Identification and DNA sequence of a human apolipoprotein E cDNA clone

Distinct Roles of Apolipoproteins A1 and E in the Modulation of High-Density Lipoprotein Composition and Function

In addition to high-density lipoprotein cholesterol (HDL-C) levels, HDL quality also appears to be very important for atheroprotection. Analysis of various clinical paradigms suggests that the lipid and apolipoprotein composition of HDL defines its size, shape, and functions and may determine its beneficial effects on human health. Previously, we reported that like apolipoprotein A-I (Apoa1), apolipoprotein E (Apoe) is also capable of promoting the de novo biogenesis of HDL with the participation of ATP binding cassette A lipid transporter member 1 (Abca1) and plasma enzyme lecithin:cholesterol acyltransferase (Lcat), in a manner independent of a functional Apoa1. Here, we performed a comparative analysis of the functions of these HDL subpopulations. Specifically, Apoe and Apoa1 double-deficient (Apoe(-/-) × Apoa1(-/-)) mice were infected with APOA1- or APOE3-expressing adenoviruses, and APOA1-containing HDL (APOA1-HDL) and APOE3-containing HDL (APOE3-HDL), respectively, were isolated and analyzed by biochemical and physicochemical methods. Western blot and lipidomic analyses indicated significant differences in the apolipoprotein and lipid composition of the two HDL species. Moreover APOE3-HDL presented a markedly reduced antioxidant potential and Abcg1-mediated cholesterol efflux capacity. Surprisingly, APOE3-HDL but not APOA1-HDL attenuated LPS-induced production of TNFα in RAW264.7 cells, suggesting that the anti-inflammatory effects of APOA1 are dependent on APOE expression. Taken together, our data indicate that APOA1 and APOE3 recruit different apolipoproteins and lipids on the HDL particle, leading to structurally and functionally distinct HDL subpopulations. The distinct role of these two apolipoproteins in the modulation of HDL functionality may pave the way toward the development of novel pharmaceuticals that aim to improve HDL functionality.

Conformational changes of non-B DNA

In contrast to B-DNA that has a right-handed double helical structure with Watson-Crick base pairing under the ordinary physiological conditions, repetitive DNA sequences under certain conditions have the potential to fold into non-B DNA structures such as hairpin, triplex, cruciform, left-handed Z-form, tetraplex, A-motif, etc. Since the non-B DNA-forming sequences induce the genetic instability and consequently can cause human diseases, the molecular mechanism for their genetic instability has been extensively investigated. On the contrary, non-B DNA can be widely used for application in biotechnology because many DNA breakage hotspots are mapped in or near the sequences that have the potential to adopt non-B DNA structures. In addition, they are regarded as a fascinating material for the nanotechnology using non-B DNAs because they do not produce any toxic byproducts and are robust enough for the repetitive working cycle. This being the case, an understanding on the mechanism and dynamics of their structural changes is important. In this critical review, we describe the latest studies on the conformational dynamics of non-B DNAs, with a focus on G-quadruplex, i-motif, Z-DNA, A-motif, hairpin and triplex (189 references).

Deficiency in apolipoprotein E has a protective effect on diet-induced nonalcoholic fatty liver disease in mice

Apolipoprotein E (apoE) mediates the efficient catabolism of the chylomicron remnants very low-density lipoprotein and low-density lipoprotein from the circulation, and the de novo biogenesis of high-density lipoprotein. Lipid-bound apoE is the natural ligand for the low-density lipoprotein receptor (LDLr), LDLr-related protein 1 and other scavenger receptors. Recently, we have established that deficiency in apoE renders mice resistant to diet-induced obesity. In the light of these well-documented properties of apoE, we sought to investigate its role in the development of diet-induced nonalcoholic fatty liver disease (NAFLD). apoE-deficient, LDLr-deficient and control C57BL/6 mice were fed a western-type diet (17.3% protein, 48.5% carbohydrate, 21.2% fat, 0.2% cholesterol, 4.5 kcal·g(-)) for 24 weeks and their sensitivity to NAFLD was assessed by histological and biochemical methods. apoE-deficient mice were less sensitive than control C57BL/6 mice to diet-induced NAFLD. In an attempt to identify the molecular basis for this phenomenon, biochemical and kinetic analyses revealed that apoE-deficient mice displayed a significantly delayed post-prandial triglyceride clearance from their plasma. In contrast with apoE-deficient mice, LDLr-deficient mice fed a western-type diet for 24 weeks developed significant accumulation of hepatic triglycerides and NAFLD, suggesting that apoE-mediated hepatic triglyceride accumulation in mice is independent of LDLr. Our findings suggest a new role of apoE as a key peripheral contributor to hepatic lipid homeostasis and the development of diet-induced NAFLD.

Neuropsychiatric Symptoms, Endophenotypes, and Syndromes in Late-Onset Alzheimer's Disease: Focus on APOE Gene

Neuropsychiatric symptoms, previously denominated as behavioural and psychological symptoms of dementia, are common features of Alzheimer's disease (AD) and are one of the major risk factors for institutionalization. At present, the role of the apolipoprotein E (APOE) gene in the development of neuropsychiatric symptoms in AD patients is unclear. In this paper, we summarized the findings of the studies of neuropsychiatric symptoms and neuropsychiatric syndromes/endophenotypes in AD in relation to APOE genotypes, with special attention to the possible underlying mechanisms. While some studies failed to find a significant association between APOE and neuropsychiatric symptoms in late-onset AD, other studies reported a significant association between the APOE ε4 allele and an increase in agitation/aggression, hallucinations, delusions, and late-life depression or anxiety. Furthermore, some negative studies that focused on the distribution of APOE genotypes between AD patients with or without neuropsychiatric symptoms further emphasized the importance of subgrouping neuropsychiatric symptoms in distinct neuropsychiatric syndromes. Explanations for the variable findings in the existing studies included differences in patient populations, differences in the assessment of neuropsychiatric symptomatology, and possible lack of statistical power to detect associations in the negative studies.

Non-apolipoprotein E and apolipoprotein E genetics of sporadic Alzheimer's disease

The genetic epidemiology of sporadic Alzheimer's disease (SAD) remains a very active area of research,making it one of the most prolifically published areas in medicine and biology. Numerous putative candidate genes have been proposed. However, with the exception of apolipoprotein E (APOE), the only confirmed genetic risk factor for SAD, all the other data appear to be not consistent. Nevertheless, the genetic risk for SAD attributable to the APOE gene in the general population is 20-0%, providing a strong evidence for the existence of additional genetic risk factors. The first part of the present article was dedicated to non-APOE genetics of SAD, reviewing chromosomes-by-chromosomes the available data concerning the major candidate genes. The second part of this article focused on some recently discovered aspects of the APOE polymorphism and their implications for SAD. An attempt to identify the future directions for non-APOE genetic research in SAD was also discussed.

Common SNPs in HMGCR in micronesians and whites associated with LDL-cholesterol levels affect alternative splicing of exon13

Background- Variation in LDL-cholesterol (LDL-C) among individuals is a complex genetic trait involving multiple genes and gene-environment interactions.

METHODS AND RESULTS

In a genome-wide association study (GWAS) to identify genetic variants influencing LDL-C in an isolated population from Kosrae, we observed associations for SNPs in the gene encoding 3hydroxy-3-methylglutaryl (HMG)-coenzyme A (CoA) reductase (HMGCR). Three of these SNPs (rs7703051, rs12654264, and rs3846663) met the statistical threshold of genome-wide significance when combined with data from the Diabetes Genetics Initiative GWAS. We followed up the association results and identified a functional SNP in intron13 (rs3846662), which was in linkage disequilibrium with the SNPs of genome-wide significance and affected alternative splicing of HMGCR mRNA. In vitro studies in human lymphoblastoid cells demonstrated that homozygosity for the rs3846662 minor allele was associated with up to 2.2-fold lower expression of alternatively spliced HMGCR mRNA lacking exon13, and minigene transfection assays confirmed that allele status at rs3846662 directly modulated alternative splicing of HMGCR exon13 (42.9+/-3.9 versus 63.7+/-1.0%Deltaexon13/total HMGCR mRNA, P=0.02). Further, the alternative splice variant could not restore HMGCR activity when expressed in HMGCR deficient UT-2 cells.

CONCLUSIONS

We identified variants in HMGCR that are associated with LDL-C across populations and affect alternative splicing of HMGCR exon13.

Apolipoprotein E predisposes to obesity and related metabolic dysfunctions in mice

Obesity is a central feature of the metabolic syndrome and is associated with increased risk for insulin resistance and typeII diabetes. Here, we investigated the contribution of human apoliproteinE3 and mouse apoliproteinE to the development of diet-induced obesity in response to western-type diet. Our data show that apolipoproteinE contributes to the development of obesity and other related metabolic disorders, and that human apolipoproteinE3 is more potent than mouse apolipoproteinE in promoting obesity in response to western-type diet. Specifically, we found that apolipoproteinE3 knock-in mice fed western-type diet for 24 weeks became obese and developed hyperglycemia, hyperinsulinemia, hyperleptinemia, glucose intolerance and insulin resistance that were more severe than in C57BL/6 mice. In contrast, apolipoproteinE-deficient mice fed western-type diet for the same period were resistant to diet-induced obesity, had normal plasma glucose, leptin and insulin levels, and exhibited normal responses to glucose tolerance and insulin resistance tests. Furthermore, low-density lipoprotein receptor-deficient mice were more sensitive to the development of diet-induced obesity and insulin resistance than apolipoprotein E-deficient mice, but were still more resistant than C57BL/6 mice, raising the possibility that low-density lipoprotein receptor mediates, at least in part, the effects of apolipoproteinE on obesity. Taken together, our findings suggest that, in addition to other previously identified mechanisms of obesity, apolipoproteinE and possibly the chylomicron pathway are also important contributors to the development of obesity and related metabolic dysfunctions in mice.

A 13 kDa carboxy-terminal fragment of ApoE stabilizes Abeta hexamers

The pathological role of ApoE4 in Alzheimer's disease (AD) is not fully elucidated yet but there is strong evidence that ApoE is involved in Abeta deposition, which is an early hallmark of AD neuropathology. Overexpression of ApoE in neuroblastoma cells (Neuro2a) leads to the generation of an intracellular 13 kDa carboxy-terminal fragment of ApoE comparable to fragments seen in brains of AD patients. ApoE4 generates more of this fragment than ApoE2 and E3 suggesting a potential pathological role of these fragments in Alzheimer's disease. Analysis of this intracellular ApoE4 fragment by protease digest followed by MALDI-TOF mass spectrometry showed the proteolytic cleavage site close to residue 187 of ApoE. We have engineered and expressed the corresponding ApoE fragments in vitro. The recombinant 13 kDa carboxy-terminal fragment inhibited fibril formation of Abeta; this contrasts with the full-length ApoE and the corresponding amino-terminal ApoE fragment. Moreover, we show that the 13 kDa carboxy-terminal fragment of ApoE stabilizes the formation of Abeta hexamers. Complexes of Abeta with the 13 kDa carboxy-terminal ApoE fragment show toxicity in PC12 cells comparable to Abeta fibrils. These data suggest that cleavage of ApoE, leading to the generation of this fragment, contributes to the pathogenic effect of ApoE4 in AD.

Non-steroidal anti-inflammatory drugs mediate increased in vitro glial expression of apolipoprotein E protein

Epidemiological studies have shown that use of non-steroidal anti-inflammatory drugs (NSAIDs) by the elderly is associated with a decreased relative risk and a delayed onset of Alzheimer's disease (AD). In contrast, the apolipoprotein E (apoE) gene has proven to be a risk factor for AD with apoE epsilon 4 AD patients having been found to show lower levels of brain apoE. In the present study, treatment of primary rat mixed glial cell cultures with the common NSAIDs, indomethacin and aspirin, induced significant increases in extracellular apoE protein levels. Similarly, treatment of primary rat astrocyte cell cultures with aspirin and a cyclooxygenase (COX)-2-selective aspirin derivative also stimulated significant increases in apoE protein. However, astrocyte and mixed glial apoE protein levels were significantly reduced following exposure to COX-2-specific indomethacin amides and an inactive indomethacin derivative. ApoE protein modulation was observed at physiological and subphysiological concentrations well below the COX inhibition IC50 values of the NSAIDs used, suggestive of a COX-independent mechanism. In contrast to these results, indomethacin and aspirin treatment failed to induce any significant changes in apoE mRNA levels. The failure of NSAIDs to significantly alter apoE expression may have been indicative of a nontranscriptional mechanism of apoE protein induction. Consequently, NSAID-induced increases in apoE protein may enhance apoE-mediated immunosuppression and compensatory synaptic plasticity, potentially resulting in decreased AD risk and delay of disease onset.

Apolipoprotein E3Basel: new insights into a highly conserved protein region

BACKGROUND

Apolipoprotein E is important for the receptor-mediated uptake of triglyceride-rich lipoproteins. Mutations in the gene encoding apolipoprotein E may cause a reduced uptake of these lipoproteins. Particular apolipoprotein E mutations have been also found to be associated with nephrologic, neurologic, and even ophthalmologic diseases. Hence, a continuously expanding role in biology is being attributed to this protein.

DESIGN

Randomly selected volunteers from of a large Swiss cohort were genotyped for the common apolipoprotein E isoforms (apolipoprotein E2, apolipoprotein E3, apolipoprotein E4).

RESULTS

In one of the volunteers, a novel C-to-T mutation causing an alanine-to-valine substitution (A106V, designated apolipoprotein E3Basel) was discovered. Alanine at residue 106 is highly conserved between mammalian species and is located in the immediate vicinity of the 112C/R polymorphism (apolipoprotein E4). Recombinant apolipoprotein E3Basel, expressed in the baculovirus system, displayed no detectable reduction in its low density lipoprotein (LDL) receptor- and heparin-binding activities. Despite normal binding functions, apolipoprotein E3Basel might cause modifications in the lipoprotein pattern. In the index case, plasma triglycerides were elevated and in two further apolipoprotein E3Basel-carriers, cholesterol, phospholipid, apolipoprotein CIII levels, LDL-cholesterol/apoB-100- and VLDL-triglyceride/VLDL-cholesterol-ratios were higher compared with apolipoprotein E3Basel-noncarriers when pair-matched for age and gender. One of the four apolipoprotein E3Basel-carriers from the index family had a personal history of Alzheimer's disease.

CONCLUSIONS

Alanine at amino acid position 106 is highly conserved but not crucial in the receptor-mediated uptake of lipoprotein particles. Nevertheless, amino acid position 106 might be involved in the apolipoprotein E-dependent regulation of the lipoprotein lipase that hydrolyzes triglycerides and in the development of Alzheimer's disease.

Inhibition of atherosclerosis in apolipoprotein-E-deficient mice following muscle transduction with adeno-associated virus vectors encoding human apolipoprotein-E

Apolipoprotein E (apoE) is a multifunctional plasma glycoprotein involved in lipoprotein metabolism and a range of cell signalling phenomena. ApoE-deficient (apoE(-/-)) mice exhibit severe hypercholesterolaemia and are an excellent model of human atherosclerosis. ApoE somatic gene transfer and bone marrow transplantation in apoE(-/-) mice results in reversal of hypercholesterolaemia, inhibition of atherogenesis and regression of atherosclerotic plaque density. Replication defective adeno-associated virus vectors (rAAVs) are an attractive system currently in clinical trial for muscle-based heterologous gene therapy to express secreted recombinant plasma proteins. Here we have applied rAAV transduction of skeletal muscle to express wild-type (epsilon3) and a defective receptor-binding mutant (epsilon2) human apoE transgene in apoE(-/-) mice. In treated animals, apoE mRNA was present in transduced muscles and, although plasma levels of recombinant apoE fell below the detection levels of our ELISA (ie <10 ng/ml), circulating antibodies to human apoE and rAAV were induced. Up to 3 months after a single administration of rAAV/apoE3, a significant reduction in atherosclerotic plaque density in aortas of treated animals was observed (approximately 30%), indicating that low-level rAAV-mediated apoE3 expression from skeletal muscle can retard atherosclerotic progression in this well-defined genetic model.

[Apolipoprotein E gene polymorphism and risk of hypercholesterolemia: a case control study in a working population of Valencia]

BACKGROUND

The apolipoprotein E (apoE) gene is polymorphic with three common alleles (epsilon 2, epsilon 3, epsilon 4) whose allelic frequency and association with lipid levels varies from population to population. The aim of this study was to estimate the association between these genetic variants and the risk of hypercholesterolemia in a Mediterranean Spanish population.

PATIENTS AND METHODS

A case-control study in a working population from Valencia was carried out. A total of 330 cases (148 men and 182 women) with moderate hypercholesterolemia (total cholesterol > 200 mg/dl or with lipid lowering treatment) and age range 20 to 60 years, were identified. 330 normocholesterolemic controls matched by age and sex were selected. From all of them data of apoE genotype, body mass index, lipid and lipoprotein levels, socioeconomic and life-style variables were obtained.

RESULTS

The epsilon 2 allele frequency was statistically lower in cases (0.033) than in controls (0.086). The epsilon 4 allele frequency was higher in cases (0.115) than in controls (0.039). In the crude logistic regression analysis, the apoE polymorphism was related (p < 0.001) to the risk of hypercholesterolemia. After adjustment by age, body mass index, educational level, tobacco smoking, alcohol consumption and physical activity the epsilon 2 allele was associated with a lower risk of hypercholesterolemia (odds ratio [OR] = 0.36; 95% confidence interval (CI): 0.20-0.64), and the epsilon 4 allele was associated with a higher risk (OR = 3.04; 95% CI: 1.82-5.06).

CONCLUSIONS

The apoE genotype was significantly related to the risk of moderate hypercholesterolemia in the Mediterranean Spanish population.

Acquisition of secretion of transforming growth factor-beta 1 leads to autonomous suppression of scavenger receptor activity in a monocyte-macrophage cell line, THP-1

Macrophage cells derived from the human monocytic leukemia cell line, THP-1, accumulate esterified cholesterol when cultivated in the presence of acetylated low density lipoprotein (Ac-LDL) through scavenger receptors (ScR). In the present study, we isolated a subtype of THP-1 cells that failed to accumulate esterified cholesterol when cultivated in the presence of Ac-LDL. The cells had negligible amounts of cell association and degradation of Ac-LDL compared with the parent THP-1 cells. The subtype THP-1 cells did not express ScR mRNA as well as that of lipoprotein lipase. In contrast, the expression of apolipoprotein E mRNA was greater than that found in parent THP-1 cells. The culture medium of subtype THP-1 cells treated with 12-O-tetradecanoylphorbol-13-acetate inhibited the uptake of Ac-LDL and the expression of ScR in parent THP-1 cells. After a 48-h incubation in the culture medium containing 12-O-tetradecanoylphorbol-13-acetate, the culture medium of differentiated subtype THP-1 cells contained 6.9 ng/ml transforming growth factor (TGF)-beta 1, while that of parent THP-1 cells secreted below detection level, which was less than 3 ng/ml. This inhibitory effect of the conditioned medium on the expression of ScR in parent THP-1 cells was abolished by pretreatment of the culture medium with anti-TGF-beta 1 antibodies. Parent THP-1 cells expressed as much TGF-beta 1 mRNA as sTHP-1 cells after stimulation of differentiation. Although the precursor forms of TGF-beta 1 that were synthesized in both parent and subtype THP-1 cells were of similar size and were expressed at similar levels, latent TGF-beta 1-binding protein, which is necessary for the secretion of TGF-beta 1, could only be co-immunoprecipitated with anti-TGF-beta 1 antibody from subtype THP-1 cells. This suggests that subtype THP-1 cells secrete TGF-beta 1 into the medium by forming a functional complex with the latent TGF-beta 1-binding protein. We conclude that subtype THP-1 cells could not take up Ac-LDL because ScR was inhibited (leading to a loss of function) caused by the secreted TGF-beta 1.

Pathogenesis of Alzheimer's disease: current status on apolipoprotein E4 gene research

Alzheimer's disease (AD) affects millions of people throughout the world. The financial and emotional costs that are inflicted by this devastating disease are enormous. Genes responsible for the early-onset familial AD have been cloned and found to be membrane associated transport proteins. Evidence suggests that apolipoprotein E4 may be the risk factor for the late-onset form of the disease. Beta-amyloid and potassium channel dysfunction have also been implicated in the development of AD. Hyperphosphorylation of "tau" has been indicated for the generation of neurofibrillary tangles in the brains of Alzheimer's patients. Since apoE4 does not bind to "tau", apoE4 may contribute to the hyperphosphorylation of "tau" which may cause the formation of neurofibrillary tangles in AD patients. Molecular biology research on AD should help in the development of drugs for the prevention and treatment of the disease.

Rational design of an animal model for Alzheimer's disease: introduction of multiple human genomic transgenes to reproduce AD pathology in a rodent

A major obstacle to understanding the pathogenesis of Alzheimer's disease is the lack of easily studied animal models. Our approach is to apply transgenic methods to humanize mice and rats, employing methods that introduce large genomic transgenes, because this improves the level of transgene protein expression and the tissue specificity of expression. Our plan is to reproduce AD pathology in rodents by making them transgenic for several human proteins involved in AD. This report describes transgenic animal lines that we have produced, and summarizes our current approach and future plans. Two human genes known to be involved in AD pathology are the amyloid precursor protein (APP) and the E4 isoform of apolipoprotein E (apoE4). So far, we have produced and analyzed a transgenic line carrying the entire human APP gene cloned in a yeast artificial chromosome. We have also produced but not yet analyzed a mouse carrying the human apoE4 gene. Work is in progress to produce a transgenic line carrying a disease-causing mutation in the human APP gene. As we produce these animals, we are breeding them together, and also breeding them with a mouse line that lacks endogenous apoE, to produce an animal model carrying several human proteins whose interaction is believed to be instrumental in development of AD pathology. These transgenic animals will be useful for dissecting the biochemical and physiological steps leading to AD, and for development of therapies for disease intervention.

Lack of association of apolipoprotein E polymorphism with plasma Lp(a) levels in the Chinese

Apolipoprotein E (apoE) polymorphism and its influence on plasma lipids, lipoproteins, lipoprotein (a) [Lp(a)] and apolipoproteins was studied in 536 (270 males and 266 females) healthy Chinese in Singapore. From analysis of variance with age and BMI as covariates, apoE genotype was found to exert a significant influence on plasma total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C) and apoB in females. Its effect in males was marginally significant only on LDL-C. In both sexes, plasma TC, LDL-C and apoB were lower in those who were E2-3 than in those who were E3-3. There was no significant difference in log-transformed Lp(a) level between the apoE genotypes after adjusting for the confounding effect of LDL-C in addition to age and BMI. The percentage variance (R2 x 100) of the lipid traits explained by apoE polymorphism in the females was 4.94% for plasma TC, 5.85% for LDL-C and 4.25% for apoB. We conclude that: 1) epsilon 2 allele had a lowering effect on plasma TC, LDL-C and apoB; 2) apoE polymorphism did not have any significant influence on Lp(a) concentration; and 3) the effect of apoE polymorphism on plasma TC, LDL-C and apoB was gender-specific, with a stronger influence in females than in males.

The modulation of apolipoprotein E gene expression by 3,3'-5-triiodothyronine in HepG2 cells occurs at transcriptional and post-transcriptional levels

The regulation of the synthesis and secretion of apolipoprotein E (apoE) is incompletely understood. This study examines the mechanisms responsible for regulating apoE gene expression in HepG2 cells by thyroid hormone (3,3'-5-triiodothyronine). The secretion rate of apoE was by thyroid hormone increased (1.5-1.8-fold) in pulse/chase experiments. Thyroid hormone doubled apoE mRNA concentration as determined by Northern-blot analysis. Inhibition of protein synthesis by cycloheximide increased the thyroid-hormone-induced stimulation of apoE mRNA. This suggests that the synthesis of new protein is not required for thyroid hormone to stimulate apoE mRNA. Actinomycin D was used to inhibit new transcription; there was a more rapid degradation of mature apoE mRNA in thyroid hormone-treated HepG2 cells than in control cells, suggesting that thyroid hormone acts post-transcriptionally to regulate apoE gene expression. Cycloheximide blocked the action of thyroid hormone, suggesting that thyroid hormone regulates the turnover of apoE mRNA via the synthesis of de novo protein. Nuclear run-on transcription assays demonstrated that thyroid hormone stimulated apoE gene transcription threefold in 24 h. These findings indicate that the expression of the apoE gene is controlled at both transcriptional and post-transcriptional loci by the thyroid hormone.

Genetic heterogeneity of apolipoprotein E and its influence on plasma lipid and lipoprotein levels

Apolipoprotein E (apoE) is one of the major protein constituents of chylomicron and very-low-density lipoprotein (VLDL) remnants and plays a central role as a ligand in the receptor-mediated uptake of these particles by the liver. Including the most common variant, apoE3, 30 apoE variants have been characterized. At present, 14 apoE variants have been found to be associated with familial dysbetalipoproteinemia, a genetic lipid disorder characterized by elevated plasma cholesterol and triglyceride levels and an increased risk for atherosclerosis. Seven apoE variants were found to be associated with other forms of hyperlipoproteinemia. This report presents an overview of all currently known apoE variants and their effects on lipoprotein metabolism.

Abundant expression of apoprotein E by macrophages in human and rabbit atherosclerotic lesions

Previous studies have demonstrated the presence of apoprotein (apo) E protein and message in arterial lesions. To determine the source of the synthesized apoE, we performed simultaneous in situ hybridization and immunocytochemistry on human and rabbit atherosclerotic tissue. Studies of serial sections of aortic atherosclerotic lesions from humans and hypercholesterolemic New Zealand White rabbits and Watanabe heritable hyperlipidemic rabbits revealed a similar pattern of macrophage-specific apoE expression in the rabbit and human lesions. In early lesions of rabbit atherosclerotic tissue, in which many macrophages were present, there was abundant expression of apoE mRNA. Northern blot analyses of total mRNA obtained from arterial macrophage-derived foam cells, freshly isolated from ballooned, cholesterol-fed New Zealand White rabbits, demonstrated positive hybridization with an apoE-specific riboprobe. Western blot analyses of conditioned media from the isolated foam cells placed in culture for up to 24 hours demonstrated the presence of secreted apoE. These studies demonstrated that in atherosclerotic lesions, arterial wall macrophages synthesize and secrete apoE and probably account for most of the apoE synthesized in the atherosclerotic artery.

Secretion of apolipoprotein E by an astrocytoma cell line

Apolipoprotein (apo) E is a predominant protein in developing mammalian brain and in damaged peripheral nerve. Of particular interest is the observation that astrocytes in the central nervous system cease to produce apoE after nerve damage, whereas an increase in apoE production results after peripheral nerve injury. Differences in the response to injury with regard to the production of apoE may be related to dissimilarities in the abilities of the central and peripheral nervous systems to regenerate. As there are few data concerning the regulation of apoE gene expression in extrahepatic tissues, we employed a human astrocytoma cell line (CCF-STTG1) as a model to study apoE production in astrocytes. CCF-STTG1 cells secreted apoE constitutively in serum-free media. Cholesterol added to the media as cholesterol:phospholipid liposomes (2-100 micrograms/ml) or as human plasma LDL increased the amount of apoE secreted into the media, but had little or no effect on the relative abundance of apoE mRNA. By contrast, the commercially available triglyceride-phospholipid emulsion Intralipid added at dilutions of 1:50 to 1:500 caused a total inhibition of apoE secretion by the cells, but again, little change was noted in the relative abundance of apoE mRNA. Insulin (5 micrograms/ml) caused a 45-55% reduction in the amount of apoE secreted by the astrocytoma cells. Glucagon (5 micrograms/ml), on the other hand, did not increase apoE secretion, and apoE mRNA concentrations were not affected by either hormone treatment. ApoE was secreted from the astrocytoma cells associated with particles of plasma VLDL to IDL and HDL size. After feeding the cells with 20 micrograms/ml cholesterol as cholesterol:phospholipid liposomes, an increased proportion of apoE was secreted associated with the larger VLDL to IDL size particles, with a concomitant decrease in the proportion associated with the smaller HDL-size particles. When cells were incubated with 5 micrograms/ml insulin, most of the apoE was associated with the HDL-size particles. When cholesterol:phospholipid liposomes were added in the presence of insulin virtually all of the secreted apoE was found associated with the VLDL to IDL size particles. In summary, the regulation of apoE production in CCF-STTG1 cells in many respects resembles that of other cells, including hepatocytes. However, it is clear that there remain to be identified cell specific factors which regulate apoE production in astrocytes. The CCF-STTG1 cell line promises to provide a suitable model to investigate these questions.

Polymorphisms in the transcribed 3' untranslated region of eukaryotic genes

In this review we present preliminary evidence for a new class of polymorphism that may be used in a systematic way to map cDNAs efficiently and to expedite the construction of a high-resolution genetic map of the human genome. Ultimately, transcribed 3' untranslated polymorphisms will warrant further study because they should be widely distributed throughout the genome within transcribed sequences, and they can be readily identified as a result of cDNA cloning and sequencing. Furthermore, these markers should be universally available on the basis of the sequence data and highly useful in linkage analyses.

A young type III hyperlipoproteinemic patient associated with apolipoprotein E deficiency

A 13-year-old female patient had noticed tuberoeruptive xanthomas since 3 years of age. Her serum, VLDL, and IDL cholesterol levels were high (348, 158, and 60 mg/dL, respectively), while LDL and HDL cholesterol levels were 56 and 62 mg/dL, respectively. VLDL-cholesterol/serum triglyceride ratio was extremely high (0.86), suggesting type III hyperlipoproteinemia (HLP). Her apo E was undetectable by the single radial immunodiffusion studies and SDS-polyacrylamide gel electrophoresis. Her parents showed hypertriglyceridemia and her two siblings were normolipidemic, and their apo E levels were normal. Genomic DNA digested with BamHI or EcoRI did not show gross differences in the restriction fragment length between the apo-E-deficient patient and normal controls. Thus, apo E deficiency may be characterized by early appearance of clinical manifestations of type III HLP and higher VLDL-cholesterol/serum triglyceride ratio.

Tight linkage between myotonic dystrophy and apolipoprotein E genes revealed with allele-specific oligonucleotides

In 16 families with myotonic dystrophy (DM) a novel approach based on use of allele-specific oligonucleotides has been employed to study the linkage relationship between the apolipoprotein E (APOE) gene and DM. Synthetic oligonucleotides, designed to discriminate between APOE alleles epsilon 3 and epsilon 4, enabled us to distinguish heterozygous carriers in a hybridization assay. In a subset of families, the relevant segment of the APOE gene was enzymatically amplified to increase the sensitivity of the method. For DM and APOE, a maximum lod score (zmax of 7.47 was obtained at a recombination frequency (theta) of 0.047 (male theta = female theta). No recombination (maximum lod score of 5.61 at theta = 0.0) was found between APOE and the apolipoprotein CII (APOC2) gene. These results suggest that, in addition to APOC2, APOE is a useful marker for presymptomatic DM diagnosis.

The baboon apolipoprotein E gene: structure, expression, and linkage with the gene for apolipoprotein C-1

To develop the baboon model for molecular genetic studies of atherosclerosis, we have cloned and sequenced the baboon apolipoprotein E (apo E) gene. The baboon apo E gene encodes the E4 isoform with respect to specific amino acid positions, suggesting that the common epsilon 3 allele is not the primal human allele. Rather than accumulating predominantly synonymous nucleotide changes, 50% of substitutions in human and baboon apo E gene coding regions cause amino acid substitutions. However, comparisons of these apo E proteins show conservation of amphipathic helices required for apo E--lipid interactions. The human and baboon apo E genes have diverged less extensively than those from rat and mouse, providing further evidence for a slowing of molecular evolution in primate species. The baboon and rhesus monkey apo E genes (intron 2) contain two Alu repeats that are absent in the human gene, indicating insertion after the divergence of human and cercopithecine lineages, but before the baboon/rhesus divergence. S1 nuclease studies show that transcription of the baboon apo E gene starts at two different positions, one of which corresponds to the human gene start site. To examine linkage of apolipoprotein genes in the baboon genome, we have used a human cDNA probe to detect apo C-I gene sequences approximately 4 kb from the 3' end of the baboon apo E gene.

Cellular promoters incorporated into the adenovirus genome. Effect of viral DNA replication on endogenous and exogenous gene transcription

Cell-specific utilization of the albumin and immunoglobulin promoters within non-replicating adenovirus genomes has been established. With such viruses plus helper viruses we now consider the impact of infection by replicating viruses on liver-specific gene expression in hepatoma cells and of the effect of replication on the previously established limits of cell-specific expression. We find that replication of any adenovirus, whether or not it contains the albumin promoter, decreases albumin and apolipoprotein A transcription in hepatoma cells but does not affect transcription of at least four other genes expressed mainly (or only) in the liver. Thus, it may be that some hepatoma-specific genes and late adenoviral genes require a factor(s) in common, and these factors become limiting during replication. In hepatoma cells, the increased copy number of the exogenous promoters resulting from viral DNA replication did not influence the cell-specific expression noted previously; only albumin and not globin or immunoglobulin promoters in the virus were active. In contrast, replication by the same viruses in HeLa cells resulted in high levels of expression from all the exogenous promoters.

Structure of mouse type IV collagen. Amino-acid sequence of the C-terminal 511-residue-long triple-helical segment of the alpha 2(IV) chain and its comparison with the alpha 1(IV) chain

The sequence of 511 residues from the C-terminal portion of the triple helix of mouse alpha 2(IV) chain was determined by using the pepsin fragment P2 of collagen IV and two cDNA clones selected from an Engelbreth-Holm-Swarm (EHS) tumor library. The sequence contains nine interruptions of the triplet repeat Gly-Xaa-Yaa ranging in size from single insertions or deletions up to stretches of eleven amino acid residues. Five of these interruptions match those present in the homologous segment of the alpha 1(IV) chain but are otherwise different in length and/or sequence. A low homology was found for the triplet regions of the alpha 1(IV) and alpha 2(IV) chain which constitute more than 90% of the sequence. The data indicate a remote evolutionary relationship of the triple-helical sequences of the two constituent chains of basement membrane collagen.

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.