Regulation of the human ApoA-II gene by the synergistic action of factors binding to the proximal and distal regulatory elements

Peroxisome Proliferator-Activated Receptor α in Lipoprotein Metabolism and Atherosclerotic Cardiovascular Disease

Peroxisome proliferator-activated receptors (PPARs) are a group of ligand-binding transcription factors with pivotal action in regulating pleiotropic signaling pathways of energetic metabolism, immune responses and cell proliferation and differentiation. A significant body of evidence indicates that the PPARα receptor is an important modulator of plasma lipid and lipoprotein metabolism, with pluripotent effects influencing the lipid and apolipoprotein cargo of both atherogenic and antiatherogenic lipoproteins and their functionality. Clinical evidence supports an important role of PPARα agonists (fibric acid derivatives) in the treatment of hypertriglyceridemia and/or low high-density lipoprotein (HDL) cholesterol levels, although the effects of clinical trials are contradictory and point to a reduction in the risk of nonfatal and fatal myocardial infarction events. In this manuscript, we provide an up-to-date critical review of the existing relevant literature.

Comparing DRD2 Promoter Methylation Between Blood and Brain in Alcohol Dependence

AIMS

The dopamine receptor D2 (DRD2) is substantially involved in several forms of addiction. In addition to genetic polymorphisms, epigenetic mechanisms have emerged as an important means of regulation. Previously, DRD2 hypo- and hyper-methylation have been observed in alcohol use disorder (AUD). Blood samples are commonly used as a surrogate marker of epigenetic alterations in epigenetic research, but few specific comparisons between blood and brain tissue samples in AUD exist.

METHODS

We used post-mortem brain tissue samples of 17 deceased patients with AUD and 31 deceased controls to investigate the relationship between blood and brain methylation of the DRD2 promoter.

RESULTS

When investigating individual cytosine methylation sites (CpG), several significant differences were found in the nucleus accumbens and hippocampus in the study population. Investigating binding sites with significant differences in methylation levels revealed hypomethylated CpGs targeting mainly activating transcription factors.

CONCLUSION

These findings support an altered transcription of the DRD2 gene in AUD specimens with a consecutively changed reward response in the brain. While methylation between specific brain regions and blood is comparable, our study further suggests that blood methylation cannot provide meaningful perspectives on DRD2 promoter methylation in the brain.

Apolipoprotein A-II, a Player in Multiple Processes and Diseases

Apolipoprotein A-II (apoA-II) is the second most abundant apolipoprotein in high-density lipoprotein (HDL) particles, playing an important role in lipid metabolism. Human and murine apoA-II proteins have dissimilar properties, partially because human apoA-II is dimeric whereas the murine homolog is a monomer, suggesting that the role of apoA-II may be quite different in humans and mice. As a component of HDL, apoA-II influences lipid metabolism, being directly or indirectly involved in vascular diseases. Clinical and epidemiological studies resulted in conflicting findings regarding the proatherogenic or atheroprotective role of apoA-II. Human apoA-II deficiency has little influence on lipoprotein levels with no obvious clinical consequences, while murine apoA-II deficiency causes HDL deficit in mice. In humans, an increased plasma apoA-II concentration causes hypertriglyceridemia and lowers HDL levels. This dyslipidemia leads to glucose intolerance, and the ensuing high blood glucose enhances apoA-II transcription, generating a vicious circle that may cause type 2 diabetes (T2D). ApoA-II is also used as a biomarker in various diseases, such as pancreatic cancer. Herein, we provide a review of the most recent findings regarding the roles of apoA-II and its functions in various physiological processes and disease states, such as cardiovascular disease, cancer, amyloidosis, hepatitis, insulin resistance, obesity, and T2D.

Association between the APOA2 rs3813627 Single Nucleotide Polymorphism and HDL and APOA1 Levels Through BMI

Background: The interaction between obesity and genetic traits on high density lipoprotein (HDL) levels has been extensively studied. The variance of serum HDL has a strong genetic heritability, although the studied variant only explains a small part of this variation. The goal of this study was to investigate the associations between the apolipoprotein type 2 (APOA2) rs3813627 single nucleotide polymorphism (SNP) and anthropometric and biochemical variables, though body mass index (BMI). Methods: This study included 153 subjects (91 overweight/obese (BMI³25 kg/m²) and 62 non-obese individuals (BMI < 25 kg/m²)). The APOA2 rs3813627 SNP was selected and genotyped. Genotype analysis was performed to analyze the associations between APOA2 SNPs and anthropometric and biochemical variables through BMI. Results: The APOA2 rs3813627 TT genotype was associated with low HDL levels in comparison with the APOA2 rs3813627 GG and GT genotype in overweight/obese individuals, but not in the non-obese subjects (p < 0.05). The same trend was observed in the apolipoprotein type 1 (APOA1) protein levels (p < 0.05). Correlation analysis revealed a negative correlation between HDL and APOA1 levels and APOA2 rs3813627 SNP under recessive model (p < 0.05). The odds ratio for low HDL levels was 3.76 and 3.94 for low APOA1 levels. The mediation analysis of APOA2 rs3813627 SNP through BMI showed a full mediation on HDL and partial mediation on APOA1 levels (p < 0.05). Bioinformatic analysis showed that rs3813627 lies in the APOA2 promoter and overlaps motifs for several bound transcription factors. Conclusion: On the basis of these data, the APOA2 rs3813627 SNP is associated with low HDL and APOA1 levels susceptibility, and this effect was mediated by an increased BMI.

Opposite regulation of the human apolipoprotein M gene by hepatocyte nuclear factor 1 and Jun transcription factors

HDL is a negative risk factor for atherosclerosis because of its multiple atheroprotective functions. Inflammation converts HDL particles from anti-atherogenic to pro-atherogenic, and this transformation is associated with changes in HDL structure and composition. Apolipoprotein M (apoM) has been recently shown to play a role in the maturation of HDL in plasma and to protect from atherosclerosis. ApoM gene is expressed primarily in the liver and kidney and is down-regulated by pro-inflammatory signals. We now show that the human apoM promoter harbors a dual specificity regulatory element in the proximal region that binds hepatocyte nuclear factor 1 (HNF-1) and members of the AP-1 family of pro-inflammatory transcription factors (c-Jun and JunB). Overexpression of c-Jun or JunB repressed both the basal and the HNF-1-mediated transactivation of the human apoM promoter. Treatment of HepG2 cells with potent inflammation-inducing phorbol esters or overexpression of PKCα was associated with a marked inhibition of apoM gene expression in a c-Jun/JunB-dependent manner. We provide evidence for a novel mechanism of inflammation-induced transcriptional repression that is based on the competition between HNF-1 and Jun proteins for binding to the same regulatory region. A similar mechanism accounts for the down-regulation of the liver-specific apolipoprotein A-II gene by Jun factors. Our studies provide novel insights on the mechanisms that control the expression of liver-specific apolipoprotein genes during inflammation and could affect the maturation and the functionality of HDL particles.

Transcriptional regulatory mechanisms of the human apolipoprotein genes in vitro and in vivo

The present review summarizes recent advances in the transcriptional regulation of the human apolipoprotein genes, focusing mostly, but not exclusively, on in-vivo studies and signaling mechanisms that affect apolipoprotein gene transcription. An attempt is made to explain how interactions of transcription factors that bind to proximal promoters and distal enhancers may bring about gene transcription. The experimental approaches used and the transcriptional regulatory mechanisms that emerge from these studies may also be applicable in other gene systems that are associated with human disease. Understanding extracellular stimuli and the specific mechanisms that underlie apolipoprotein gene transcription may in the long run allow us to selectively switch on antiatherogenic genes, and switch off proatherogenic genes. This may have beneficial effects and may confer protection from atherosclerosis to humans.

SREBP-1 binds to multiple sites and transactivates the human ApoA-II promoter in vitro : SREBP-1 mutants defective in DNA binding or transcriptional activation repress ApoA-II promoter activity

-Screening of an expression human liver cDNA library resulted in the isolation of several cDNA clones homologous to sterol regulatory element-binding protein-1 (SREBP-1) that recognize the regulatory element AIIAB and AIIK of the human apoA-II promoter. DNaseI footprinting of the apoA-II promoter using SREBP-1 (1 to 460) expressed in bacteria identified 5 overall protected regions designated AIIAB (-64 to -48), AIICD (-178 to -154), AIIDE (-352 to -332), AIIHI (-594 to -574), and AIIK (-760 to -743). These regions contain inverted E-box palindromic or direct repeat motifs and bind SREBP-1 with different affinities. Transient cotransfection experiments in HepG2 cells showed that SREBP-1 transactivated the -911/29 apoA-II promoter 3.5-fold as well as truncated apoA-II promoter segments that contain 1, 2, 3, or 4 SREBP binding sites. Mutagenesis analysis showed that transactivation by SREBP was mainly affected by mutations in element AIIAB. Despite the strong transactivation of the apoA-II promoter by SREBP-1 we could not demonstrate significant changes on the endogenous apoA-II mRNA levels of HepG2 cells after cotransfection with SREBP-1 or in the presence or absence of cholesterol and 25-OH-cholesterol. An SREBP-1 mutant lacking the amino-terminal activation domain bound normally to its cognate sites and repressed the apoA-II promoter activity. Repression was also caused by specific amino acid substitutions of Leu, Val, or Gly for Lys359, which affected DNA binding. Repression by the DNA binding-deficient mutants was abolished by deletion of the amino-terminal activation domain (1 to 90) of SREBP-1. Overall, the findings suggest that the wild-type SREBP-1 can bind and transactivate efficiently the apoA-II promoter in cell culture. SREBP-1 mutants lacking the activation domain bind to their cognate sites and directly repress the apoA-II promoter whereas mutants defective in DNA binding indirectly repress the apoA-II promoter activity, possibly by a squelching mechanism.

A bifunctional regulatory element of the human ApoA-I gene responsive to a distal enhancer

Promoter elements located up to 2 kb upstream of the apolipoprotein A-I (apoA-I) gene are necessary for apoA-I expression in liver and intestine cells in tissue culture. In transgenic mice, a distal enhancer located between the apoA-IV and apoC-III genes is additionally necessary for tissue-specific expression of apoA-I in liver and intestine. We have identified a previously uncharacterized regulatory element between 746 and 856 nucleotides 5' of the apoA-I transcription start site that differentially affects the expression of apoA-I reporter plasmids in intestine cells dependent on the presence of the distal apolipoprotein enhancer. Deletion of the -856/-746 sequence strongly repressed transcription in the presence of the apolipoprotein enhancer, but in the absence of the enhancer, deletion of the -856/-746 element increased transcription. By contrast, in liver cells, deletion of the -856/-746 element strongly repressed transcription in the presence of the distal enhancer but had no detectable effect on transcription in the absence of the distal enhancer. Electrophoretic mobility shift analysis revealed tissue-specific and sequence-specific protein-DNA complexes formed by the -856/-746 element in intestine, liver, and HeLa cell nuclear extracts. The complexes formed by extracts of intestinal cells differed from those of liver and HeLa cells by their sensitivity to DNase digestion and their pattern of protein footprints. Collectively, the data suggest that the -856/-746 sequence is a composite regulatory element that interacts with multiple proteins and the apolipoprotein distal enhancer to achieve tissue-specific expression of apoA-I.

Cooperative binding of upstream stimulatory factor and hepatic nuclear factor 4 drives the transcription of the human apolipoprotein A-II gene

The activity of the human apoA-II promoter is controlled by a synergistic interaction of the distal enhancer and the proximal promoter. An important role in apoA-II promoter activity is exerted by a transcription factor, designated CIIIB1, which binds to the proximal element AB and the distal elements of the enhancer, K and L. In the present communication we establish that CIIIB1 corresponds to the previously described factor, upstream stimulatory factor (USF) using the following criteria. (a) Purification of CIIIB1 by affinity chromatography provided a heat-stable protein with an apparent molecular mass of 45 kDa that cross-reacted with anti-USF1 and -USF2a antibodies; (b) CIIIB1 bound to the elements AB, K, and L was supershifted by these antibodies; (c) the heterodimer USF1/2a is the predominant form that corresponds to CIIIB1. Cotransfection experiments in HepG2 cells established the functional significance of USF in apoA-II transcription. It was found that the minimal promoter AB was transactivated by USF2a. In addition, all three E-box motifs present in elements AB, K and L are necessary for maximum transactivation by USF2a. A dominant negative form of USF2a inhibits the activity of apoA-II promoter. The USF1/2a heterodimer, which is naturally expressed in the liver, is as efficient as the USF2a homodimer in the transactivation of apoA-II promoter/enhancer constructs. Cotransfection experiments in COS-1 cells showed that hepatic nuclear factor 4 (HNF-4) synergized with USF2a in the transactivation of the apoA-II promoter. In addition, we showed that HNF-4 and USF2a bind to the enhancer cooperatively. This may account for the transcriptional synergism observed between USF and HNF-4 in the transactivation of the apoA-II promoter.

Interaction of nascent ApoE2, ApoE3, and ApoE4 isoforms expressed in mammalian cells with amyloid peptide beta (1-40). Relevance to Alzheimer's disease

Population studies have established that one of the common isoforms of apolipoprotein E, the apoE4, is associated with higher incidence and earlier age of onset of late onset familial Alzheimer's disease (AD), whereas apoE2 may have the opposite effect. The apoE3 and apoE4 isoforms were shown to display different binding reactivities with amyloid beta peptide (Abeta) and tau protein in vitro. On the basis of these findings, it has been proposed that the apoE isoforms may modulate positively or negatively the formation of either the neurofibrillary tangles or the amyloid deposits in the brain of patients with AD. To study the interaction of Abeta with nascent apoE isoforms we have expressed their cDNAs in baby hamster kidney (BHK-21) cells using the Semliki Forest Virus expression system. Analysis of the secreted apoE by one- and two-dimensional gel electrophoresis and immunoblotting showed that the nascent apoE is heavily modified with carbohydrate chains containing sialic acid. A dimeric form of apoE is formed with apoE2 and apoE3 but not with apoE4 isoforms. Analysis of the interaction of nascent apoE2, apoE3, and apoE4 produced by BHK-21 cells with Abeta (1-40) under physiological conditions (pH 7.4, 37 degrees C) showed that the efficiency of the apoE monomer-Abeta complex formation follows the order apoE2 > apoE3 >> apoE4. In addition, the apoE2 dimer formed a complex with Abeta more efficiently than the apoE3 dimer. The isoform-specific differences in binding were temperature-dependent and are attenuated upon decrease of the temperature. The binding behavior of the monomeric apoE is different from that reported for plasma apoE3 and apoE4 or commercially available apoE3 and apoE4 preparations and similar to that described for apoE3 and apoE4 produced by human embryonic kidney (HEK-293) cells. It appears that the efficiency of binding between each of three main apoE isoforms and Abeta correlates inversely with the risk of developing late-onset familial AD and may indicate possible involvement of apoE in the binding and clearance of Abeta in vivo.

Orphan nuclear receptor HNF-4 binds to the human coagulation factor VII promoter

The human coagulation protease factor VII plays a pivotal role in the initiation of the coagulation cascade by both the extrinsic and the intrinsic pathway. Although the gene, encoding factor VII, is expressed predominantly in the liver, the mechanisms underlying this tissue-specific expression have not been elucidated. In this study, we have analyzed the contribution of 5 kilobases upstream of the ATG translational initiation codon upon hepatic factor VII gene transcription. Transient transfection assays of a set of nested deletions in both liver and non-liver cell lines, HepG2 and HeLa respectively, indicate that several regions are involved in liver-specific expression. A slight negative effect on factor VII promoter activity in HepG2 cells is mediated by sequences upstream of position -1212. DNase I protection experiments reveal six footprints, FPVII1 through FPVII6, within the proximal 714 base pairs but a minimal promoter of 165 base pairs containing only FPVII3-6 is sufficient to confer liver-specific expression in HepG2 cells. Interestingly, FPVII6, at position -14 to +10 on the sense strand, would indicate that an as yet unknown transcription factor covers the ATG translational initiation codon. Gel retardation experiments show that the liver-enriched transcription factor HNF-4 binds specifically to footprint FPVII4 at position -71 to -49. Furthermore, a T --> A transversion, that in the HNF-4 binding site of factor IX causes a severe bleeding disorder, was introduced into the HNF-4-binding site of factor VII and reduced promoter activity by 20-50%. Coordinate HNF-4-mediated regulation of several blood protease genes as well as genes involved in lipid metabolism might account for the positive correlation of these factors with increased risk of occlusive heart diseases.

Purification and characterization of nuclear factors binding to the negative regulatory element D of human apolipoprotein A-II promoter: a negative regulatory effect is reversed by GABP, an Ets-related protein

We have previously shown that transcription of the human apolipoprotein A-II (apoA-II) gene is controlled by a complex set of regulatory elements [Cardot et al. (1993) Biochemistry 32, 9080-9093]. We have also identified previously described, as well as new activities which bind to these elements and influence to varying degrees the transcription of the apoA-II gene. DNA binding and competition assays indicated that element D binds three new activities, designated AIID1, AIID2, and AIID4, as well as an activity related to C/EBP. Activities AIID1, AIID2, and AIID4 were purified and characterized further in order to determine their function on the transcriptional regulation of human apoA-II gene. SDS-PAGE analysis as well as photoaffinity cross-linking of the affinity-purified AIID2 showed that it consists of three proteins with molecular masses ranging between 54 and 63 kDa. The amino acid sequence of tryptic peptides obtained from AIID2 protein bands revealed that it is homologous to GABP, an Ets-related protein. Similar analysis showed that affinity-purified AIID4 has an apparent molecular mass of 130 kDa. AIID1 activity was purified partially; in addition to binding to the apoA-II promoter, AIID1 also binds to the regulatory element C of apoCIII and may play a role in the transcriptional regulation of both genes. Methylation interference of G residues and permanganate modification of T residues indicated that the binding sites of AIID2 and AIID4 were contiguous on element D. However, the binding site of AIID1 overlaps with the binding sites of both AIID2 and AIID4. This suggests that the binding of AIID1 and AIID2 or of AIID1 and AIID4 may be mutually exclusive, whereas AIID2 and AIID4 may bind simultaneously. Transcription from a minimal promoter containing elements AB, C, and D of apoA-II increased 1.5- to 1.6-fold when element D is deleted, as well as by promoter mutations which eliminated the binding of both AIID1 and/or AIID4 to element D, but permitted the binding of AIID2/GABP. The findings suggest that element D has a negative regulatory role on apoA-II gene transcription when it is occupied by protein AIID1 and/or AIID4. This negative effect is reversed when element D is occupied only by the regulatory factor AIID2/GABP.