Liver X Receptors Interact with Corepressors to Regulate Gene Expression

Abstract Liver X receptors (LXRs) are members of the nuclear receptor superfamily that regulate gene expression in response to oxysterols and play a critical role in cholesterol homeostasis by regulating genes that are involved in cholesterol transport, catabolism, and triglyceride synthesis. Oxysterols and synthetic agonists bind LXRs and activate transcription by recruiting coactivator proteins. The role of LXRs in regulating target gene expression in the absence of ligand is unknown. Here we show that LXRs interact with corepressors, N-CoR (nuclear receptor corepressor) and SMRT (silent mediator of retinoic acid receptor and thyroid receptor), which are released upon binding agonists. The LXR-corepressor interaction is isoform selective, wherein LXRα has a very strong interaction with corepressors and LXRβ only shows weak interaction. LXRs also exhibit a preference for interacting with N-CoR vs. SMRT. Similar to other nuclear receptors, mutations in the LXR helix 3 and 4 region abolish corepressor interaction. Using a transient transfection assay, we demonstrate that LXR represses transcription that can be further increased by cotransfecting N-CoR into cells. Chromatin immunoprecipitation experiments further indicated that N-CoR is recruited onto endogenous LXR target genes, and addition of LXR agonists releases N-CoR from their promoters. Collectively, these results suggest that corepressors play an important role in regulating LXR target gene expression.

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Notch Activation Is an Early and Critical Event during T-Cell Leukemogenesis in Ikaros-Deficient Mice

Molecular and Cellular Biology ◽

10.1128/mcb.26.1.209-220.2006 ◽

2006 ◽

Vol 26 (1) ◽

pp. 209-220 ◽

Cited By ~ 116

Author(s):

Alexis Dumortier ◽

Robin Jeannet ◽

Peggy Kirstetter ◽

Eva Kleinmann ◽

MacLean Sellars ◽

...

Keyword(s):

Gene Expression ◽

Tumor Suppressor ◽

Target Gene ◽

Critical Role ◽

Ectopic Expression ◽

Notch Pathway ◽

Notch Target Gene ◽

Target Gene Expression ◽

T Cell Leukemogenesis ◽

Notch Activation

ABSTRACT The Ikaros transcription factor is both a key regulator of lymphocyte differentiation and a tumor suppressor in T lymphocytes. Mice carrying a hypomorphic mutation (IkL/L) in the Ikaros gene all develop thymic lymphomas. IkL/L tumors always exhibit strong activation of the Notch pathway, which is required for tumor cell proliferation in vitro. Notch activation occurs early in tumorigenesis and may precede transformation, as ectopic expression of the Notch targets Hes-1 and Deltex-1 is detected in thymocytes from young IkL/L mice with no overt signs of transformation. Notch activation is further amplified by secondary mutations that lead to C-terminal truncations of Notch 1. Strikingly, restoration of Ikaros activity in tumor cells leads to a rapid and specific downregulation of Notch target gene expression and proliferation arrest. Furthermore, Ikaros binds to the Notch-responsive element in the Hes-1 promoter and represses Notch-dependent transcription from this promoter. Thus, Ikaros-mediated repression of Notch target gene expression may play a critical role in defining the tumor suppressor function of this factor.

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Activated Liver X Receptors Stimulate Adipocyte Differentiation through Induction of Peroxisome Proliferator-Activated Receptor γ Expression

Molecular and Cellular Biology ◽

10.1128/mcb.24.8.3430-3444.2004 ◽

2004 ◽

Vol 24 (8) ◽

pp. 3430-3444 ◽

Cited By ~ 187

Author(s):

Jong Bae Seo ◽

Hyang Mi Moon ◽

Woo Sik Kim ◽

Yun Sok Lee ◽

Hyun Woo Jeong ◽

...

Keyword(s):

Gene Expression ◽

Target Gene ◽

Adipocyte Differentiation ◽

Specific Gene ◽

Peroxisome Proliferator ◽

Liver X Receptors ◽

Gene Promoters ◽

Peroxisome Proliferator Activated Receptor ◽

Specific Gene Expression ◽

X Receptors

ABSTRACT Liver X receptors (LXRs) are nuclear hormone receptors that regulate cholesterol and fatty acid metabolism in liver tissue and in macrophages. Although LXR activation enhances lipogenesis, it is not well understood whether LXRs are involved in adipocyte differentiation. Here, we show that LXR activation stimulated the execution of adipogenesis, as determined by lipid droplet accumulation and adipocyte-specific gene expression in vivo and in vitro. In adipocytes, LXR activation with T0901317 primarily enhanced the expression of lipogenic genes such as the ADD1/SREBP1c and FAS genes and substantially increased the expression of the adipocyte-specific genes encoding PPARγ (peroxisome proliferator-activated receptor γ) and aP2. Administration of the LXR agonist T0901317 to lean mice promoted the expression of most lipogenic and adipogenic genes in fat and liver tissues. It is of interest that the PPARγ gene is a novel target gene of LXR, since the PPARγ promoter contains the conserved binding site of LXR and was transactivated by the expression of LXRα. Moreover, activated LXRα exhibited an increase of DNA binding to its target gene promoters, such as ADD1/SREBP1c and PPARγ, which appeared to be closely associated with hyperacetylation of histone H3 in the promoter regions of those genes. Furthermore, the suppression of LXRα by small interfering RNA attenuated adipocyte differentiation. Taken together, these results suggest that LXR plays a role in the execution of adipocyte differentiation by regulation of lipogenesis and adipocyte-specific gene expression.

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Alternative Mechanisms by Which Mediator Subunit MED1/TRAP220 Regulates Peroxisome Proliferator-Activated Receptor γ-Stimulated Adipogenesis and Target Gene Expression

Molecular and Cellular Biology ◽

10.1128/mcb.00967-07 ◽

2007 ◽

Vol 28 (3) ◽

pp. 1081-1091 ◽

Cited By ~ 65

Author(s):

Kai Ge ◽

Young-Wook Cho ◽

Hong Guo ◽

Teresa B. Hong ◽

Mohamed Guermah ◽

...

Keyword(s):

Gene Expression ◽

Rna Polymerase Ii ◽

Nuclear Receptor ◽

Transcriptional Activity ◽

Target Gene ◽

Molecular Mechanisms ◽

Peroxisome Proliferator ◽

Peroxisome Proliferator Activated Receptor ◽

Cultured Fibroblasts ◽

Target Gene Expression

ABSTRACT Mediator is a general coactivator complex connecting transcription activators and RNA polymerase II. Recent work has shown that the nuclear receptor-interacting MED1/TRAP220 subunit of Mediator is required for peroxisome proliferator-activated receptor γ (PPARγ)-stimulated adipogenesis of mouse embryonic fibroblasts (MEFs). However, the molecular mechanisms remain undefined. Here, we show an intracellular PPARγ-Mediator interaction that requires the two LXXLL nuclear receptor recognition motifs on MED1/TRAP220 and, furthermore, we show that the intact LXXLL motifs are essential for optimal PPARγ function in a reconstituted cell-free transcription system. Surprisingly, a conserved N-terminal region of MED1/TRAP220 that lacks the LXXLL motifs but gets incorporated into Mediator fully supports PPARγ-stimulated adipogenesis. Moreover, in undifferentiated MEFs, MED1/TRAP220 is dispensable both for PPARγ-mediated target gene activation and for recruitment of Mediator to a PPAR response element on the aP2 target gene promoter. However, PPARγ shows significantly reduced transcriptional activity in cells deficient for a subunit (MED24/TRAP100) important for the integrity of the Mediator complex, indicating a general Mediator requirement for PPARγ function. These results indicate that there is a conditional requirement for MED1/TRAP220 and that a direct interaction between PPARγ and Mediator through MED1/TRAP220 is not essential either for PPARγ-stimulated adipogenesis or for PPARγ target gene expression in cultured fibroblasts. As Mediator is apparently essential for PPARγ transcriptional activity, our data indicate the presence of alternative mechanisms for Mediator recruitment, possibly through intermediate cofactors or other cofactors that are functionally redundant with MED1/TRAP220.

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Tissue- and Nuclear Receptor-Specific Function of the C-Terminal LXXLL Motif of Coactivator NCoA6/AIB3 in Mice

Molecular and Cellular Biology ◽

10.1128/mcb.00451-07 ◽

2007 ◽

Vol 27 (23) ◽

pp. 8073-8086 ◽

Cited By ~ 18

Author(s):

Qingtian Li ◽

Mei-Jin Chu ◽

Jianming Xu

Keyword(s):

Gene Expression ◽

Mammary Gland ◽

Bile Acid ◽

Nuclear Receptor ◽

Target Gene ◽

Acid Synthesis ◽

Estrogen Receptor Α ◽

High Cholesterol Diet ◽

Target Gene Expression ◽

Lxxll Motif

ABSTRACT Although the LXXLL motif of nuclear receptor (NR) coactivators is essential for interaction with NRs, its role has not been assessed in unbiased animal models. The nuclear receptor coactivator 6 (NCoA6; also AIB3, PRIP, ASC-2, TRBP, RAP250, or NRC) is a coactivator containing an N-terminal LXXLL-1 (L1) and a C-terminal L2. L1 interacts with many NRs, while L2 interacts with the liver X receptor α (LXRα) and the estrogen receptor α (ERα). We generated mice in which L2 was mutated into AXXAL (L2m) to disrupt its interaction with LXRα and ERα. NCoA6L2m/L2m mice exhibited normal reproduction, mammary gland morphogenesis, and ERα target gene expression. In contrast, when treated with an LXRα agonist, lipogenesis and the LXRα target gene expression were significantly reduced in NCoA6L2m/L2m mice. The induction of Cyp7A1 expression by a high-cholesterol diet was impaired in NCoA6L2m/L2m mice, which reduced bile acid synthesis in the liver and excretion in the feces and resulted in cholesterol accumulation in the liver and blood. These results demonstrate that L2 plays a tissue- and NR-specific role: it is required for NCoA6 to mediate LXRα-regulated lipogenesis and cholesterol/bile acid homeostasis in the liver but not required for ERα function in the mammary gland.

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Liver X receptors alpha and beta promote myelination and remyelination in the cerebellum

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1424951112 ◽

2015 ◽

Vol 112 (24) ◽

pp. 7587-7592 ◽

Cited By ~ 42

Author(s):

Delphine Meffre ◽

Ghjuvan’Ghjacumu Shackleford ◽

Mehdi Hichor ◽

Victor Gorgievski ◽

Eleni T. Tzavara ◽

...

Keyword(s):

Gene Expression ◽

Transcriptional Control ◽

Motor Coordination ◽

Cholesterol Homeostasis ◽

Liver X Receptors ◽

Protein Levels ◽

Myelin Sheaths ◽

X Receptors ◽

Myelin Gene

The identification of new pathways governing myelination provides innovative avenues for remyelination. Liver X receptors (LXRs) α and β are nuclear receptors activated by oxysterols that originated from the oxidation of cholesterol. They are crucial for cholesterol homeostasis, a major lipid constituent of myelin sheaths that are formed by oligodendrocytes. However, the role of LXRs in myelin generation and maintenance is poorly understood. Here, we show that LXRs are involved in myelination and remyelination processes. LXRs and their ligands are present in oligodendrocytes. We found that mice invalidated for LXRs exhibit altered motor coordination and spatial learning, thinner myelin sheaths, and reduced myelin gene expression. Conversely, activation of LXRs by either 25-hydroxycholesterol or synthetic TO901317 stimulates myelin gene expression at the promoter, mRNA, and protein levels, directly implicating LXRα/β in the transcriptional control of myelin gene expression. Interestingly, activation of LXRs also promotes oligodendroglial cell maturation and remyelination after lysolecithin-induced demyelination of organotypic cerebellar slice cultures. Together, our findings represent a conceptual advance in the transcriptional control of myelin gene expression and strongly support a new role of LXRs as positive modulators in central (re)myelination processes.

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MicroRNA Biomarkers in Traumatic Brain Injury

Neurotrauma ◽

10.1093/med/9780190279431.003.0022 ◽

2018 ◽

pp. 261-268

Author(s):

Manish Bhomia ◽

Nagaraja S. Balakathiresan ◽

Kevin K. W. Wang ◽

Barbara Knollmann-Ritschel

Keyword(s):

Gene Expression ◽

Traumatic Brain Injury ◽

Brain Injury ◽

Critical Role ◽

Regulate Gene Expression ◽

Rna Molecules ◽

Posttranscriptional Level ◽

Potential Use ◽

Regulate Gene

Traumatic brain injury (TBI) is currently considered one of the major causes of disability and death worldwide. The cellular and molecular changes of TBI pathology are dynamic and complex in nature. MicroRNAs (miRNA) are small endogenous RNA molecules that regulate gene expression at the posttranscriptional level. Several studies have shown a critical role of miRNAs in the development of long- and short-term TBI pathology. Circulating miRNAs are of great interest as blood-based biomarkers in TBI diagnosis. In this chapter, the authors review recent reports that aim to understand the role of miRNAs in TBI pathophysiology and their potential use as a therapeutic target. Additionally, the authors discuss the potential use of miRNAs as blood-based diagnostic markers for TBI and their possible association with other neurodegenerative diseases.

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Cloning of the gene encoding cucumber lumazine synthase and an analysis of its promoter activity in cucumber

Canadian Journal of Plant Science ◽

10.4141/cjps10001 ◽

2010 ◽

Vol 90 (6) ◽

pp. 809-818 ◽

Cited By ~ 1

Author(s):

R. Wang ◽

M. Chen ◽

F. Liao ◽

F. Jiang ◽

B. Ma ◽

...

Keyword(s):

Gene Expression ◽

Amino Acids ◽

Genetic Engineering ◽

Promoter Activity ◽

Target Gene ◽

Critical Role ◽

35S Promoter ◽

Target Gene Expression ◽

Lumazine Synthase ◽

Cucumber Fruit

Promoters are an important regulatory element controlling the temporal and spatial expression of genes; thus, they play a critical role in genetic engineering by controlling target gene expression. Cucumber is a widely planted vegetable with a pleasant flavor and high economic value. However, most genetic engineering studies involving cucumber have utilized the CaMV 35S promoter, which mediates ubiquitous target gene expression. To identify a promoter that is highly expressed in cucumber fruit, total proteins from cucumber fruit were analyzed by two-dimensional gel electrophoresis. One spot which is highly expressed in fruit was sequenced from its N-terminus using the Edman degradation method. A total of 10 amino acids (Ala-Val-Arg-His-Ile-Ala-Gly-Ser-Leu-Ala) were sequenced. Based on these 10 residues, a cDNA fragment 905 bp in length was cloned using 3′- and 5′-RACE. The corresponding gene, which encodes 220 amino acids, showed 65-73% similarity to other plant lumazine synthases without the signal peptide. We also cloned the 2.1-kb upstream promoter sequence of this Cucumis sativus lumazine synthase (CsLS) and analyzed its promoter activity by GUS histochemical and fluorometric assays. Our results indicate that CsLS is highly expressed in cucumber fruit, whereas it is expressed at low levels in cucumber stems and leaves.

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