Duplicated Downstream Enhancers Control Expression of the Human Apolipoprotein E Gene in Macrophages and Adipose Tissue

ApoE is expressed in multiple mammalian cell types in which it supports cellular differentiated function. In this report we demonstrate that apoE expression in adipocytes is regulated by factors involved in modulating systemic insulin sensitivity. Systemic treatment with pioglitazone increased systemic insulin sensitivity and increased apoE mRNA levels in adipose tissue by 2-3-fold. Treatment of cultured 3T3-L1 adipocytes with ciglitazone increased apoE mRNA levels by 2-4-fold in a dose-dependent manner and increased apoE secretion from cells. Conversely, treatment of adipocytes with tumor necrosis factor (TNF) ␣ reduced apoE mRNA levels and apoE secretion by 60%. Neither insulin nor a peroxisome proliferator-activated receptor (PPAR) ␣ agonist regulated adipocyte apoE gene expression. In addition, treatment of human monocyte-derived macrophages with ciglitazone did not regulate expression of apoE. Additional analyses using reporter genes indicated that the effect of TNF␣ and PPAR␥ agonists on the apoE gene was mediated via distinct gene control elements. The TNF␣ effect was mediated by elements within the proximal promoter, whereas the PPAR␥ effect was mediated by elements within a downstream enhancer. However, the addition of TNF␣ substantially reduced the absolute levels of apoE reporter gene response even in the presence of ciglitazone. These results indicate for the first time that adipose tissue expression of apoE is modulated by physiologic regulators of insulin sensitivity.

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

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Divergent Effects of Peroxisome Proliferator-activated Receptor γ Agonists and Tumor Necrosis Factor α on Adipocyte ApoE Expression

Yue

Rasouli

Ranganathan

et al. 2004

“…The regulatory complexity emerges from interactions of a number of proteins which bind to proximal regions of the APOE gene promoter, as well as to far downstream elements involved in its tissue-specific expression (23)(24)(25)(26)(27)(28)(29)(30). In brain, the regulation of this gene remains largely unexplored, despite its importance in processes of degeneration and regeneration of the nervous system.…”

Section: Apolipoprotein E (Apoe)mentioning

confidence: 99%

Transcription Factors Zic1 and Zic2 Bind and Transactivate the Apolipoprotein E Gene Promoter

Salero¹,

Pérez-Sen²,

Aruga³

et al. 2001

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.

“…Tissue-specific control elements in the Apoe gene restrict its expression to hepatocytes (10), astrocytes (11), skin fibroblasts (12), adipocytes, and macrophages (13). Hepatocyte-derived apoE, the major source of plasma apoE (14), is responsible for receptor-mediated uptake of remnant lipoproteins in the liver by the secretion-capture pathway (15,16).…”

mentioning

confidence: 99%

Hypomorphic Apolipoprotein E Mice

Raffaı̈

Weisgraber

2002

In creating an allelic variant of mouse Apoe designed to resemble human apolipoprotein E4 (apoE4), we generated hypomorphic apoE (hypoE) mice that express only ϳ5% of normal apoE mRNA levels in all tissues. Insertion of a neo cassette flanked by loxP sites in the third intron of Apoe reduced expression of the Arg-61 allelic variant in hypoE mice and resulted in plasma apoE levels that were ϳ2-5% of normal. Unlike other mouse models with low levels of circulating apoE, hypoE mice had a nearly normal lipoprotein cholesterol profile when fed a chow diet. Further reduction of apoE expression in hypoE/Apoe ؊/؊ heterozygous mice led to an increase in remnant lipoprotein-associated cholesterol levels, demonstrating that hypoE mice express close to the threshold level of Arg-61 apoE required for a normal lipoprotein profile. Unlike wild type mice, hypoE mice were susceptible to diet-induced hypercholesterolemia, which was fully reversed within 3 weeks after resumption of a chow diet. In Mx1-Cre transgenic hypoE mice, plasma apoE levels returned to normal within 10 days after gene repair and removal of the neo cassette following induction of Cre recombinase. HypoE mice provide the opportunity for conditional gene repair by crossing with inducible or lineage/cell type-specific Cre transgenic mice, generating new models to dissect the roles of apoE in atherosclerosis regression, immunoregulation, and neurodegeneration. Apolipoprotein E (apoE)1 is an important structural and functional protein component of lipoproteins that plays a prominent role in lipid metabolism in plasma and in the central nervous system (1, 2). As a high affinity ligand for the low density lipoprotein (LDL) receptor, the LDL receptor-related protein, and heparan sulfate proteoglycans, apoE mediates the uptake of plasma remnant lipoproteins by the liver (3, 4). In addition, apoE participates in diverse biological processes, such as intracellular cholesterol utilization (5), cell growth (6), immunoregulation (7-9), and neuronal growth and repair (2).Tissue-specific control elements in the Apoe gene restrict its expression to hepatocytes (10), astrocytes (11), skin fibroblasts (12), adipocytes, and macrophages (13). Hepatocyte-derived apoE, the major source of plasma apoE (14), is responsible for receptor-mediated uptake of remnant lipoproteins in the liver by the secretion-capture pathway (15,16). ApoE secreted by hepatocytes into the space of Disse associates with incoming remnant lipoproteins and with heparan sulfate proteoglycans bound to hepatic sinusoidal surfaces. This local enrichment in apoE facilitates remnant clearance through receptor-mediated processes. In the brain, astrocytes are the major source of apoE, which serves in lipid homeostasis in the central nervous system (17-19). Macrophage-derived apoE promotes remnant lipoprotein uptake and retards the development of atherosclerosis in Apoe Ϫ/Ϫ mice (20 -22). ApoE also participates in the regression of atherosclerosis (23, 24), contributes to the production of very low density lipoprotein (...