Both variants were defective, exhibiting 13 and 27% of normal binding, respectively. In contrast, a double mutant in which arginine was substituted for serine 139 and alanine for leucine 149 displayed slightly enhanced receptor binding activity. These studies confirm that the middle of the apoE molecule is important in receptor binding and indicate that only certain amino acid substitutions in this region interfere with receptor binding activity.

Metabolism of canine beta-very low density lipoproteins in normal and cholesterol-fed dogs

Cholesteryl ester-rich beta-very low density lipoproteins (beta-VLDL) are beta-migrating lipoproteins that accumulate in the plasma of cholesterol-fed animals and of patients with type III hyperlipoproteinemia. There are two distinct fractions: fraction I beta-VLDL are chylomicron remnants of intestinal origin, and fraction II beta-VLDL are cholesterol-rich VLDL of hepatic origin. The liver rapidly clears fraction I beta-VLDL from the plasma of both normal and cholesterol-fed dogs. The liver also clears fraction II beta-VLDL rapidly and efficiently from the plasma of normal dogs by receptor-mediated uptake. In cholesterol-fed dogs the clearance is biphasic: an initial rapid die-away of about 30% to 40% of the injected dose within 5 minutes, followed by a slow clearance of plasma radioactivity (a half-life of more than 20 hours). The rapid, initial phase of fraction II beta-VLDL clearance appears to be related to sequestration of the lipoproteins presumably on endothelial cells and is apparently associated with lipolytic processing. Treatment of the fraction II beta-VLDL with lipoprotein lipase abolishes this rapid phase. In the cholesterol-fed dog, the slow, late phase of clearance corresponds to the conversion of fraction II beta-VLDL to the smaller, denser intermediate and low density lipoproteins (IDL and LDL), which are slowly cleared from the plasma. It is concluded that fraction II beta-VLDL are catabolized in the normal dog by rapid uptake mediated at least in part by the apo B,E(LDL) receptor of hepatic parenchymal cells. In cholesterol-fed dogs, in which these receptors are markedly down-regulated, fraction II beta-VLDL are apparently initially bound to endothelial cells and converted to IDL and LDL by lipolytic processing.

Structural relationship of human apolipoprotein B48 to apolipoprotein B100

Although the complete amino acid sequence of human apolipoprotein (apo) B100 is known (4536 amino acids), the structure of apo B48 has not been defined. The objective of our study was to define the structure of apo B48 and its relationship to apo B100. Antibodies were produced against 22 synthetic peptides corresponding to sequences in human apo B100. The levels of immunoreactivity of the antipeptides to apo B100 and apo B48 were used to define the structural relationship between these two species of apo B. Six antibodies from sequences in the amino-terminal half of apo B100, including antipeptide 2110-2129, bound to both apo B100 and apo B48. 15 other apo B-specific antipeptides from sequences carboxyl-terminal to residue 2152 bound to apo B100, but not to apo B48. Immunoblots of cyanogen bromide digests of apo B100 and apo B48 with antipeptides 2068-2091 and 2110-2129 detected a 16-KD fragment (residues 2016-2151) in the apo B100 digest and a fragment of identical size in the apo B48 digest. Because apo B48 appears to contain the apo B100 cyanogen bromide fragment 2016-2151 and because an antiserum specific for the peptide 2152-2168 does not bind to apo B48, we conclude that apo B48 represents the amino-terminal 47% of apo B100 and that the carboxyl terminus of apo B48 is in the vicinity of residue 2151 of apo B100.

Receptor binding activity of lipid recombinants of apolipoprotein B-100 thrombolytic fragments

Apolipoprotein (apo) B-100, the protein constituent of low density lipoproteins (LDL), is the determinant responsible for LDL binding to the apoB,E(LDL) receptor on cells. The current study was designed to identify the region(s) of apoB-100 that interact with the apoB,E(LDL) receptor. Apolipoprotein B-100 was fragmented by thrombin digestion, and the isolated fragments (T2, T3, T4) were recombined with cholesterol-induced canine high density lipoproteins (HDLc). Before the recombination, the receptor binding activity of apoE of the HDLc was abolished by reductive methylation and extensive trypsin treatment. This treatment permitted almost complete replacement of the small residual apoE fragments by the large apoB fragments. Recombinant apoB particles were isolated by ultracentrifugation and tested for binding to receptors on cultured human fibroblasts. The recombinant particles had chemical and physical properties similar to those of native HDLc. Recombinants of both the whole thrombolytic digest and of isolated fragments displayed specific binding to the apoB,E (LDL) receptor. Anti-apoB,E(LDL) receptor antibodies abolished 90% of the binding, and there was almost no specific binding to receptor-negative fibroblasts or to cells in which the receptors had been down-regulated. The binding of apoB-100 recombinants to the receptor also demonstrated calcium dependency; in addition, the surface binding of the recombinants was released by polyanionic compounds. All these recombinants had binding affinities comparable to one another but less than that of native LDL. Although T2, T3 and T4 recombinants can all bind specifically to the apoB,E(LDL) receptor, it remains to be established whether their activity represents physiologically relevant binding. Nevertheless, the present findings illustrate the potential of the recombinant method using HDLc lipids to reconstitute biological activity.

Characterization of lipoprotein receptors on rat Fu5AH hepatoma cells

The rat hepatoma cell line Fu5AH has the unusual property of accumulating massive amounts of cholesteryl ester upon incubation with hypercholesterolemic serum, and especially when incubated with beta-very low density lipoproteins (beta-VLDL) from cholesterol-fed dogs. The present study was designed to identify and characterize the lipoprotein receptors that mediate the cholesteryl ester accumulation. The beta-VLDL and cholesterol-induced apolipoprotein (apo) E-containing high density lipoproteins (apoE HDLc) bound to Fu5AH cells with very high affinity (Kd approximately equal to 10(-10) M), whereas low density lipoproteins (LDL) bound with unusually low affinity (Kd approximately equal to 10(-8) M). Receptor binding activity of 125I-labeled beta-VLDL, 125I-labeled apoE HDLc, and 125I-labeled LDL was abolished by incubation in the presence of an excess of unlabeled LDL or of a polyclonal antibody to the bovine adrenal apoB,E(LDL) receptor. The receptors were completely down-regulated by preincubating Fu5AH cells with beta-VLDL, but much higher levels of beta-VLDL were required than for down-regulation of fibroblast apoB,E(LDL) receptors. Receptor binding was abolished by reductive methylation of the lysyl residues of the apolipoprotein of the beta-VLDL and by an apoE monoclonal antibody (1D7) that blocks receptor binding. The Fu5AH receptor was further characterized by using the bovine adrenal apoB,E(LDL) receptor antibody. A single protein (Mr approximately equal to 130,000) was identified in Triton extracts of whole cells, and two proteins (Mr approximately equal to 130,000 and 115,000) were found in Fu5AH cell membranes disrupted by homogenization. The Mr approximately equal to 115,000 protein was released from the membranes and did not react with an antibody to the carboxyl-terminal (cytoplasmic) domain of the apoB,E(LDL) receptors. These studies indicate that Fu5AH cells express apoB,E(LDL) receptors that have unusually low affinity for apoB-continuing lipoproteins, require large amounts of cholesterol to induce down-regulation, and are susceptible to specific proteolysis in cell homogenates. These apoB,E(LDL) receptors are responsible for the receptor-mediated uptake of beta-VLDL and chylomicron remnants by Fu5AH cells.

Familial defective apolipoprotein B-100: low density lipoproteins with abnormal receptor binding

Previous in vivo turnover studies suggested that retarded clearance of low density lipoproteins (LDL) from the plasma of some hypercholesterolemic patients is due to LDL with defective receptor binding. The present study examined this postulate directly by receptor binding experiments. The LDL from a hypercholesterolemic patient (G.R.) displayed a reduced ability to bind to the LDL receptors on normal human fibroblasts. The G.R. LDL possessed 32% of normal receptor binding activity (approximately equal to 9.3 micrograms of G.R. LDL per ml were required to displace 50% of 125I-labeled normal LDL, vs. approximately equal to 3.0 micrograms of normal LDL per ml). Likewise, the G.R. LDL were much less effective than normal LDL in competing with 125I-labeled normal LDL for cellular uptake and degradation and in stimulating intracellular cholesteryl ester synthesis. The defect in LDL binding appears to be due to a genetic abnormality of apolipoprotein B-100: two brothers of the proband possess LDL defective in receptor binding, whereas a third brother and the proband's son have normally binding LDL. Further, the defect in receptor binding does not appear to be associated with an abnormal lipid composition or structure of the LDL: the chemical and physical properties of the particles were normal, and partial delipidation of the LDL did not alter receptor binding activity. Normal and abnormal LDL subpopulations were partially separated from plasma of two subjects by density-gradient ultracentrifugation, a finding consistent with the presence of a normal and a mutant allele. The affected family members appear to be heterozygous for this disorder, which has been designated familial defective apolipoprotein B-100. These studies indicate that the defective receptor binding results in inefficient clearance of LDL and the hypercholesterolemia observed in these patients.

DNA sequence of the human apolipoprotein B gene

The sequence of the human apolipoprotein B gene comprises 43 kb divided into 29 exons, one of which is unusually long and contains 7572 bp. Comparison of the gene sequence with four complete and three partial cDNA sequences published elsewhere reveals a total of 60 nucleotide substitutions and 39 amino acid substitutions and one small deletion in the signal peptide.

Lipoprotein uptake by neuronal growth cones in vitro

Macrophages that rapidly enter injured peripheral nerve synthesize and secrete large quantities of apolipoprotein E. This protein may be involved in the redistribution of lipid, including cholesterol released during degeneration, to the regenerating axons. To test this postulate, apolipoprotein E-associated lipid particles released from segments of injured rat sciatic nerve and apolipoprotein E-containing lipoproteins from plasma were used to determine whether sprouting neurites, specifically their growth cones, possessed lipoprotein receptors. Pheochromocytoma (PC12) cells, which can be stimulated to produce neurites in vitro, were used as a model system. Apolipoprotein E-containing lipid particles and lipoproteins, which had been labeled with fluorescent dye, were internalized by the neurites and their growth cones; the unmetabolized dye appeared to be localized to the lysosomes. The rapid rate of accumulation in the growth cones precludes the possibility of orthograde transport of the fluorescent particles from the PC12 cell bodies. Thus, receptor-mediated lipoprotein uptake is performed by the apolipoprotein B,E(LDL) (low density lipoprotein) receptors, and in the regenerating peripheral nerve apolipoprotein E may deliver lipids to the neurites and their growth cones for membrane biosynthesis.

Mapping of human apolipoprotein B antigenic determinants

A minimum of 16 epitopes which provide a group of topographical markers to study the conformation of apolipoprotein (apo) B have been mapped in relation to elements of the sequence of apo B-100. Six of these epitopes are identified by monoclonal antibodies (Mabs) directed against low density lipoprotein (LDL) apo B, while at least 10 others react with Mabs obtained by immunization with delipidated and solubilized apo B. Five epitopes which are also expressed on apo B-48 have been assigned to the thrombolytic fragment T4 on the N-terminal side of apo B-100. None of these five epitopes requires the presence of lipids for its expression, suggesting that the conformation of the T4 region of apo B is more dependent on peptide-chain interactions than on peptide-lipid interactions. Four distinct epitopes have been assigned to the median thrombolytic fragment T3 of apo B-100, all of which require the presence of lipids for their expression; those epitopes closer to the C-terminus of T3 require specific interaction with cholesteryl esters. The same lipid dependence also characterizes a cluster of epitopes mapped to the N-terminal region of fragment T2. The epitopes that are close to the T2/T3 cleavage site and depend on the presence of cholesteryl esters for their expression are also those that react with the Mabs that inhibit the binding of LDL to its receptor. Therefore this region, which in addition contains two sequences with structural homology to the apo E receptor binding domain, probably constitutes a physiologically important receptor binding site for apo B. Finally, four other distinct epitopes which do not require the presence of lipids for their expression have been mapped on T2. In conclusion, the present report presents evidence that the immunochemical analogy of apo B-48 and apo B-100 is on the N-terminal half of apo B-100, whereas the apo B receptor binding domain is localized on the C-terminal half of apo B-100 close to the T2/T3 cleavage site.

Apolipoprotein E synthesis in neurofibrosarcoma and schwannoma cell cultures from two individuals with neurofibromatosis

Apolipoprotein E is expressed in neurofibrosarcoma and Schwannoma cell cultures derived from two patients with different types of neurofibromatosis, but not in six cell cultures derived from the benign neurofibromatosis neurofibromas. In addition, a cell culture derived from a nonneurofibromatosis human malignant astrocytoma showed apolipoprotein E expression. Although all cells in either the neurofibrosarcoma or Schwannoma cultures appeared morphologically similar (suggesting homogeneity), apolipoprotein E was immunochemically detected in the perinuclear region of only half of the cells. Thus, production of apolipoprotein E in neurofibromatosis-associated neurofibroma tumors may be a marker for a specific subclass of transformed cells. The expression of apolipoprotein E in glial cell neoplasms is possibly related to an alteration in their lipid metabolism.

Astrocytes synthesize apolipoprotein E and metabolize apolipoprotein E-containing lipoproteins

We have previously demonstrated that astrocytes synthesize and secrete apolipoprotein E in situ. In the present work, primary cultures of rat brain astrocytes were used to study apolipoprotein E synthesis, secretion, and metabolism in vitro. The astrocytes in culture contained immunoreactive apolipoprotein E in the area of the Golgi apparatus. Incubation of the astrocytes with [35S]methionine resulted in the secretion of labeled immunoprecipitable apolipoprotein E, which constituted 1-3% of the total secreted proteins. The apolipoprotein E secreted in culture and the apolipoprotein E in rat brain extracts differed from serum apolipoprotein E in two respects: both had a slightly higher apparent molecular weight (approx. 36,000) and more acidic isoforms than serum apolipoprotein E. Sialylation of the newly secreted apolipoprotein accounted for the difference in both the apparent molecular weight and isoelectric focusing pattern of newly secreted apolipoprotein E and plasma apolipoprotein E. The astrocytes possessed apolipoprotein B,E(LDL) receptors capable of binding and internalizing apolipoprotein E-containing lipoproteins. The uptake of lipoproteins by the cells led to a reduction in the number of cell surface receptors and to the intracellular accumulation of cholesteryl esters. Since apolipoprotein E is present within the brain, and since brain cells can express apolipoprotein B,E(LDL) receptors, apolipoprotein E-containing lipoproteins may function to redistribute lipid and regulate cholesterol homeostasis within the brain.

Expression of the human apolipoprotein E gene in cultured mammalian cells

The gene for human apolipoprotein (apo-) E was isolated from a human genomic library constructed in the cosmid shuttle vector pCV108. The transient expression of the apo-E gene was examined in cultured mammalian cells 48 h following calcium phosphate-mediated gene transfer. The expression of the cloned human apo-E gene, which contained between 0.7 and 29 kilobases of 5'-flanking DNA, was not restricted to human cells or to cultured cells derived from tissues that have been shown to synthesize apo-E. Several independent mouse L cell stable transfectants with the human apo-E gene integrated into their genome were selected on the basis of G418 resistance, which is conferred by the selectable gene marker in the cosmid vector. The levels of human apo-E mRNA found in the stable transfected mouse L cells ranged from undetectable to a level comparable to that found in the human liver. The size of the apo-E mRNA observed in the stable transfectants was identical to that found in the liver, indicating that the mouse L cells were capable of correctly processing the human apo-E gene transcripts. The integrated human apo-E genes had not undergone major rearrangements or deletions during transfer, and the level of apo-E mRNA found in the different stable transfectants correlated directly with the number of integrated copies of the human apo-E gene. The stable transfected L cells secreted authentic human apo-E into the medium. The secreted protein interacted specifically with antibodies to human plasma apo-E and had an apparent Mr = 35,000 to 36,000, which is slightly larger than that of plasma apo-E. The secreted human apo-E was associated with lipid (presumably phospholipids), floated at d approximately 1.09 g/ml, and bound with high affinity to the apo-B,E(LDL) receptor on fibroblasts.

Identification of homozygosity for a human apolipoprotein A-I variant

An apolipoprotein (apo) A-I variant, previously described in two Norwegian families (Schamaun et al. 1983. Hum. Genet. 64: 380-383), represents a mutation in apoA-I in which a single amino acid substitution of lysine for glutamic acid has taken place at residue 136. An offspring resulting from intermarriage between the two families is genotypically homozygous for this variant. He is the first individual discovered to be homozygous for any of the apoA-I variants. Analysis of lipid data collected from these families indicates one or more lipid abnormalities. The low density lipoproteins (LDL) of subjects having this apoA-I variant demonstrate a compositional abnormality. The plasma cholesterol concentration in the homozygous subject is low because of the extremely reduced levels of LDL and apoB, a property shared by some of his first-degree relatives. However, because of the presence of apoE2 in this family, it is not possible to definitively link these lipid abnormalities to the presence of the A-I variant.

Isolation and characterization of the apolipoprotein E receptor from canine and human liver

Previous results have demonstrated that liver membranes possess two distinct lipoprotein receptors: a low density lipoprotein (LDL) receptor that binds lipoproteins containing either apolipoprotein (apo-) B or apo-E, and an apo-E-specific receptor that binds apo-E-containing lipoproteins, but not the apo-B-containing LDL. This study reports the isolation and purification of apo-B,E(LDL) and apo-E receptors from canine and human liver membranes. The receptors were solubilized with the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate and were partially purified by DEAE-cellulose chromatography. The apo-B,E(LDL) receptor was isolated by affinity chromatography on LDL-Sepharose. The apo-E receptor, which did not bind to the LDL-Sepharose column, was then purified by using an HDLc (cholesterol-induced high density lipoprotein)-Sepharose affinity column and an immunoaffinity column. Characterization of the receptors revealed that the hepatic apo-B,E(LDL) receptor is similar to the extrahepatic LDL receptor with an apparent Mr = 130,000 on non-reducing sodium dodecyl sulfate-polyacrylamide gels. The apo-E receptor was found to be distinct from the apo-B,E(LDL) receptor, with an apparent Mr = 56,000. The purified apo-E receptor displayed Ca2+-dependent binding to apo-E-containing lipoproteins and did not bind to LDL or chemically modified apo-E HDLc. Antibodies raised against the apo-B,E(LDL) receptor cross-reacted with the apo-E receptor. However, an antibody prepared against the apo-E receptor did not react with the apo-B,E(LDL) receptor. The apo-E receptor also differed from the apo-B,E(LDL) receptor in amino acid composition, indicating that the apo-E receptor and the apo-B,E(LDL) receptor are two distinct proteins. Immunoblot characterization with anti-apo-E receptor immunoglobulin G indicated that the apo-E receptor is present in the hepatic membranes of man, dogs, rats, and mice and is localized to the rat liver parenchymal cells.

Human apolipoprotein E. Determination of the heparin binding sites of apolipoprotein E3

The interaction of human apolipoprotein (apo-) E3 with heparin was examined using heparin-Sepharose as a model system. The approach taken to determine the region of apo-E that is responsible for binding to heparin was to identify apo-E monoclonal antibodies that inhibited heparin binding, to determine the epitopes of the inhibiting antibodies, and finally to examine the heparin binding of fragments containing the inhibiting antibody epitopes. Three antibodies, designated 1D7, 6C5, and 3H1, were found to inhibit binding, suggesting that multiple heparin binding sites were present on apo-E. The epitopes of the inhibiting antibodies were determined by immunoblot analysis of synthetic or proteolytic fragments of apo-E. Measurement of the heparin binding activity of fragments containing epitopes of the inhibiting antibodies demonstrated that apo-E3 contains two heparin binding sites. The first site is located in the vicinity of residues 142-147 and coincides with the 1D7 epitope. The second binding site is contained in the carboxyl-terminal region of apo-E and is inhibited by 3H1, the epitope of which is located between residues 243 and 272. The epitope of the third inhibiting antibody, 6C5, is located at the amino terminus of apo-E; however, this antibody inhibits the second heparin binding site located in the carboxyl-terminal region. A head-to-tail association of apo-E, in which the 6C5 epitope and the second heparin binding site would be in close proximity, is proposed to account for this observation. In the lipid-free state both heparin binding sites on apo-E are expressed; however, when apo-E is complexed to phospholipid or on the surface of a lipoprotein particle, only the first binding site (residues 142-147) is expressed.

Fat feeding in humans induces lipoproteins of density less than 1.006 that are enriched in apolipoprotein [a] and that cause lipid accumulation in macrophages

Formula diets containing lard or lard and egg yolks were fed to six normolipidemic volunteers to investigate subsequent changes in the composition of lipoproteins of d less than 1.006 g/ml and in their ability to bind and be taken up by receptors on mouse macrophages. Both formulas induced the formation of d less than 1.006 lipoproteins that were approximately 3.5-fold more active than fasting very low density lipoproteins (VLDL) in binding to the receptor for beta-VLDL on macrophages. Subfractionation of postprandial d less than 1.006 lipoproteins by agarose chromatography yielded two subfractions, fraction I (chylomicron remnants) and fraction II (hepatic VLDL remnants), which bound to receptors on macrophages. However, fraction I lipoproteins induced a 4.6-fold greater increase in macrophage triglyceride content than fraction II lipoproteins or fasting VLDL. Fraction I lipoproteins were enriched in apolipoproteins (apo) B48, E, and [a]. Fraction II lipoproteins lacked apo[a] but possessed apo B100 and apo E. The apo[a] was absent in normal fasting VLDL, but was present in the d less than 1.006 lipoproteins (beta-VLDL) of fasting individuals with type III hyperlipoproteinemia. The apo[a] from postprandial d less than 1.006 lipoproteins was larger than either of two apo[a] subspecies obtained from lipoprotein (a) [Lp(a)] isolated at d = 1.05-1.09. However, all three apo[a] subspecies were immunochemically identical and had similar amino acid compositions: all were enriched in proline and contained relatively little lysine, phenylalanine, isoleucine, or leucine. The association of apo[a] with dietary fat-induced fraction I lipoproteins suggests that the previously observed correlation between plasma Lp(a) concentrations and premature atherosclerosis may be mediated, in part, by the effect of apo[a] on chylomicron remnant metabolism.

Apolipoprotein E is the determinant that mediates the receptor uptake of beta-very low density lipoproteins by mouse macrophages

Beta very low density lipoproteins (beta-VLDL) from cholesterol-fed animals from patients with Type III hyperlipoproteinemia are internalized by a receptor-mediated process in mouse macrophages. Once internalized, the cholesteryl esters of beta-VLDL are hydrolyzed in lysosomes, and the released cholesterol is re-esterified, resulting in a massive accumulation of cholesteryl esters. In the present study, competitive binding experiments demonstrated that canine apo E HDLc (lipoproteins that contain almost exclusively apolipoprotein E) inhibited the receptor-mediated uptake of 125I-beta-VLDL. The incorporation of human apo E into beta-VLDL was also shown to modulate binding. Reductively methylated beta-VLDL (methyl beta-VLDL) were not taken up by macrophages and did not stimulate cholesteryl ester synthesis. When unmodified human apo E-3 was incorporated into the lipoprotein in place of the canine methyl apo E, these hybrid beta-VLDL (methyl beta-VLDL [E-3]) were internalized and degraded and were as effective as native beta-VLDL in stimulating cholesteryl ester synthesis in macrophages. In the reverse experiment, the incorporation of methyl apo E-3 into native canine beta-VLDL (beta-VLDL [methyl E-3]) reduced the binding activity of the beta-VLDL and abolished their ability to stimulate cellular cholesteryl ester synthesis. Canine beta-VLDL into which apo E-2(Arg158----Cys) had been incorporated had less ability to stimulate cholesteryl ester synthesis (20%) than did native beta-VLDL, but they were more active than beta-VLDL [methyl E-2] or beta-VLDL [methyl E-3], which had virtually no activity. These results demonstrate that apo E is the determinant mediating the receptor binding and uptake of beta-VLDL by mouse macrophages.

Preparative immobiline isoelectric focusing of plasma apolipoproteins on vertical slab gels

An effective preparative isoelectric focusing method has been developed using the LKB Immobiline system in a vertical slab gel apparatus. Advantages of this procedure are ease of sample application, excellent resolution, and the direct visualization of focused bands. Narrow pH gradients have been used to separate apolipoprotein E3 isoforms (pH gradient 4.9-5.9) and to resolve the apolipoprotein C mixture (pH gradient 4.0-5.0). Recoveries ranged from 40 to 70%. The method should be valuable for protein and isoform purification.

Obligatory role of cholesterol and apolipoprotein E in the formation of large cholesterol-enriched and receptor-active high density lipoproteins

The formation of large cholesterol-enriched high density lipoproteins (HDL1/HDLc) from typical HDL3 requires lecithin:cholesterol acyltransferase activity, additional cholesterol, and a source of apolipoprotein (apo-) E. The present study explores the role of apo-E in promoting HDL1/HDLc formation and in imparting to these lipoprotein particles the ability to interact with the apo-B,E(low density lipoprotein (LDL] receptor. Incubation of normal canine serum with cholesterol-loaded mouse peritoneal macrophages resulted in the formation of HDL1/HDLc that competed with 125I-LDL for binding to the apo-B,E(LDL) receptors on cultured human fibroblasts. Cholesterol efflux from macrophages was necessary because incubation of normal canine serum with nonloaded macrophages did not cause HDL1/HDLc formation. However, cholesterol delivery to the serum was not sufficient to result in HDL1/HDLc formation. Apolipoprotein E had to be available. Incubation of apo-E-depleted canine serum with cholesterol-loaded J774 cells, a macrophage cell line that does not synthesize apo-E, demonstrated that no HDL1/HDLc formation was detected even in the presence of significant cholesterol efflux. However, addition of exogenous apo-E to the serum during the incubation with cholesterol-loaded J744 cells promoted the formation of large receptor-active HDL1/HDLc. The receptor binding activity of these particles produced in vitro correlated with the amount of apo-E incorporated into the HDL1/HDLc. Apolipoproteins A-I and C-III were ineffective in promoting HDL1/HDLc formation; thus, apo-E was unique in allowing HDL1/HDLc formation. These results demonstrate that when lecithin:cholesterol acyltransferase activity, cholesterol, and apo-E are present in serum, typical HDL can be transformed in vitro into large cholesterol-rich HDL1/HDLc that are capable of binding to lipoprotein receptors.

Lipoproteins of special significance in atherosclerosis. Insights provided by studies of type III hyperlipoproteinemia

In summary, the study of type III hyperlipoproteinemia has provided important insights into lipoprotein metabolism that have helped to elucidate several functional roles for apo E and have provided a better understanding of the mechanisms whereby specific lipoproteins may be atherogenic or anti-atherogenic. The molecular defect in type III hyperlipoproteinemia and dysbetalipoproteinemia is the presence of a mutant form of apo E, usually apo E2, that is defective in binding to both apo B,E(LDL) and apo E receptors. The receptor-defective apo E results in an impaired clearance of remnant lipoproteins (beta-VLDL). In addition, the abnormal apo E may impair the lipolytic processing of hepatic beta-VLDL through its involvement in lipid transfer or exchange processes. The accumulation of beta-VLDL may provide the most direct mechanism responsible for the accelerated atherosclerosis observed in type III hyperlipoproteinemia, a mechanism that involves the receptor mediated uptake of beta-VLDL by macrophages, which are then converted to arterial foam cells. Alterations in the HDL of patients with type III hyperlipoproteinemia further support the concept that HDL are anti-atherogenic. The increase in HDL-with apo E provides insight into the role of these cholesterol-enriched HDL in reverse cholesterol transport and in the cellular redistribution of cholesterol, processes whereby cholesterol deposition may be reversed. It should be stressed that both the accumulation of beta-VLDL and alterations in HDL (reduction in typical HDL and an increase in HDL-with apo E) are associated with accelerated atherogenesis in animals fed high levels of fat and cholesterol. Although valuable information has been gained concerning the mechanisms involved in type III hyperlipoproteinemia by the study of the disease, the clinical expression of this disorder is variable, ranging from hypocholesterolemia to marked hypercholesterolemia in subjects with the same molecular defect (E2/2). This variability in expression is more easily understood when one considers the various factors that can promote the hyperlipoproteinemia and when one considers the mechanisms of action whereby these factors may exacerbate the effects of the presence of an abnormal apo E. In most cases, development of type III hyperlipoproteinemia requires that a second event (a predisposing environmental factor or a second genetic defect) be associated with the primary genetic defect (an abnormal form of apo E).

Apolipoprotein E associated with astrocytic glia of the central nervous system and with nonmyelinating glia of the peripheral nervous system

The plasma protein apolipoprotein (apo) E is an important determinant of lipid transport and metabolism in mammals. In the present study, immunocytochemistry has been used to identify apo E in specific cells of the central and peripheral nervous systems of the rat. Light microscopic examination revealed that all astrocytes, including specialized astrocytic cells (Bergmann glia of the cerebellum, tanycytes of the third ventricle, pituicytes of the neurohypophysis, and Müller cells of the retina), possessed significant concentrations of apo E. In all of the major subdivisions of the central nervous system, the perinuclear region of astrocytic cells, as well as their cell processes that end on basement membranes at either the pial surface or along blood vessels, were found to be rich in apo E. Extracellular apo E was present along many of these same surfaces. The impression that apo E is secreted by astrocytic cells was confirmed by electron microscopic immunocytochemical studies, which demonstrated the presence of apo E in the Golgi apparatus. Apo E was not present in neurons, oligodendroglia, microglia, ependymal cells, and choroidal cells. In the peripheral nervous system, apo E was present within the glia surrounding sensory and motor neurons; satellite cells of the dorsal root ganglia and superior cervical sympathetic ganglion as well as the enteric glia of the intestinal ganglia were reactive. Apo E was also present within the non-myelinating Schwann cells but not within the myelinating Schwann cells of peripheral nerves. These results suggest that apo E has an important, previously unsuspected role in the physiology of nervous tissue.

Topographic study of sudanophilic lesions in cholesterol-fed minipigs by image analysis

A new morphometric technique using image analysis has been developed to express the topographic distribution of atherosclerotic lesions in unambiguous statistical terms. Computer-stored images of opened Sudan IV-stained aortas and iliac and coronary arteries from hypercholesterolemic minipigs (n = 39) were used in this study. The image processing methods included transformation of the data to standard templates, automated image segmentation, and creation of probability-of-occurrence maps. These maps have shown that sudanophilic lesions are localized with a characteristic topography along the aortas and iliac and coronary arteries. Areas of high probability are associated with the entrance regions of vessels and the lateral leading edges of the major flow dividers. Regions immediately distal to large branches were found to be areas of low probability. Despite the association of areas of sudanophilia with entrance regions and branch points, a major portion of sudanophilic lesions was not associated with any orifice region (e.g., ductus scar, dorsolateral surface of abdominal aorta, and ventral surface of terminal aorta). The present study provides the necessary information for the development of a rational sampling strategy for the experimental study of the distribution of localizing factors (e.g., hemodynamic, biochemical, cellular, mass transport, histological) and their relationships to putative atherogenic mechanisms.