Identification and characterization of transcriptional regulatory regions associated with expression of the human apolipoprotein E gene

The Impact of Apolipoprotein E (APOE) Epigenetics on Aging and Sporadic Alzheimer's Disease

Sporadic Alzheimer's disease (AD) derives from an interplay among environmental factors and genetic variants, while epigenetic modifications have been expected to affect the onset and progression of its complex etiopathology. Carriers of one copy of the apolipoprotein E gene (APOE) ε4 allele have a 4-fold increased AD risk, while APOE ε4/ε4-carriers have a 12-fold increased risk of developing AD in comparison with the APOE ε3-carriers. The main longevity factor is the homozygous APOE ε3/ε3 genotype. In the present narrative review article, we summarized and described the role of APOE epigenetics in aging and AD pathophysiology. It is not fully understood how APOE variants may increase or decrease AD risk, but this gene may affect tau- and amyloid-mediated neurodegeneration directly or indirectly, also by affecting lipid metabolism and inflammation. For sporadic AD, epigenetic regulatory mechanisms may control and influence APOE expression in response to external insults. Diet, a major environmental factor, has been significantly associated with physical exercise, cognitive function, and the methylation level of several cytosine-phosphate-guanine (CpG) dinucleotide sites of APOE.

Unraveling Molecular and Genetic Insights into Neurodegenerative Diseases: Advances in Understanding Alzheimer's, Parkinson's, and Huntington's Diseases and Amyotrophic Lateral Sclerosis

Neurodegenerative diseases are, according to recent studies, one of the main causes of disability and death worldwide. Interest in molecular genetics has started to experience exponential growth thanks to numerous advancements in technology, shifts in the understanding of the disease as a phenomenon, and the change in the perspective regarding gene editing and the advantages of this action. The aim of this paper is to analyze the newest approaches in genetics and molecular sciences regarding four of the most important neurodegenerative disorders: Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. We intend through this review to focus on the newest treatment, diagnosis, and predictions regarding this large group of diseases, in order to obtain a more accurate analysis and to identify the emerging signs that could lead to a better outcome in order to increase both the quality and the life span of the patient. Moreover, this review could provide evidence of future possible novel therapies that target the specific genes and that could be useful to be taken into consideration when the classical approaches fail to shed light.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer’s disease pathology and brain diseases

ApoE is the major lipid and cholesterol carrier in the CNS. There are three major human polymorphisms, apoE2, apoE3, and apoE4, and the genetic expression of APOE4 is one of the most influential risk factors for the development of late-onset Alzheimer's disease (AD). Neuroinflammation has become the third hallmark of AD, together with Amyloid-β plaques and neurofibrillary tangles of hyperphosphorylated aggregated tau protein. This review aims to broadly and extensively describe the differential aspects concerning apoE. Starting from the evolution of apoE to how APOE's single-nucleotide polymorphisms affect its structure, function, and involvement during health and disease. This review reflects on how APOE's polymorphisms impact critical aspects of AD pathology, such as the neuroinflammatory response, particularly the effect of APOE on astrocytic and microglial function and microglial dynamics, synaptic function, amyloid-β load, tau pathology, autophagy, and cell–cell communication. We discuss influential factors affecting AD pathology combined with the APOE genotype, such as sex, age, diet, physical exercise, current therapies and clinical trials in the AD field. The impact of the APOE genotype in other neurodegenerative diseases characterized by overt inflammation, e.g., alpha- synucleinopathies and Parkinson's disease, traumatic brain injury, stroke, amyotrophic lateral sclerosis, and multiple sclerosis, is also addressed. Therefore, this review gathers the most relevant findings related to the APOE genotype up to date and its implications on AD and CNS pathologies to provide a deeper understanding of the knowledge in the APOE field.

The role of APOE in transgenic mouse models of AD

Identified in 1993, APOE4 is the greatest genetic risk factor for Alzheimer's disease (AD), increasing risk up to 15-fold compared to the common variant APOE3. Since the mid 1990's, transgenic (Tg) mice have been developed to model AD pathology and progression, primarily via expression of the familial AD (FAD) mutations in the presence of mouse-APOE (m-APOE). APOE4, associated with enhanced amyloid-β (Aβ) accumulation, has rarely been the focus in designing FAD-Tg mouse models. Initially, FAD-Tg mice were crossed with human (h)-APOE driven by heterologous promoters to identify an APOE genotype-specific AD phenotype. These models were later supplemented with FAD-Tg mice crossed with APOE-knockouts (APOE-/- or APOE-KO) and h-APOE-targeted replacement (h-APOE-TR) mice, originally generated to study the role of APOE genotype in peripheral lipid metabolism and atherosclerotic lesion development. Herein, we compare the m- and h-APOE multi-gene clusters, and then critically review the relevant history and approaches to developing a Tg mouse model to characterize APOE-dependent AD pathology, in combination with genetic (sex, age) and modifiable (e.g., inflammation, obesity) risk factors. Finally, we present recent data from the EFAD mice, which express 5xFAD mutations with the expression of the human apoE isoforms (E2FAD, E3FAD and E4FAD). This includes a study of 6- and 18-month-old male and female E3FAD and E4FAD, a comparison that enables examination of the interaction among the main AD risk factors: age, APOE genotype and sex. While no single transgenic mouse can capture the effects of all modifiable and genetic risk factors, going forward, a conscious effort needs to be made to include the factors that most significantly modulate AD pathology.

Cell-specific production, secretion, and function of apolipoprotein E

Apolipoprotein E (apoE) is a 34-kDa glycoprotein that is secreted from many cells throughout the body. ApoE is best known for its role in lipoprotein metabolism. Recent studies underline the association of circulating lipoprotein-associated apoE levels and the development for cardiovascular disease (CVD). Besides its well-established role in pathology of CVD, it is also implicated in neurodegenerative diseases and recent new data on adipose-produced apoE point to a novel metabolic role for apoE in obesity. The regulation of apoE production and secretion is remarkably cell and tissue specific. Here, we summarize recent insights into the differential regulation apoE production and secretion by hepatocytes, monocytes/macrophages, adipocytes, and the central nervous system and relevant variations in apoE biochemistry and function.

Regulation of HDL genes: transcriptional, posttranscriptional, and posttranslational

HDL regulation is exerted at multiple levels including regulation at the level of transcription initiation by transcription factors and signal transduction cascades; regulation at the posttranscriptional level by microRNAs and other noncoding RNAs which bind to the coding or noncoding regions of HDL genes regulating mRNA stability and translation; as well as regulation at the posttranslational level by protein modifications, intracellular trafficking, and degradation. The above mechanisms have drastic effects on several HDL-mediated processes including HDL biogenesis, remodeling, cholesterol efflux and uptake, as well as atheroprotective functions on the cells of the arterial wall. The emphasis is on mechanisms that operate in physiologically relevant tissues such as the liver (which accounts for 80% of the total HDL-C levels in the plasma), the macrophages, the adrenals, and the endothelium. Transcription factors that have a significant impact on HDL regulation such as hormone nuclear receptors and hepatocyte nuclear factors are extensively discussed both in terms of gene promoter recognition and regulation but also in terms of their impact on plasma HDL levels as was revealed by knockout studies. Understanding the different modes of regulation of this complex lipoprotein may provide useful insights for the development of novel HDL-raising therapies that could be used to fight against atherosclerosis which is the underlying cause of coronary heart disease.

Znf202 affects high density lipoprotein cholesterol levels and promotes hepatosteatosis in hyperlipidemic mice

Background

The zinc finger protein Znf202 is a transcriptional suppressor of lipid related genes and has been linked to hypoalphalipoproteinemia. A functional role of Znf202 in lipid metabolism in vivo still remains to be established.

Methodology and Principal Findings

We generated mouse Znf202 expression vectors, the functionality of which was established in several in vitro systems. Next, effects of adenoviral znf202 overexpression in vivo were determined in normo- as well as hyperlipidemic mouse models. Znf202 overexpression in mouse hepatoma cells mhAT3F2 resulted in downregulation of members of the Apoe/c1/c2 and Apoa1/c3/a4 gene cluster. The repressive activity of Znf202 was firmly confirmed in an apoE reporter assay and Znf202 responsive elements within the ApoE promoter were identified. Adenoviral Znf202 transfer to Ldlr−/− mice resulted in downregulation of apoe, apoc1, apoa1, and apoc3 within 24 h after gene transfer. Interestingly, key genes in bile flux (abcg5/8 and bsep) and in bile acid synthesis (cyp7a1) were also downregulated. At 5 days post-infection, the expression of the aforementioned genes was normalized, but mice had developed severe hepatosteatosis accompanied by hypercholesterolemia and hypoalphalipoproteinemia. A much milder phenotype was observed in wildtype mice after 5 days of hepatic Znf202 overexpression. Interestingly and similar to Ldl−/− mice, HDL-cholesterol levels in wildtype mice were lowered after hepatic Znf202 overexpression.

Conclusion/Significance

Znf202 overexpression in vivo reveals an important role of this transcriptional regulator in liver lipid homeostasis, while firmly establishing the proposed key role in the control of HDL levels.

Sterol-dependent nuclear import of ORP1S promotes LXR regulated trans-activation of apoE

Oxysterol binding protein related protein 1S (ORP1S) is a member of a family of sterol transport proteins. Here we present evidence that ORP1S translocates from the cytoplasm to the nucleus in response to sterol binding. The sterols that best promote nuclear import of ORP1S also activate the liver X receptor (LXR) transcription factors and we show that ORP1S binds to LXRs, promotes binding of LXRs to LXR response elements (LXREs) and specifically enhances LXR-dependent transcription via the ME.1 and ME.2 enhancer elements of the apoE gene. We propose that ORP1S is a cytoplasmic sterol sensor, which transports sterols to the nucleus and promotes LXR-dependent gene transcription through select enhancer elements.

Functional analysis of APOE locus genetic variation implicates regional enhancers in the regulation of both TOMM40 and APOE

Genetic variation within the apolipoprotein E gene (APOE) locus is associated with late-onset Alzheimer's disease risk and quantitative traits as well as apoE expression in multiple tissues. The aim of this investigation was to explore the influence of APOE locus cis-regulatory element enhancer region genetic variation on regional gene promoter activity. Luciferase reporter constructs containing haplotypes of APOE locus gene promoters; APOE, APOC1, and TOMM40, and regional putative enhancers; TOMM40 IVS2-4, TOMM40 IVS6 poly-T, as well as previously described enhancers; ME1, or BCR, were evaluated for their effects on luciferase activity in 3 human cell lines. Results of this investigation demonstrate that in SHSY5Y cells, the APOE promoter is significantly influenced by the TOMM40 IVS2-4 and ME1 and the TOMM40 promoter is significantly influenced by the TOMM40 IVS6 poly-T, ME1 and BCR. In HepG2 cells, theTOMM40 promoter is significantly influenced by all four enhancers, whereas the APOE promoter is not influenced by any of the enhancers. The main novel finding of this investigation was that multiple APOE locus cis-elements influence both APOE and TOMM40 promoter activity according to haplotype and cell type suggesting that a complex transcriptional regulatory structure modulates regional expression.

Proteomics mapping of cord blood identifies haptoglobin "switch-on" pattern as biomarker of early-onset neonatal sepsis in preterm newborns

Background

Intra-amniotic infection and/or inflammation (IAI) are important causes of preterm birth and early-onset neonatal sepsis (EONS). A prompt and accurate diagnosis of EONS is critical for improved neonatal outcomes. We sought to explore the cord blood proteome and identify biomarkers and functional protein networks characterizing EONS in preterm newborns.

Methodology/Principal Findings

We studied a prospective cohort of 180 premature newborns delivered May 2004-September 2009. A proteomics discovery phase employing two-dimensional differential gel electrophoresis (2D-DIGE) and mass spectrometry identified 19 differentially-expressed proteins in cord blood of newborns with culture-confirmed EONS (n = 3) versus GA-matched controls (n = 3). Ontological classifications of the proteins included transfer/carrier, immunity/defense, protease/extracellular matrix. The 1^st-level external validation conducted in the remaining 174 samples confirmed elevated haptoglobin and haptoglobin-related protein immunoreactivity (Hp&HpRP) in newborns with EONS (presumed and culture-confirmed) independent of GA at birth and birthweight (P<0.001). Western blot concurred in determining that EONS babies had conspicuous Hp&HpRP bands in cord blood (“switch-on pattern”) as opposed to non-EONS newborns who had near-absent “switch-off pattern” (P<0.001). Fetal Hp phenotype independently impacted Hp&HpRP. A Bayesian latent-class analysis (LCA) was further used for unbiased classification of all 180 cases based on probability of “antenatal IAI exposure” as latent variable. This was then subjected to 2^nd-level validation against indicators of adverse short-term neonatal outcome. The optimal LCA algorithm combined Hp&HpRP switch pattern (most input), interleukin-6 and neonatal hematological indices yielding two non-overlapping newborn clusters with low (≤20%) versus high (≥70%) probability of IAI exposure. This approach reclassified ∼30% of clinical EONS diagnoses lowering the number needed to harm and increasing the odds ratios for several adverse outcomes including intra-ventricular hemorrhage.

Conclusions/Significance

Antenatal exposure to IAI results in precocious switch-on of Hp&HpRP expression. As EONS biomarker, cord blood Hp&HpRP has potential to improve the selection of newborns for prompt and targeted treatment at birth.

Association between an intronic apolipoprotein E polymorphism and bone mineral density in Singaporean Chinese females

INTRODUCTION

Apolipoprotein E (ApoE) is implicated in the pathogenesis of osteoporosis.

OBJECTIVE

To investigate possible association of the non-classical APOE gene +113C/G (rs440446) intron 1 enhancer polymorphism with bone mineral density (BMD) in a homogeneous Chinese population in Singapore.

METHODS

A total of 655 volunteers, males and females, aged between 31 and 72 years, from the public participated. BMD was measured using dual-energy X-ray absorptiometry and APOE +113C/G (rs440446) genotypes were determined by Sequenom MassARRAY system. To adjust for potential confounders, anthropometric, demographic, and lifestyle determinants were obtained, and serum lipids and E(2) were measured.

RESULTS

The +113C/G (rs440446) polymorphism within the APOE gene was associated with BMD in Chinese Singaporean females only. Females with the heterozygous CG genotype were significantly associated with reduced total, lumbar spine, and femoral neck of hip BMD, after multilevel adjustment of confounders. The association was stronger in the spine than in the hip. When females were stratified according to WHO classification for osteoporosis, those with CG and GG genotypes had increased risk (OR 3.50 and 2.22, respectively) of developing osteopenia/osteoporosis in the lumbar spine. Serum lipids did not explain the influence of APOE +113 C/G (rs440446) on BMD.

CONCLUSION

This study demonstrated an association between APOE +113C/G (rs440446) polymorphism with measures of BMD in Singaporean Chinese females.

Interactions of functional apolipoprotein E gene promoter polymorphisms with smoking on aortic atherosclerosis

BACKGROUND

Apolipoprotein E gene (APOE) interacts with environmental factors in defining risk for atherosclerosis. We studied whether the APOE epsilon2/epsilon3/epsilon4 genotype or APOE promoter polymorphisms -219G/T and +113G/C might interact with smoking on the development of fatty streaks. We also studied the previously unknown effects of +113G/C on transcriptional activity.

METHODS AND RESULTS

The fatty streak areas of aorta were measured morphometrically in subjects of the Helsinki Sudden Death Study. Within APOE epsilon3/epsilon3 subjects, there was a strong interaction between smoking and both -219G/T (P=0.009) and +113G/C (P=0.003) promoter polymorphisms on abdominal aorta fatty streak area: the -219T- and +113C-allele carriers had larger lesion areas compared with G/G (12.7% versus 5.9%, P=0.007; 12.9% versus 6.3%, P=0.010, respectively) within nonsmokers. Within smokers, the associations were inverse. Moreover, smoking increased the fatty streak area within -219G/G or +113G/G genotypes and -219G/+113G/epsilon3 haplotype carriers. Functional studies in reporter assay showed that in comparison with the +113G allele, the +113C allele had higher transcriptional activity and bound transcription factors from liver cell nuclear extract with significantly lower affinity.

CONCLUSIONS

In middle-aged Finnish men with APOE epsilon3/epsilon3 genotype, the APOE promoter polymorphisms -219G/T and +113G/C interact with smoking in modulating aortic atherosclerosis. The +113G/C polymorphism has an effect on transcriptional activity.

Inflammatory signaling pathways regulating ApoE gene expression in macrophages

The atheroprotective role of apolipoprotein E (apoE) is well established. During inflammation, expression of apoE in macrophages is reduced leading to enhanced atheromatous plaque development. In the present study, we investigated the signaling pathways involved in the repression of apoE gene expression in response to lipopolysaccharide (LPS) treatment, a condition that mimics the inflammatory stress, in mouse macrophages RAW 264.7. We identified Tpl-2 and MEKK1 as the kinases that are primarily responsible for the down-regulation of apoE promoter activity by LPS. Using a dominant negative form of IkappaB, we established that Tpl-2 and MEKK1 signaling pathways converge to NF-kappaB acting on the apoE core promoter -55/+73. In addition to NF-kappaB activation, LPS also activated c-Jun via its phosphorylation by JNK. The activity of the apoE promoter was repressed by c-Jun, whereas small interference RNA-mediated inhibition of endogenous c-Jun expression reversed the inhibitory effect of Tpl-2 on the apoE promoter. Transfection experiments and DNA binding assays showed that the binding site for c-Jun is in the -55/+73 region of the apoE promoter. Finally, we showed that LPS inhibited apoE gene expression via activation of the Tpl-2/MEK/ERK pathway acting on a different apoE promoter region. In summary, LPS represses apoE gene expression in macrophages via signaling pathways that involve the upstream kinases Tpl-2 and MEKK1, the intermediate mitogen-activated protein kinases ERK and JNK, and the downstream transcription factors AP-1 and NF-kappaB that inhibit the apoE promoter activity via distinct regions.

Relations of APOE promoter polymorphisms to LDL cholesterol and markers of subclinical atherosclerosis in young adults

The common apolipoprotein E (apoE) gene (APOE) epsilon2/epsilon3/epsilon4 polymorphism explains part of serum lipid variation, and polymorphisms in the APOE promoter region have been proposed to participate in the regulation of serum lipid levels within the most common APOE epsilon3/epsilon3 genotype group. We determined APOE -219G/T and +113G/C promoter genotypes and estimated APOE haplotypes in 525 participants of the Cardiovascular Risk in Young Finns Study. We studied the associations of the APOE promoter polymorphisms and their haplotypes with cross-sectional and longitudinal serum lipid and apolipoprotein concentrations as well as with flow-mediated dilatation (FMD), carotid artery compliance (CAC), and intima-media thickness (IMT) within the APOE epsilon3/epsilon3 carriers. We found no significant association between the APOE promoter genotypes and serum lipids [low density lipoprotein-cholesterol (LDL-C), HDL-C, and triglycerides], apolipoproteins (apoA-I and apoB), or brachial artery FMD, CAC, or carotid IMT in either men or women. In longitudinal analyses in males, the carriers of heterozygous genotypes (-219G/T or +113G/C) and, furthermore, carriers of the -219T/+113C/epsilon3 haplotype had significantly higher LDL-C and total cholesterol concentrations throughout the 21 year follow-up period compared with homozygous G allele carriers or noncarriers of the -219T/+113C/epsilon3 haplotype. Such associations were not found in females. In summary, the APOE promoter polymorphisms -219G/T and +113G/C as well as their haplotype are associated with longitudinal changes in LDL-C and total cholesterol concentrations in young Finnish males but do not seem to be major determinants for FMD, CAC, or carotid IMT in males or females.

ADLAPH: A molecular haplotyping method based on allele-discriminating long-range PCR

We present a method, called Allele-Discriminating Long and Accurate PCR Haplotyping (ADLAPH), for directly determining haplotypes from an extended genomic region. This method uses allele-discriminating primers in long-range PCR to amplify only one of the two chromosome homologues for the region of interest. Haplotypes are then determined from these phase-separated PCR fragments by conventional single nucleotide polymorphism (SNP) genotyping methods. This simple robust procedure makes it practical for high-throughput haplotyping of unrelated individuals, and potentially allows direct observation of haplotype information for up to 40 kb or more. We demonstrate the feasibility of this molecular haplotyping procedure by generating apolipoprotein E (APOE) haplotypes from 100 unrelated subjects.

Evidence for Non-additive Influence of Single Nucleotide Polymorphisms within the Apolipoprotein E Gene

We analyzed 13 single nucleotide polymorphisms (SNPs) within the apolipoprotein E (APOE) gene, to identify pairs of SNPs that interact in a non-additive manner to influence genotypic mean levels of the ApoE protein in blood. An overparameterized general linear model of two-SNP genotype means was applied to data from 456 female and 398 male unrelated European Americans from Rochester, MN, USA. We found statistically significant evidence for non-additivity between SNPs within the male sample, but not within the female sample. We observed nine pairs of SNPs with evidence of non-additivity at the alpha=0.05 level of statistical significance within the male sample, when approximately three were expected by chance. Five of the nine pairs involved three SNPs (560, 624 and 1163) that did not have a statistically significant influence when considered separately in a single-site analysis. Three of the nine pairs involving four SNPs (832, 1998, 3937 and 4951) showed significant evidence for non-additivity in at least one of two other male samples from Jackson, MS, USA and North Karelia, Finland. Although all four of these SNPs had a statistically significant influence in Rochester when considered separately, only SNP 3937 gave a significant result in the other male samples. The four SNPs are located in the promoter, intronic and exonic regions, and 3' to the polyadenylation signal in the APOE gene. Our study suggests that analyses that only consider SNPs located in exons and ignore contexts such as those indexed by gender and population, and disregard non-additivity of SNP effects, may inappropriately model the contribution of a gene to the genetic architecture of a trait that has a complex multifactorial etiology.

Apolipoprotein E gene polymorphisms and thrombosis and restenosis after coronary artery stenting

Experimental data support a protective function of apolipoprotein E (apoE) against restenosis, the main factor limiting the long-term benefit of percutaneous coronary interventions. We investigated the possibility that the single nucleotide polymorphisms (SNPs)--219G/T, 113G/C, 334T/C, and 472C/T of the gene encoding apoE (APOE) are associated with the incidence of death and myocardial infarction or restenosis after stenting in coronary arteries. In addition, we asked whether the apoE isotype-related epsilon2/epsilon3/epsilon4 polymorphism, defined by specific allele combinations (haplotypes) of the 334T/C and 472C/T polymorphism, and other APOE haplotypes, derived from all four SNPs investigated, are associated with adverse clinical and angiographic outcomes after stenting. Our study included 1,850 consecutive patients with symptomatic coronary artery disease (CAD) who underwent stent implantation. Follow-up angiography was performed in 1,556 patients (84.1%) at 6 months after the intervention. We found that none of the APOE SNPs is associated with death and myocardial infarction or restenosis after stenting. In addition, we observed no relationship between APOE haplotypes and adverse outcomes. In conclusion, the APOE -219G/T, 113G/C, 334T/C, and 472C/T polymorphisms, either alone or in combination, do not represent genetic markers of the risk of thrombotic and restenotic complications in patients with CAD treated with coronary stenting.

Identification of a novel enhancer of brain expression near the apoE gene cluster by comparative genomics

Comparative analysis of the human and mouse genomic sequences downstream of the apolipoprotein E gene (APOE) revealed a highly conserved element with previously undefined function. In reporter gene transfection studies, this element which is located approximately 42 kb distal to APOE was found to have silencer activity in a subset of cell lines examined. Analysis of transgenic mice containing a fusion construct linking this distal 631 bp conserved element to a reporter gene comprised of the human APOE gene with its proximal promoter resulted in robust brain expression of the transgenic human apoE mRNA in three independent transgenic lines, supporting the identification of a novel brain controlling region (BCR). Further studies using immunohistochemistry revealed widespread human apoE localization throughout the brains of the BCR-apoE transgenic mice with prominent expression in the cortex and diencephalon. In addition, double-label immunofluorescence performed on brain sections and cultures of primary cortical cells localized human apoE protein to cortical neurons and microglia. These studies demonstrate that comparative sequence analysis is a successful strategy to predict candidate regulatory regions in vivo, although they do not imply that this element controls apoE expression physiologically.

Astroglial regulation of apolipoprotein E expression in neuronal cells. Implications for Alzheimer's disease

Although apolipoprotein (apo) E is synthesized in the brain primarily by astrocytes, neurons in the central nervous system express apoE, albeit at lower levels than astrocytes, in response to various physiological and pathological conditions, including excitotoxic stress. To investigate how apoE expression is regulated in neurons, we transfected Neuro-2a cells with a 17-kilobase human apoE genomic DNA construct encoding apoE3 or apoE4 along with upstream and downstream regulatory elements. The baseline expression of apoE was low. However, conditioned medium from an astrocytic cell line (C6) or from apoE-null mouse primary astrocytes increased the expression of both isoforms by 3-4-fold at the mRNA level and by 4-10-fold at the protein level. These findings suggest that astrocytes secrete a factor or factors that regulate apoE expression in neuronal cells. The increased expression of apoE was almost completely abolished by incubating neurons with U0126, an inhibitor of extracellular signal-regulated kinase (Erk), suggesting that the Erk pathway controls astroglial regulation of apoE expression in neuronal cells. Human neuronal precursor NT2/D1 cells expressed apoE constitutively; however, after treatment of these cells with retinoic acid to induce differentiation, apoE expression diminished. Cultured mouse primary cortical and hippocampal neurons also expressed low levels of apoE. Astrocyte-conditioned medium rapidly up-regulated apoE expression in fully differentiated NT2 neurons and in cultured mouse primary cortical and hippocampal neurons. Thus, neuronal expression of apoE is regulated by a diffusible factor or factors released from astrocytes, and this regulation depends on the activity of the Erk kinase pathway in neurons.

Effect of atorvastatin on ApoE and ApoC-I synthesis and secretion by THP-1 macrophages

Apolipoprotein (apo) E and C-I are plasma apolipoproteins that have been implicated in the etiology of atherosclerosis and obesity, respectively. Both proteins are synthesized and secreted by macrophages, though pharmacological regulation of their production is poorly understood. The authors compared the effect of 2 HMG-CoA reductase inhibitors, atorvastatin and cerivastatin, on the synthesis and secretion of apoE and apoC-I by THP-1 macrophages. Atorvastatin reduced medium apoE and cellular apoE mRNA of PMA-activated THP-1 cells in a dose-dependent manner (-24% and -22%, respectively, at 1-micromol/L, P < 0.01). ApoC-I in the medium was also reduced by atorvastatin in a dose-dependent manner, though to a lesser extent (-15% at 1-micromol/L, P < 0.05). Cerivastatin similarly reduced medium apoE (-20% at 1-micromol/L, P < 0.05) and cellular apoE mRNA (-31% at 1-micromol/L, P < 0.05), and significantly lowered cellular apoC-I mRNA (-15%, P < 0.05), but not apoC-I in the medium. In experiments with THP-1 macrophages loaded with cholesterol (ie, 24-hour incubation with acetyl-LDL), atorvastatin and cerivastatin (1-micromol/L) significantly (P < 0.05) reduced both medium apoE (-30% and -25%, respectively) and cellular apoE mRNA (-25% and -17%, respectively). A lower and less consistent effect was observed on medium apoC-I (-6% and -18%, respectively) and cellular apoC-I mRNA (-13% and -19%, respectively). These data demonstrate that statins have the capacity to reduce the synthesis and secretion of both apoE and apoC-I in THP-1 macrophages loaded or unloaded with cholesterol.

Expanding the association between the APOE gene and the risk of Alzheimer's disease: possible roles for APOE promoter polymorphisms and alterations in APOE transcription

Alzheimer's disease (AD) is the most commonly diagnosed form of dementia in the elderly. Predominantly this disease is sporadic in nature with only a small percentage of patients exhibiting a familial trait. Early-onset AD may be explained by single gene defects; however, most AD cases are late onset (> 65 years) and, although there is no known definite cause for this form of the disease, there are several known risk factors. Of these, the epsilon4 allele of the apolipoprotein E (apoE) gene (APOE) is a major risk factor. The epsilon4 allele of APOE is one of three (epsilon2 epsilon3 and epsilon4) common alleles generated by cysteine/arginine substitutions at two polymorphic sites. The possession of the epsilon 4 allele is recognized as the most common identifiable genetic risk factor for late-onset AD across most populations. Unlike the pathogenic mutations in the amyloid precursor or those in the presenilins, APOE epsilon4 alleles increase the risk for AD but do not guarantee disease, even when present in homozygosity. In addition to the cysteine/arginine polymorphisms at the epsilon2/epsilon3/epsilon4 locus, polymorphisms within the proximal promoter of the APOE gene may lead to increased apoE levels by altering transcription of the APOE gene. Here we review the genetic and biochemical evidence supporting the hypothesis that regulation of apoE protein levels may contribute to the risk of AD, distinct from the well known polymorphisms at the epsilon2/epsilon3/epsilon4 locus.

TaqMan systems for genotyping of disease-related polymorphisms present in the gene encoding apolipoprotein E

Polymorphisms of the gene encoding apolipoprotein E have been implicated in the pathogenesis of peripheral and coronary artery disease and neurodegenerative disorders such as sporadic and late-onset familial forms of Alzheimer's disease. We have developed TaqMan assay systems for the single nucleotide polymorphisms -219G/T, located in the promoter of the apolipoprotein E gene, 113G/C, present in the transcriptional enhancer element of intron 1, 334T/C, determining Cys or Arg as amino acid residue 112 of mature apolipoprotein E, and 472C/T, determining Arg or Cys as residue 158. The accuracy of genotype determination with the TaqMan systems was demonstrated by analyses with restriction endonucleases. We determined the genotypes of the apolipoprotein E polymorphisms in 2349 study subjects. The genotypes were distributed as: -219GG = 27.3%, -219GT = 49.1%, and -219TT = 23.6% (p = 0.435); 113GG = 41.3%, 113GC = 45.2%, and 113CC = 13.5% (p = 0.343); 334TT = 73.4%, 334TC = 24.7%, and 334CC = 1.9% (p = 0.539); 472CC = 86.3%, 472CT=12.8%, and 472TT= 0.9% ( p = 0.004) (Hardy-Weinberg equilibrium estimates are given in parentheses). The allele combinations which define the three major isoforms of apolipoprotein E, namely apoE2, apoE3, and apoE4, had the following allele frequencies: 334T/472T (epsilon2; 112Cys/158Cys) = 7.3%, 334T/472C (epsilon3; 112Cys/158Arg) = 78.4%, and 334C/472C (epsilon4; 112Arg/158Arg) = 14.2%, respectively. ApoE genotypes were distributed as: epsilon2epsilon2 = 0.9%, epsilon2epsilon3 = 11.2%, epsilon2epsilon4 = 1.6%, epsilon3epsilon3 = 61.3%, epsilon3epsilon4 = 23.1%, and epsilon4epsilon4 = 1.9% (p = 0.014). The TaqMan assays allow for fast and sensitive genotyping and are especially suitable for studies including large numbers of participants.

Transcription factors Zic1 and Zic2 bind and transactivate the apolipoprotein E gene promoter

We have used the yeast one-hybrid system to identify transcription factors that bind to specific sequences in proximal regions of the apolipoprotein E gene promoter. The sequence between -163 and -124, that has been previously defined as a functional promoter element, was used as a bait to screen a human brain cDNA library. Ten cDNA clones that encoded portions of the human Zic1 (five clones) and Zic2 (five clones) transcription factors were isolated. Electrophoretic mobility shift assays confirmed the presence of a binding site for Zic1 and Zic2 in the -136/-125 region. Displacement of binding with oligonucleotides derived from adjacent sequences within the APOE promoter revealed the existence of two additional Zic-binding sequences in this promoter. These sequences were identified by electrophoretic mobility shift assays and mutational analysis in regions -65/-54 and -185/-174. Cotransfection of Zic1 and Zic2 expression vector and different APOE promoter-luciferase reporter constructs in U87 glioblastoma cell line showed that the three binding sites partially contributed to the trans-stimulation of the luciferase reporter. Ectopic expression of Zic1 and Zic2 in U87 cells also trans-stimulated the expression of the endogenous gene, increasing the amount of apolipoprotein E produced by glial cells. These data indicate that Zic proteins might contribute to the transcriptional activity of the apolipoprotein E gene and suggest that apolipoprotein E could mediate some of the developmental processes in which Zic proteins are involved.

Sequence diversity and large-scale typing of SNPs in the human apolipoprotein E gene

A common strategy for genotyping large samples begins with the characterization of human single nucleotide polymorphisms (SNPs) by sequencing candidate regions in a small sample for SNP discovery. This is usually followed by typing in a large sample those sites observed to vary in a smaller sample. We present results from a systematic investigation of variation at the human apolipoprotein E locus (APOE), as well as the evaluation of the two-tiered sampling strategy based on these data. We sequenced 5.5 kb spanning the entire APOE genomic region in a core sample of 72 individuals, including 24 each of African-Americans from Jackson, Mississippi; European-Americans from Rochester, Minnesota; and Europeans from North Karelia, Finland. This sequence survey detected 21 SNPs and 1 multiallelic indel, 14 of which had not been previously reported. Alleles varied in relative frequency among the populations, and 10 sites were polymorphic in only a single population sample. Oligonucleotide ligation assays (OLA) were developed for 20 of these sites (omitting the indel and a closely-linked SNP). These were then scored in 2179 individuals sampled from the same three populations (n = 843, 884, and 452, respectively). Relative allele frequencies were generally consistent with estimates from the core sample, although variation was found in some populations in the larger sample at SNPs that were monomorphic in the corresponding smaller core sample. Site variation in the larger samples showed no systematic deviation from Hardy-Weinberg expectation. The large OLA sample clearly showed that variation in many, but not all, of OLA-typed SNPs is significantly correlated with the classical protein-coding variants, implying that there may be important substructure within the classical epsilon 2, epsilon 3, and epsilon 4 alleles. Comparison of the levels and patterns of polymorphism in the core samples with those estimated for the OLA-typed samples shows how nucleotide diversity is underestimated when only a subset of sites are typed and underscores the importance of adequate population sampling at the polymorphism discovery stage. [The sequence data described in this paper have been submitted to the GenBank data library under accession no. AF261279.]

Apolipoprotein E gene promoter polymorphisms in Alzheimer's disease

Alzheimer's disease, the most frequent form of senile dementia, presents in the vast majority of cases as a multifactorial trait, where a series of genetic and environmental risk factors converge. The increasing body of data, both epidemiological and functional, is strengthening the evidence that apolipoprotein E (APOE, gene; apoE, protein) is a true susceptibility factor for the onset of the common form of Alzheimer's disease. The E4 isoform of apoE remains to date as the main genetic risk factor for the disease, although the mechanisms responsible for this association are not well understood. It is also clear that apoE4 is not necessary or sufficient to cause the disease, indicating that other risk and protecting factors exist. ApoE is upregulated in response to nervous system injury, suggesting that it could have a neuroprotective role; on the other hand, there is evidence indicating that apoE is neurotoxic when present at high levels. Thus, apoE levels seem to be relevant for the functionality of the protein. The APOE proximal promoter hosts numerous regulatory elements, raising the possibility that polymorphisms in this region could produce variation in apoE levels by altering APOE transcriptional activity, which could finally result in AD susceptibility. We will review here the current evidence on the relationship between APOE proximal promoter polymorphisms, APOE gene transcriptional activity and apoE protein levels, and risk for AD.

Conserved protein motifs and structural organization of a fish gene homologous to mammalian apolipoprotein E

Apolipoprotein E (apoE) plays a central role in lipid metabolism from its ability to interact with lipoprotein receptors. Besides its role in cardiovascular diseases, apoE polymorphism contributes to susceptibility to neurodegenerative diseases, such as Alzheimer's disease. The statistical significance of the combined match scores obtained after apoE motif-based protein sequence database searches, the structural features of the deduced protein, and the phylogenetic analysis, support the evidence that a homologue to mammalian apoE can be found in teleost fish. Isolation and characterization of the first nonmammalian APOE revealed that the zebrafish gene spans 2555/2692 bp instead of 3597 bp in human and has the same splice junctions and exon/intron organization as found in mammals, except that there is an additional intron that splits the last exon (exon 4) into two exons (exons 4 and 5). Enlargement of APOE size in the mammalian lineage occurs mainly by Alu repeats insertion. The additional intron found in zebrafish gene was also identified at the same splicing site in trout APOE and is located in the corresponding linker region following the conserved low density lipoprotein receptor binding domain. Primer extension and reverse transcriptase PCR (RT-PCR) assays demonstrated that two transcription start sites are located 26 and 28 bp upstream of the first intron and 22 or 24 bp downstream from a canonical TATA box. Sequence inspection of the 5'-flanking region upstream of the TATA box revealed potential regulatory DNA elements. These results will serve as a basis for comparative studies on transcriptional and post-transcriptional mechanisms of APOE regulation in vertebrates.

Transcriptional regulatory sequences within the first intron of the chicken apolipoproteinAI (apoAI) gene

Previous studies demonstrated that the -82 to +87 nucleotides (nt) 5'-upstream region of the chicken apolipoprotein (apoAI) gene are necessary for maximum reporter chloramphenicol acetyl transferase (cat) gene activation in chicken hepatocarcinoma (LMH) cells [Bhattacharyya, N., Chattapadhyay, R., Oddoux, C., Banerjee, D., 1993. Characterisation of the chicken apolipoprotein A-I gene 5'-flanking region. DNA Cell Biol. 12, 597-604]. The -82 to +87nt contain the 5'-untranslated nt, part of the first intron, and the upstream regulatory sequences. In this study, we examined the role of the first intron in the transcriptional regulation of the chicken apoAI gene. Six different reporter cat gene constructs with or without part of the first intron were prepared and transfected into LMH, normal rat kidney (NRK) and human hepatocarcinoma (HepG2) cells. Cell extracts were prepared from each transfected cell line, and CAT activities were measured. All three cell-lines readily expressed CAT, indicating that transcriptional regulatory sequences are present within the first intron region of the chicken apoAI gene. In an enhancer assay, the first intron containing cat construct exhibited a 5.4-fold increase of reporter activity in NRK cells when compared to a SV 40 promoter containing cat plasmid, suggesting the presence of a moderate enhancer element within +29 to +87nt of the first intron. DNase I protection assays, electrophoretic mobility shift assays and binding experiments with nuclear proteins isolated from different chicken tissues and LMH cells showed interaction with +29 to +87nt. Nuclear proteins isolated from tissues like liver and intestine, that actively express apoAI gene, failed to interact with +29 to +87nt, whereas nuclear proteins isolated from tissues that are less active in apoAI gene expression readily interacted with this region. To show the binding of the LMH-specific trans-acting factors to the +50 to +68nt intron region, DNA-affinity chromatography step was performed by using 3H-labeled nuclear proteins. These studies demonstrate that the first intron region of the apoAI gene interacts with trans-acting proteins and plays an important role in transcriptional regulation of the apoAI gene.

Oxysterols and atherosclerosis

Oxysterols are present in human atherosclerotic plaque and are suggested to play an active role in plaque development. Moreover, the oxysterol:cholesterol ratio in plaque is much higher than in normal tissues or plasma. Oxysterols in plaque are derived both non-enzymically, either from the diet and/or from in vivo oxidation, or (e.g. 27-hydroxycholesterol) are formed enzymically during cholesterol catabolism. While undergoing many of the same reactions as cholesterol, such as being esterified by cells and in plasma, certain oxysterols in some animal and in vitro models exhibit far more potent effects than cholesterol per se. In vitro, oxysterols perturb several aspects of cellular cholesterol homeostasis (including cholesterol biosynthesis, esterification, and efflux), impair vascular reactivity and are cytotoxic and/or induce apoptosis. Injection of relatively large doses of oxysterols into animals causes acute angiotoxicity whereas oxysterol-feeding experiments have yielded contrary results as far as their atherogenicity is concerned. There is no direct evidence yet in humans that oxysterols contribute to atherogenesis. However, oxysterol levels are elevated in human low-density lipoprotein (LDL) subfractions that are considered potentially atherogenic and two recent studies have indicated that raised plasma levels of a specific oxysterol (7beta-hydroxycholesterol) may be associated with an increased risk of atherosclerosis. At the present time there are a number of significant and quite widespread problems with current literature which preclude more than a tentative suggestion that oxysterols have a causal role in atherogenesis. Further studies are necessary to definitively determine the role of oxysterols in atherosclerosis, and considering the wide-ranging tissue levels reported in the literature, special emphasis is needed on their accurate analysis, especially in view of the susceptibility of the parent cholesterol to artifactual oxidation.

Risk for Alzheimer's disease correlates with transcriptional activity of the APOE gene

While the straightepsilon4 allele of apolipoprotein E ( APOE, gene; ApoE, protein) is widely accepted as a major genetic risk factor for the late onset form of Alzheimer's disease (AD), recent evidence points to variations in ApoE levels as another important factor. We have previously reported that a common variant in the regulatory region of APOE (-491A) is associated with risk for late onset AD. In this report we analyze the association of another APOE promoter polymorphism (-427T/C) with AD in two case-control clinical samples and demonstrate a correlation between APOE promoter transcriptional activity and risk for AD. The association studies show that the allelic variant (-427C) and the haplotype [-491A-427C] of the APOE promoter are associated with increased risk for AD. Study of the transcriptional activity of the common haplotypes defined by combination of the -491 and -427 alleles indicated that the risk for late onset AD positively correlates with transcriptional activity of the APOE gene, suggesting that increases in the local expression of ApoE could be responsible for the association of APOE promoter polymorphism with AD.

Inducible transcriptional activity of bcn-1 element from laminin gamma1-chain gene promoter in renal and nonrenal cells

Laminin is a major component of the extracellular matrix whose expression is regulated by growth factors. The laminin gamma1-chain promoter contains a newly identified transcriptional element denoted bcn-1 that is both active and inducible in mesangial cells. In this study, we explored activation of the bcn-1 element in other renal and nonrenal cells. Treatment of rat glomerular epithelial cells (GEC) with phorbol 12-myristate 13-acetate (PMA) increased activity of the bcn-1 transcriptional element, within the context of the native laminin gamma1-chain promoter or when cloned upstream of a heterologous promoter. Treatment of GEC with PMA induced nuclear DNA-binding activity, BCN-1, which was recognized by the bcn-1 motif in a gel shift assay. These results provide evidence that the bcn-1 motif and its cognate BCN-1 factor(s) may regulate transcription of the laminin gamma1-chain in GEC. The bcn-1 element and its cognate BCN-1 DNA-binding activity were also inducible in monkey kidney COS-7 and in human T cell Jurkat lines. SDS-PAGE of in situ ultraviolet cross-linked nucleoproteins from GEC, COS, and Jurkat cells revealed one major 110-115 kDa adduct in all three cell lines. These results demonstrate that the bcn-1 element is active in renal and nonrenal cells from different mammalian species where the same protein contributes to the inducible BCN-1 DNA-binding activity.

Quantitation of apolipoprotein epsilon gene expression by competitive polymerase chain reaction in a patient with familial apolipoprotein E deficiency

A simple method of obtaining semiquantitative and reliable data on apolipoprotein (apo) sigma gene expression is described. We detected apo sigma specific sequences by reverse transcription (rT)-PCR. For quantitative measurement, an apo sigma DNA standard was produced allowing the development of a competitive PCR-method. The efficiency of RNA extraction and cDNA synthesis was controlled by quantitation of a housekeeping gene (glyceraldehyde-3-phosphatedehydrogenase, G3PDH) in separate reactions. To imitate a defined induction of apo sigma gene expression, serial twofold dilutions of total RNA were reversely transcribed and the respective cDNAs used to perform a competitive apo sigma and G3PDH PCR. The change in apo sigma cDNA and G3PDH cDNA was 1.7-2.3-fold with an expected value of 2.0-fold. Standard deviations in three independently performed experiments were within a range of < 15% of the mean, indicating low intra-assay variation and high reproducibility. To illustrate this method, apo sigma gene expression was measured in a patient with complete lack of functional active apo E in comparison to healthy controls. The method presented here might be valuable in assessment of apo sigma gene expression in human disease.

Allelic polymorphisms in the transcriptional regulatory region of apolipoprotein E gene

In this work, we explored the existence of genetic variants within the apolipoprotein E gene transcriptional regulatory region, using a denaturing gradient gel electrophoresis screening of a region comprising nucleotides -1017 to +406. Upon a population study, three new polymorphic sites (-491, -427 and -219) and two mutations were found. Functional effects of the polymorphisms, assayed by transient transfection and electrophoretic mobility shift assays in a human hepatoma cell line, showed that polymorphisms at sites -491 and -219 of the APOE promoter produce variations in the transcriptional activity of the gene, most probably through differential binding of nuclear proteins.

Targeted replacement of the mouse apolipoprotein E gene with the common human APOE3 allele enhances diet-induced hypercholesterolemia and atherosclerosis

Apolipoprotein (apo) E, a constituent of several lipoproteins, is a ligand for the low density lipoprotein receptor, and this interaction is important for maintaining cholesterol and triglyceride homeostasis. We have used a gene replacement strategy to generate mice that express the human apoE3 isoform in place of the mouse protein. The levels of apoE mRNA in various tissues are virtually the same in the human apoE3 homozygous (3/3) mice and their littermates having the wild type mouse allele (+/+). Total cholesterol and triglyceride levels in fasted plasma from the 3/3 mice were not different from those in the +/+ mice, when maintained on a normal (low fat) chow diet. We found, however, notable differences in the distribution of plasma lipoproteins and apolipoprotein E between the two groups: beta-migrating lipoproteins and plasma apoB100 levels are decreased in the 3/3 mice, and the apoE distribution is shifted from high density lipoproteins to larger lipoprotein particles. In addition, the fractional catabolic rate of exogenously administered remnant particles without apoE was 6-fold slower in the 3/3 mice compared with the +/+ mice. When the 3/3 and +/+ animals were fed a high fat/high cholesterol diet, the 3/3 animals responded with a dramatic increase (5-fold) in total cholesterol compared with the +/+ mice (1.5-fold), and after 12 weeks on this same diet the 3/3 animals developed significantly (at least 13-fold) larger atherosclerotic plaques in the aortic sinus area than the +/+ animals. Thus the structural differences between human APOE3 and mouse ApoE proteins are sufficient to cause an increased susceptibility to dietary-induced hypercholesterolemia and atherosclerosis in the 3/3 mice.

Transcriptional regulation of apolipoprotein E expression by cyclic AMP

Incubation of HepG2 cells in the presence of dibutyryl cAMP (db-cAMP), a cell permeable analogue of cyclic AMP, or forskolin, an agent which elevates intracellular cAMP, resulted in a 50% decrease in apoE mRNA levels within 24 h. Results of nuclear run-on transcription assays showed that db-cAMP down-regulates apoE gene expression at the transcriptional level. By transfection analysis with a plasmid containing the -614/+804 human apoE gene fused to the secreted placental alkaline phosphatase (SPAP) reporter gene, we showed that the SPAP activity was decreased by 50% when HepG2 cells were incubated in the presence of db-cAMP or forskolin, indicating that this promoter region mediated this negative effect. In contrast, when the smaller fragment -200/+1 of apoE promoter was linked to the CAT reporter gene, db-cAMP treatment of HepG2 cells resulted in a 2-fold increase in CAT activity, suggesting that positive cAMP-responsive elements were present in the proximal apoE promoter. These data indicate that transcriptional modulation of apoE gene expression by agents known to elevate the intracellular cAMP level is complex and involves several negative and positive elements located in the -614 to +804 region of the apoE gene whose global effect is negative on apoE gene transcription.

In vivo footprinting analysis of the hepatic control region of the human apolipoprotein E/C-I/C-IV/C-II gene locus

Expression of both the apolipoprotein (apo)E and apoC-I genes in the liver is specified by a 319-nucleotide hepatic control region (HCR-1) that is located 15 kilobase pairs downstream of the apoE gene and 5 kilobase pairs downstream of the apoC-I gene. In vivo footprint analysis of HCR-1 in intact nuclei revealed several liver-specific protein-binding sites that were not detectable by in vitro methods. In addition to three previously identified in vitro footprints, four in vivo footprints were identified in a region of HCR-1 that is required for directing gene expression to hepatocytes. Prominent liver-specific DNase I-hypersensitive sites were associated with these footprints. Liver-specific nuclear protein binding to these sites was confirmed by oligonucleotide gel-retention assays. The in vivo analysis also identified a cluster of nuclear protein-binding sites in the Alu family repeat segment adjacent to the domain required for liver expression. Micrococcal nuclease digestion indicated the presence of a nucleosome in the central domain of HCR-1 in liver chromatin that was in phase with the nucleosome location in tissues that did not express the transgene. These results suggest that HCR-1 functions in a highly structured chromatin environment requiring a complex interaction of liver-enriched transcription factors.

Trans-repressor BEF-1 phosphorylation. A potential control mechanism for human ApoE gene regulation

Human apolipoprotein E is a plasma lipoprotein that appears to play an important protective role in the development of atherosclerosis. While little is known about the regulation of apoE, recent studies have shown that cytokines repress apoE synthesis both in vivo and in vitro. Furthermore, we have recently shown that the endogenous apoE gene is negatively regulated by the nuclear trans-repressor BEF-1 in the human HepG2 cell line. In this study we demonstrate that treatment of HepG2 cells with the cytokine interleukin-1 and interleukin-6 resulted in the induction of an isoform of BEF-1, designated B1. The induction of the B1 isoform could be blocked by the protein kinase inhibitor staurosporine, suggesting that B1 is a phosphorylated form of BEF-1. As further support, the B1 isoform could also be induced by phorbol ester, and subsequently inhibited by staurosporine, implicating a role for protein kinase C-mediated phosphorylation. Quantitation of the levels of the BEF-1 isoforms, and studies in the presence of cyclohexamide, provided evidence for the phosphorylation of an existing intracellular pool of BEF-1, with no change in the total intracellular level. Under conditions that generated increased levels of the B1 isoform, there was a concomitant and proportional decrease in the level of apoE mRNA. The effect did not appear to be the result of improved binding to the apoE regulatory region as the DNA binding affinity of B1 was identical to native BEF-1. Our data suggest that the regulation of apoE by BEF-1 is modulated by differential phosphorylation, possibly through the protein kinase C pathway.

Structure of the hepatic control region of the human apolipoprotein E/C-I gene locus

The specificity of expression in the liver of the human apolipoprotein (apo) E/C-I gene locus is determined by a hepatic control region (HCR) that is located 15 kilobases downstream of the apoE gene. DNase I footprint studies of this sequence using nuclear extracts identified a region of the HCR that is enriched in nuclear protein-binding sites. Nuclease analysis of chromatin revealed liver-specific DNase I-hypersensitive sites that were associated with this region, and additional liver-specific nuclease-sensitive sites associated with the apoE gene were identified. The HCR domain has a limited binding affinity for the nuclear scaffold. The specific domain required for liver expression was tested by ligating subfragments of the HCR to the apoE gene and examining their activity in transgenic mice. A segment of 319 nucleotides that contained several potential regulatory sequences was required for full activity of liver-specific transcription with shorter segments yielding much lower levels of expression in the liver. All constructs that contained a fully active HCR were expressed in approximately a copy-dependent manner, suggesting that transgene expression was independent of integration position. Taken together, the properties of the HCR are consistent with its function as a locus control region for the liver-specific expression of the apoE gene.

The human apolipoprotein E gene is negatively regulated in human liver HepG2 cells by the transcription factor BEF-1

Apolipoprotein E (apoE) is a major constituent of plasma lipoprotein that functions in lipid transport and redistribution (reverse cholesterol transport) and probably plays an important role in inhibiting the development and/or progression of atherosclerosis. While cis-acting regions involved in basal and tissue-specific control of the apoE gene have been identified by promoter mapping studies, much less is known about factors that regulate the gene. In this study, we demonstrate that the region between -94 and -84 upstream of transcriptional start site of the human apoE gene contains a binding site for the transcriptional repressor factor BEF-1, a tyrosine-phosphorylated nuclear protein that was first identified in HeLa cells. Using gel retardation assays, we show that HeLa cell-derived BEF-1 binds the apoE BEF-1 homology, and this binding can be competed with the prototype BEF-1 sequence, but not by a mutated sequence. Furthermore, we demonstrate that the apoE- producing human liver HepG2 cell produces significant levels of BEF-1, which could bind to both the prototype BEF-1 sequence and the apoE homology, and be competed equivalently with cold BEF-1 or apoE homology. To determine if BEF-1 affected the expression of apoE, we performed competition experiments using plasmids containing the intact or mutated BEF-1 homology. The introduction of the intact BEF-1 site into HepG2 cells resulted in an induction of apoE mRNA, whereas control and mutated BEF-1-containing plasmids had no significant effect. We also found that increasing the level of nuclear BEF-1 by treatment of cells with orthovanadate resulted in a reduction in the level of apoE mRNA. Overall, our data suggest that the endogenous apoE gene in the human HepG2 cell line is repressed by the trans-acting influence of nuclear factor BEF-1.

Characterization of an upstream regulatory sequence and its binding protein in the mouse apolipoprotein E gene

The mouse apolipoprotein (apo) E gene from strain C57BL/6 was isolated from a genomic DNA library and its complete nucleotide sequence, together with 1.3 kilobase of 5' flanking DNA and 300 base pairs of the 3' flanking DNA, was determined. Regulatory sequences in the proximal 5' flanking region of the gene were identified. Using a chloramphenicol acetyltransferase transient assay system, positive and negative cis-acting sequences were mapped within 380 base pairs of the 5' flanking region of the mouse apoE gene. Two nuclear protein binding sites were identified within this region by DNase I footprinting. We have characterized one of these regions, termed mouse apoE regulatory sequence (MARS-2), which spans nucleotides -151 to -133. Gel mobility shift assays using oligonucleotides of the MARS-2 sequence having specific deletions or substitutions as probes or competitors showed that the essential sequence of MARS-2 required for nuclear protein binding consists of 16 nucleotides encompassing -151 to -136. When nuclear extracts from different cells were examined, L cells and mouse liver nuclear protein contained the highest levels of binding protein for the MARS-2 probe. This protein, termed MARS-2 binding protein, was purified from mouse liver nuclear extracts to homogeneity using gel filtration and MARS-2 oligonucleotide-specific column chromatographic procedures. The Mr = 66,000 binding protein showed a gel mobility shift band that was identical to that of crude nuclear extracts.

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.

ApoA-IV is secreted on discrete HDL particles by the rat hepatoma cell line McA-RH7777 transfected with ApoA-IV cDNA

In the present study, the synthesis and secretion of transfected apolipoprotein (apo) A-IV was investigated in rat hepatoma McA-RH7777, a cell line that does not express apoA-IV mRNA or protein. An expression plasmid that contained the rat apoA-IV cDNA was transfected into the cells; five stable transformants were selected that harbor different copy numbers of the apoA-IV construct and secrete different amounts of apoA-IV. Gel filtration column chromatography and density gradient ultracentrifugation, combined with gel electrophoresis and electron microscopy techniques, demonstrated that (1) the secreted apoA-IV associated mainly with high-density lipoproteins (HDLs) and only a trace amount of apoA-IV was associated with very-low-density lipoproteins; (2) overexpression of apoA-IV resulted in an increased number of disk-shaped structures (thickness, approximately 8.0 nm and diameter, approximately 22 nm); and (3) the electrophoretic mobilities of the apoA-IV-containing particles differed from those of apoA-I-containing HDL. Expression of apoA-IV exerted no discernible effect on the density distribution or the secretion efficiency of apoB-100. Additionally, secretion of apoB-100 and apoA-IV exhibited opposite responses to serum: apoB-100 secretion was stimulated eightfold after addition of serum, whereas apoA-IV secretion was inhibited by 40%. These results suggest that synthesis of apoA-IV may lead to the formation of a subclass of HDL with a different metabolic fate than that of lipoproteins containing either apoA-I or apoB.

The effect of 25-hydroxycholesterol on the regulation of apolipoprotein E mRNA levels and secretion in the human hepatoma HepG2

The human hepatoma cell line, HepG2, was cultured with 25 OH cholesterol, a potent inhibitor of HMG-CoA reductase, in order to examine the effect of the oxysterol on apo E synthesis and secretion. Treatment of cells with oxysterol (2.5 microM) resulted in a greater than 90% inhibition of HMG-CoA reductase activity and a 3-fold reduction in its cognate mRNA level. However, apo E mRNA level and secretion were not affected after 24 h of drug treatment. This drug treatment was associated with a reduction in both cellular free and esterified cholesterol levels by 50% and 40%, respectively. Exposure of HepG2 cells to an ACAT inhibitor, the Sandoz compound (58-035) for 24 h, at a concentration of 5 micrograms/ml, resulted in a 30% increase and 70% decrease in the intracellular levels of free and esterified cholesterol, respectively. Under this regimen of drug treatment, the level of apo E mRNA was increased by approximately 70%, while HMG-CoA reductase mRNA level was decreased by 35%. When the cells were exposed to the combination of the ACAT inhibitor and 25 OH cholesterol, the cellular levels of free and esterified cholesterol were reduced by 30% and 80%, respectively. This combination of drugs had no effect on apo E mRNA; however, the level of HMG-CoA reductase mRNA was decreased by 3.5-fold. Taken together, the data suggested that reduction in the intracellular levels of either free or esterified cholesterol had no effect on apo E mRNA level. By contrast, a small increment in cellular free cholesterol content was associated with a significant induction in apo E mRNA level. Furthermore, 25 OH cholesterol caused a significant redistribution (50%) of apo E from the HDL fraction to the d greater than 1.21 g/ml infranatant. By using high performance liquid chromatography and molecular sieve columns, it was found that the appearance of a lipid-poor apo E particle was not an artifact of ultracentrifugation. This particle contained 85 wt% protein and 15 wt% of free cholesterol and phospholipid. The results suggested that a lipid-poor apo E particle was secreted by the HepG2 cells under certain circumstances.