Apolipoprotein E (apoE), a 34 kDa glycoprotein, mediates hepatic and extrahepatic uptake of plasma lipoproteins and cholesterol efflux from lipid-laden macrophages. In humans, three structural different apoE isoforms occur, with subsequent functional changes and pathological consequences. Here, we review data supporting the involvement of apoE structural domains and isoforms in normal and altered lipid metabolism, cardiovascular and neurodegenerative diseases, as well as stress-related pathological states. Studies using truncated apoE forms provided valuable information regarding the regions and residues responsible for its properties. ApoE3 renders protection against cardiovascular diseases by maintaining lipid homeostasis, while apoE2 is associated with dysbetalipoproteinemia. ApoE4 is a recognized risk factor for Alzheimer's disease, although the exact mechanism of the disease initiation and progression is not entirely elucidated. ApoE is also implicated in infections with herpes simplex type-1, hepatitis C and human immunodeficiency viruses. Interacting with both viral and host molecules, apoE isoforms differently interfere with the viral life cycle. ApoE exerts anti-inflammatory effects, switching macrophage phenotype from the proinflammatory M1 to the anti-inflammatory M2, suppressing CD4+ and CD8+ lymphocytes, and reducing IL-2 production. The anti-oxidative properties of apoE are isoform-dependent, modulating the levels of various molecules (Nrf2 target genes, metallothioneins, paraoxonase). Mimetic peptides were designed to exploit apoE beneficial properties. The “structure correctors” which convert apoE4 into apoE3-like molecules have pharmacological potential. Despite no successful strategy is yet available for apoE-related disorders, several promising candidates deserve further improvement and exploitation.

In atherogenesis, macrophage-derived apolipoprotein E (apoE) has an athero-protective role by a mechanism that is not fully understood. We investigated the regulatory mechanisms involved in the modulation of apoE expression in macrophages. The experiments showed that the promoters of all genes of the apoE/apoCI/apoCIV/apoCII gene cluster are enhanced by multienhancer 2 (ME.2), a regulatory region that is located 15.9 kb downstream of the apoE gene. ME.2 interacts with the apoE promoter in a macrophage-specific manner. Transient transfections in RAW 264.7 macrophages showed that the activity of ME.2 was strongly decreased by deletion of either 87 bp from the 5 end or 131 bp from the 3 end. We determined that the minimal fragment of this promoter that can be activated by ME.2 is the proximal ؊100/؉73 region. The analysis of the deletion mutants of ME.2 revealed the importance of the 5 end of ME.2 in apoE promoter transactivation. Chromatin conformational capture assays demonstrated that both ME.2 and ME.1 physically interacted with the apoE promoter in macrophages. Our data showed that phorbol 12-myristate 13-acetate-induced differentiation of macrophages is accompanied by a robust induction of apoE and STAT1 expression. In macrophages (but not in hepatocytes), STAT1 up-regulated apoE gene expression via ME.2. The STAT1 binding site was located in the 174 -182 region of ME.2. In conclusion, the specificity of the interactions between the two multienhancers (ME.1 and ME.2) and the apoE promoter indicates that these distal regulatory elements play an important role in the modulation of apoE gene expression in a cell-specific manner.Apolipoprotein E (apoE), a glycoprotein of 35 kDa, is associated with the chylomicron remnants, very low density lipoproteins, low density lipoproteins (LDL), and high density lipoproteins (HDL) and plays an important role in lipid metabolism (1-6). Deficiency in apoE results in atherosclerosis in humans and in animal models (7-12). ApoE knock-out mice are the best-characterized animal models of atherosclerosis (13). ApoE is a ligand for the LDL receptor found in the liver and other tissues and for the LDL receptor-related protein found in hepatocytes and as such it facilitates the clearance of lipoprotein remnants from the circulation (14 -16). Malfunction of the mechanisms of cholesterol clearance leads to the accumulation of remnants in the plasma, a process associated with premature atherosclerosis (8,11,12). ApoE regulates plasma cholesterol levels, also having an important role in cholesterol efflux, as documented by studies in patients and animal models with apoE deficiency or mutated apoE genes (17-24). Recently, antioxidant and anti-inflammatory functions within the atherosclerotic plaque were attributed to apoE (23,24).ApoE is mainly synthesized by the liver and also by various cells and peripheral tissues (25). At the site of atherosclerotic lesion, apoE is provided by macrophages. Transgenic mice expressing apoE only in macrophages are protected against atherosclerosis even th...

Apolipoprotein A-I (apoA-I) is the major protein component of high-density lipoproteins (HDL), mediating many of its atheroprotective properties. Increasing data reveal the pro-atherogenic effects of bisphenol A (BPA), one of the most prevalent environmental chemicals. In this study, we investigated the mechanisms by which BPA exerts pro-atherogenic effects. For this, LDLR −/− mice were fed with a high-fat diet and treated with 50 µg BPA/kg body weight by gavage. After two months of treatment, the area of atherosclerotic lesions in the aorta, triglycerides and total cholesterol levels were significantly increased, while HDL-cholesterol was decreased in BPA-treated LDLR −/− mice as compared to control mice. Real-Time PCR data showed that BPA treatment decreased hepatic apoA-I expression. BPA downregulated the activity of the apoA-I promoter in a dose-dependent manner. This inhibitory effect was mediated by MEKK1/NF-κB signaling pathways. Transfection experiments using apoA-I promoter deletion mutants, chromatin immunoprecipitation, and protein-DNA interaction assays demonstrated that treatment of hepatocytes with BPA induced NF-κB signaling and thus the recruitment of p65/50 proteins to the multiple NF-κB binding sites located in the apoA-I promoter. In conclusion, BPA exerts pro-atherogenic effects downregulating apoA-I by MEKK1 signaling and NF-κB activation in hepatocytes. Int. J. Mol. Sci. 2019, 20, 6281 2 of 15 and in apoA-I −/− /LDLR −/− mice [6]. On the other hand, overexpression of human apoA-I reduced atherogenesis in apoE −/− or in LDLR −/− mice [7][8][9][10][11][12][13][14], providing strong evidence for the antiatherogenic role of apoA-I. Recently, it was proposed that the ratio of HDL-cholesterol to apoA-I gives additional insights as a risk marker for cardiovascular disease [15]. To exploit the anti-atherogenic properties of HDL-apoA-I, different approaches, such as pharmacological interventions using HDL-apoA-I mimetic peptides or infusions of apoA-I-containing particles were proposed for the reduction of atherogenesis [16,17].In humans, the APOAI gene is 1.8 Kb in length and it is located on chromosome 11 in the APOAI/APOCIII/APOAIV gene cluster [18]. ApoA-I is synthesized mainly by the liver and the small intestine. APOAI gene expression is mainly regulated at transcriptional level [19]. The APOAI gene promoter contains a TATA box and several cis elements that regulate its gene expression in either a positive or negative manner. Acidosis caused apoA-I downregulation by promoting binding of repressor proteins to a pH-responsive element that overlaps the TATA box within the apoA-I promoter [20]. Several hormones, such as glucocorticoids, thyroid hormones, and insulin, induced APOAI gene expression through a direct mechanism interacting with their specific hormone response elements [21,22]. In humans, treatment with fibrates that interact with a PPAR-responsive element located in the apoA-I promoter increased APOAI gene expression, but opposite effects of fibrates on apoA-I expression were found in r...

Apolipoprotein CII (apoCII) is a specific activator of lipoprotein lipase and plays an important role in triglyceride metabolism. The aim of our work was to elucidate the regulatory mechanisms involved in apoCII gene modulation in macrophages. Using Chromosome Conformation Capture we demonstrated that multienhancer 2 (ME.2) physically interacts with the apoCII promoter and this interaction facilitates the transcriptional enhancement of the apoCII promoter by the transcription factors bound on ME.2. We revealed that the transcription factor STAT1, previously shown to bind to its specific site on ME.2, is functional for apoCII gene upregulation. We found that siRNA-mediated inhibition of STAT1 gene expression significantly decreased the apoCII levels, while STAT1 overexpression in RAW 264.7 macrophages increased apoCII gene expression. Using transient transfections, DNA pull down and chromatin immunoprecipitation assays, we revealed a novel STAT1 binding site in the −500/−493 region of the apoCII promoter, which mediates apoCII promoter upregulation by STAT1. Interestingly, STAT1 could not exert its upregulatory effect when the RXRα/T3Rβ binding site located on the apoCII promoter was mutated, suggesting physical and functional interactions between these factors. Using GST pull-down and co-immunoprecipitation assays, we demonstrated that STAT1 physically interacts with RXRα. Taken together, these data revealed that STAT1 bound on ME.2 cooperates with RXRα located on apoCII promoter and upregulates apoCII expression only in macrophages, due to the specificity of the long-range interactions between the proximal and distal regulatory elements. Moreover, we showed for the first time that STAT1 and RXRα physically interact to exert their regulatory function.

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