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Lee WS, McColl B, Maksimovic J, Vadolas J. Epigenetic interplay at the β-globin locus. BIOCHIMICA ET BIOPHYSICA ACTA-GENE REGULATORY MECHANISMS 2017; 1860:393-404. [DOI: 10.1016/j.bbagrm.2017.01.014] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/02/2016] [Revised: 01/28/2017] [Accepted: 01/30/2017] [Indexed: 02/02/2023]
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Ding XF, Yu SC, Chen BD, Lin SJ, Chang C, Li GH. Recent advances in the study of testicular nuclear receptor 4. J Zhejiang Univ Sci B 2013; 14:171-7. [PMID: 23463759 DOI: 10.1631/jzus.b1200357] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Testicular nuclear receptor 4 (TR4), also known as NR2C2 (nuclear receptor subfamily 2, group C, member 2), is a transcriptional factor and a member of the nuclear receptor family. TR4 was initially cloned from human and rat hypothalamus, prostate, and testes libraries. For almost two decades, its specific tissue distribution, genomic organization, and chromosomal assignment have been well investigated in humans and animals. However, it has been very difficult to study TR4's physiological functions due to a lack of specific ligands. Gene knock-out animal techniques provide an alternative approach for defining the biological functions of TR4. In vivo studies of TR4 gene knockout mice (TR4(-/-)) found that they display severe spinal curvature, subfertility, premature aging, and prostate prostatic intraepithelial neoplasia (PIN) development. Upstream modulators, downstream target gene regulation, feedback mechanisms, and differential modulation mediated by the recruitment of other nuclear receptors and coregulators have been identified in studies using the TR4(-/-) phenotype. With the establishment of a tissue-specific TR4(-/-) mouse model, research on TR4 will be more convenient in the future.
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Affiliation(s)
- Xian-fan Ding
- Department of Urology, Sir Run Run Shaw Hospital, School of Medicine, Zhejiang University, Hangzhou, China
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3
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Park SS, Kim E. Nur77 inhibits TR4-induced PEPCK expression in 3T3-L1 adipocytes. Anim Cells Syst (Seoul) 2012. [DOI: 10.1080/19768354.2011.603748] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
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4
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The roles of testicular orphan nuclear receptor 4 (TR4) in cerebellar development. THE CEREBELLUM 2009; 7:9-17. [PMID: 18418664 DOI: 10.1007/s12311-008-0006-3] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Since Testicular Receptor 4 (TR4) was cloned, efforts have been made to elucidate its physiological function. To examine the putative functions of TR4, the conventional TR4 knockout (TR4(-/-)) mouse model was generated. Throughout postnatal and adult stages, TR4(-/-) mice exhibited behavioral deficits in motor coordination, suggesting impaired cerebellar function. Histological examination of the postnatal and adult TR4(-/-) cerebellum revealed gross abnormalities in foliation. Further analyses demonstrated changes in the lamination of the TR4(-/-) cerebellar cortex, including reduction in the thickness of both the molecular layer (ML) and the internal granule layer (IGL). Analyses of the developing TR4(-/-) cerebellum indicate that the lamination irregularities observed may result from disrupted granule cell proliferation within the external granule cell layer (EGL), delayed inward migration of post-mitotic granule cells, and increased apoptosis during cerebellar development. In addition, abnormal development of Purkinje cells was observed in the postnatal TR4(-/-) cerebellum, as indicated by aberrant dendritic arborization. In postnatal, neuronal-specific TR4 knockout mice, architectural changes in the cerebellum were similar to those seen in TR4(-/-) animals, suggesting that TR4 function in neuronal lineages might be important for cerebellar morphogenesis, and that the effect on Purkinje cell development is likely mediated by changes elsewhere, such as in granule cells, or is highly dependent on developmental stage. Together, our findings from various TR4 knockout mouse models suggest that TR4 is required for normal cerebellar development and that failure to establish proper cytoarchitecture results in dysfunction of the cerebellum and leads to abnormal behavior.
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Lin TJ, Yang RY, Lee HJ. Collective repression of the hepatitis B virus enhancer II by human TR4 and TR2 orphan receptors. Hepatol Res 2008; 38:79-84. [PMID: 17645519 DOI: 10.1111/j.1872-034x.2007.00208.x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
AIM The human testicular receptor 4 and 2 (TR4 and TR2) orphan receptors are members of the nuclear receptor superfamily that regulate target gene expression via binding to the consensus AGGTCA direct repeats of the hormone response elements. Previous studies have reported that TR4 and other nuclear receptors could bind to the direct repeat 1 element of the hepatitis B virus (HBV) core promoter. METHODS Differential gene expression of HBV caused by TR4 and TR2 was determined by gel retardation and functional assays. RESULTS Electrophoretic mobility shift assay demonstrated that TR4 and TR2 might bind to the direct repeat 6 element of the HBV enhancer II region. RESULTS of the dual-luciferasereporter gene assay showed that TR4 and TR2 might significantly suppress HBV gene expression through this direct repeat 6 element in the enhancer II. CONCLUSION These results implied that TR4 together with its heterodimer partner TR2 could collectively play a significant role in the transcriptional suppression of HBV gene expression via the direct repeat 6 element in the enhancer II. Therefore, the application of nuclear receptors potentially may be antiviral agents in chronic HBV infection.
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Affiliation(s)
- Tzu-Jon Lin
- Department of Life Science, National Dong Hwa University, Hualien, Taiwan
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Liu NC, Lin WJ, Kim E, Collins LL, Lin HY, Yu IC, Sparks JD, Chen LM, Lee YF, Chang C. Loss of TR4 orphan nuclear receptor reduces phosphoenolpyruvate carboxykinase-mediated gluconeogenesis. Diabetes 2007; 56:2901-9. [PMID: 17827404 DOI: 10.2337/db07-0359] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
OBJECTIVE Regulation of phosphoenolpyruvate carboxykinase (PEPCK), the key gene in gluconeogenesis, is critical for glucose homeostasis in response to quick nutritional depletion and/or hormonal alteration. RESEARCH DESIGN/METHODS AND RESULTS Here, we identified the testicular orphan nuclear receptor 4 (TR4) as a key PEPCK regulator modulating PEPCK gene via a transcriptional mechanism. TR4 transactivates the 490-bp PEPCK promoter-containing luciferase reporter gene activity by direct binding to the TR4 responsive element (TR4RE) located at -451 to -439 in the promoter region. Binding to TR4RE was confirmed by electrophoretic mobility shift and chromatin immunoprecipitation assays. Eliminating TR4 via knockout and RNA interference (RNAi) in hepatocytes significantly reduced the PEPCK gene expression and glucose production in response to glucose depletion. In contrast, ectopic expression of TR4 increased PEPCK gene expression and hepatic glucose production in human and mouse hepatoma cells. Mice lacking TR4 also display reduction of PEPCK expression with impaired gluconeogenesis. CONCLUSIONS Together, both in vitro and in vivo data demonstrate the identification of a new pathway, TR4 --> PEPCK --> gluconeogenesis --> blood glucose, which may allow us to modulate metabolic programs via the control of a new key player, TR4, a member of the nuclear receptor superfamily.
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Affiliation(s)
- Ning-Chun Liu
- Department of Pathology, Cancer Center, University of Rochester, Rochester, NY 14642, USA
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Barzon L, Maffei P, Sonino N, Pilon C, Baldazzi L, Balsamo A, Del Maschio O, Masi G, Trevisan M, Pacenti M, Fallo F. The role of 21-hydroxylase in the pathogenesis of adrenal masses: review of the literature and focus on our own experience. J Endocrinol Invest 2007; 30:615-23. [PMID: 17848847 DOI: 10.1007/bf03346358] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
An exaggerated response of 17- hydroxyprogesterone (17-OHP) to exogenous ACTH stimulation has been found in 30 to 70% of patients with incidentally discovered adrenal tumors, supporting the concept that congenital 21- hydroxylase deficiency may be a predisposing factor for adrenocortical tumorigenesis. Decreased expression of 21-hydroxylase gene has been observed in sporadic non-functioning adrenocortical adenomas and adrenocortical carcinomas, in agreement with the reduced steroidogenic activity found in these types of tumors. Screening studies for the presence of mutations in CYP21A2 gene, encoding 21-hydroxylase, in patients with sporadic adrenocortical tumors yielded discordant results. Overall, a higher frequency of germline 21-hydroxylase mutation carriers has been found among patients with adrenal tumors, including incidentalomas, than in the general population. However, the presence of mutations did not correlate with endocrine test results and tumor mass features, suggesting that 21-hydroxylase deficiency does not represent a relevant mechanism in adrenal tumorigenesis. Mechanisms leading to reduced 21-hydroxylase expression and activity are still unknown.
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Affiliation(s)
- L Barzon
- Department of Histology, Microbiology and Medical Biotechnologies, University of Padua, Padua, Italy
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Benoit G, Cooney A, Giguere V, Ingraham H, Lazar M, Muscat G, Perlmann T, Renaud JP, Schwabe J, Sladek F, Tsai MJ, Laudet V. International Union of Pharmacology. LXVI. Orphan nuclear receptors. Pharmacol Rev 2007; 58:798-836. [PMID: 17132856 DOI: 10.1124/pr.58.4.10] [Citation(s) in RCA: 154] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
Half of the members of the nuclear receptors superfamily are so-called "orphan" receptors because the identity of their ligand, if any, is unknown. Because of their important biological roles, the study of orphan receptors has attracted much attention recently and has resulted in rapid advances that have helped in the discovery of novel signaling pathways. In this review we present the main features of orphan receptors, discuss the structure of their ligand-binding domains and their biological functions. The paradoxical existence of a pharmacology of orphan receptors, a rapidly growing and innovative field, is highlighted.
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Affiliation(s)
- Gérard Benoit
- Unité Mixte de Recherche 5161 du Centre National de la Recherche Scientifique, Institut National de la Recherche Agronomique 1237, Institut Fédératif de Recherche 128 BioSciences Lyon-Gerland, Ecole Normale Supérieure de Lyon, Lyon, France
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Wang CP, Lee YF, Chang C, Lee HJ. Transactivation of the proximal promoter of human oxytocin gene by TR4 orphan receptor. Biochem Biophys Res Commun 2006; 351:204-8. [PMID: 17054912 DOI: 10.1016/j.bbrc.2006.10.021] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2006] [Accepted: 10/06/2006] [Indexed: 11/16/2022]
Abstract
The human testicular receptor 4 (TR4) shares structural homology with members of the nuclear receptor superfamily. Some other members of this superfamily were able to regulate the transcriptional activity of the human oxytocin (OXT) promoter by binding to the first DR0 regulatory site. However, little investigation was conducted systematically in the study of the second dDR4 site of OXT proximal promoter, and the relationship between the first and the second sites of OXT promoter. Here, we demonstrated for the first time that TR4 could increase the proximal promoter activity of the human OXT gene via DR0, dDR4, and OXT (both DR0 and dDR4) elements, respectively. TR4 might induce OXT gene expression through the OXT element in a dose-dependent manner. However, there is no synergistic effect between DR0 and dDR4 elements during TR4 transactivation. Taken together, these results suggested that TR4 should be one of important regulators of OXT gene expression.
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Affiliation(s)
- Chih-Pin Wang
- Department of Life Science, National Dong Hwa University, Hualien 97401, Taiwan
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Lin YL, Wang YH, Lee HJ. Transcriptional regulation of the human TR2 orphan receptor gene by nuclear factor 1-A. Biochem Biophys Res Commun 2006; 350:430-6. [PMID: 17010934 DOI: 10.1016/j.bbrc.2006.09.061] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2006] [Accepted: 09/14/2006] [Indexed: 11/15/2022]
Abstract
The human testicular receptor 2 (TR2), a member of the nuclear hormone receptor superfamily, has no identified ligand yet. Previous evidence demonstrated that a 63bp DNA fragment, named the promoter activating cis-element (PACE), has been identified as a positive regulatory region in the 5' promoter region of the human TR2 gene. In the present report, the human nuclear factor 1-A (NF1-A) was identified as a transcriptional activator to recognize the center of the PACE, called the PACE-C. NF1-A could bind to the 18bp PACE-C region, and enhance about 13- to 17-fold of the luciferase reporter gene activity via the PACE-C in dose-dependent and orientation-independent manners. This transcriptional activation was further confirmed by real-time RT-PCR assay. In conclusion, our results indicated that NF1-A transcription factor plays an important role in the transcriptional activation of the TR2 gene expression via the PACE-C in the minimal promoter region.
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Affiliation(s)
- Yun-Lu Lin
- Department of Life Science, National Dong Hwa University, Hualien 97401, Taiwan
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Nakajima T, Fujino S, Nakanishi G, Kim YS, Jetten AM. TIP27: a novel repressor of the nuclear orphan receptor TAK1/TR4. Nucleic Acids Res 2004; 32:4194-204. [PMID: 15302918 PMCID: PMC514368 DOI: 10.1093/nar/gkh741] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
The nuclear orphan receptor TAK1/TR4 functions as a positive as well as a negative regulator of transcription; however, little is known about the factors regulating or mediating its activity. Yeast two-hybrid analysis using the ligand-binding domain (LBD) of TAK1 as bait identified a novel TAK1-interacting protein, referred to as TIP27, which functions as a repressor of TAK1-mediated transactivation. TIP27 is a 27 kDa protein containing two zinc finger motifs. Mammalian two-hybrid analysis showed that TIP27 interacts specifically with TAK1 and not with several other nuclear receptors tested. The region between Asp39 and Lys79 of TIP27, referred to as TAK1-interaction domain (TID), is critical for its interaction with TAK1 while the TAK1-LBD from helix 3 until the C-terminus is required for the optimal interaction with TIP27. Pull-down assays demonstrated that the TIP27 physically interacts with TAK1 and supported the critical importance of the TID. Confocal microscopy showed that in the nucleus, TIP27 and TAK1 co-localize. TIP27 acts as a strong repressor of DR1-dependent transcriptional activation by TAK1. This repression does not involve the inhibition of TAK1 homodimerization or DR1 binding but may be due to an effect on co-activator recruitment by TAK1. Our results indicate that TIP27 functions as a TAK1-selective repressor.
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Affiliation(s)
- Takeshi Nakajima
- Cell Biology Section, Division of Intramural Research, National Institute of Environmental Health Sciences, National Institutes of Health, 111 T.W. Alexander Drive, Research Triangle Park, NC 27709, USA
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Abstract
Nuclear orphan receptors represent a large and diverse subgroup in the nuclear receptor superfamily. Although putative ligands for these orphan members remain to be identified, some of these receptors possess intrinsic activating, inhibitory, or dual regulatory functions in development, differentiation, homeostasis, and reproduction. In particular, gene-silencing events elicited by chicken ovalbumin upstream promoter-transcription factors (COUP-TFs); dosage-sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene 1 (DAX-1); germ cell nuclear factor (GCNF); short heterodimer partner (SHP); and testicular receptors 2 and 4 (TR2 and TR4) are among the best characterized. These orphan receptors are critical in controlling basal activities or hormonal responsiveness of numerous target genes. They employ multiple and distinct mechanisms to mediate target gene repression. Complex cross-talk exists between these orphan receptors at their cognate DNA binding elements and an array of steroid?nonsteroid hormone receptors, other transcriptional activators, coactivators and corepressors, histone modification enzyme complexes, and components of basal transcriptional components. Therefore, perturbation induced by these orphan receptors at multiple levels, including DNA binding activities, receptor homo- or heterodimerization, recruitment of cofactor proteins, communication with general transcriptional machinery, and changes at histone acetylation status and chromatin structures, may contribute to silencing of target gene expression in a specific promoter or cell-type context. Moreover, the findings derived from gene-targeting studies have demonstrated the significance of these orphan receptors' function in physiologic settings. Thus, COUP-TFs, DAX-1, GCNF, SHP, and TR2 and 4 are known to be required for multiple physiologic and biologic functions, including neurogenesis and development of the heart and vascular system steroidogenesis and sex determination, gametogenesis and embryonic development, and cholesterol?lipid homeostasis.
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MESH Headings
- Animals
- COUP Transcription Factor I
- COUP Transcription Factors
- DAX-1 Orphan Nuclear Receptor
- DNA-Binding Proteins/metabolism
- Gametogenesis/physiology
- Gene Expression/physiology
- Gene Silencing/physiology
- Humans
- Models, Molecular
- Nuclear Receptor Subfamily 2, Group C, Member 1
- Nuclear Receptor Subfamily 6, Group A, Member 1
- Receptors, Cytoplasmic and Nuclear/metabolism
- Receptors, Cytoplasmic and Nuclear/physiology
- Receptors, Retinoic Acid/metabolism
- Receptors, Steroid/metabolism
- Receptors, Thyroid Hormone/metabolism
- Repressor Proteins/metabolism
- Transcription Factors/metabolism
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Affiliation(s)
- Ying Zhang
- Section on Molecular Endocrinology, Endocrinology, and Reproduction Research Branch, National Institutes of Health, Bethesda, Maryland 20892, USA
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Kim E, Xie S, Yeh SD, Lee YF, Collins LL, Hu YC, Shyr CR, Mu XM, Liu NC, Chen YT, Wang PH, Chang C. Disruption of TR4 orphan nuclear receptor reduces the expression of liver apolipoprotein E/C-I/C-II gene cluster. J Biol Chem 2003; 278:46919-26. [PMID: 12954636 DOI: 10.1074/jbc.m304088200] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Apolipoprotein E (apoE) is synthesized in many tissues, and the liver is the primary site from which apoE redistributes cholesterol and other lipids to peripheral tissues. Here we demonstrate that the TR4 orphan nuclear receptor (TR4) can induce apoE expression in HepG2 cells. This TR4-mediated regulation of apoE gene expression was further confirmed in vivo using TR4 knockout mice. Both serum apoE protein and liver apoE mRNA levels were significantly reduced in TR4 knockout mice. Gel shift and luciferase reporter gene assays further demonstrated that TR4 can induce apoE gene expression via a TR4 response element located in the hepatic control region that is 15 kb downstream of the apoE gene. Furthermore our in vivo data from TR4 knockout mice prove that TR4 can also regulate apolipoprotein C-I and C-II gene expression via the TR4 response element within the hepatic control region. Together our data show that loss of TR4 down-regulates expression of the apoE/C-I/C-II gene cluster in liver cells, demonstrating important roles of TR4 in the modulation of lipoprotein metabolism.
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Affiliation(s)
- Eungseok Kim
- George Whipple Laboratory for Cancer Research, University of Rochester Medical Center, Rochester, NY 14642, USA
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Yang Y, Wang X, Dong T, Kim E, Lin WJ, Chang C. Identification of a novel testicular orphan receptor-4 (TR4)-associated protein as repressor for the selective suppression of TR4-mediated transactivation. J Biol Chem 2003; 278:7709-17. [PMID: 12486131 DOI: 10.1074/jbc.m207116200] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Although many co-activators have been identified for various nuclear receptors, relatively fewer co-repressors have been isolated and characterized. Here we report the identification of a novel testicular orphan nuclear receptor-4 (TR4)-associated protein (TRA16) that is mainly localized in the nucleus of cells as a repressor to suppress TR4-mediated transactivation. The suppression of TR4-mediated transactivation is selective because TRA16 shows only a slight influence on the transactivation of androgen receptor, glucocorticoid receptor, and progesterone receptor. Sequence analysis shows that TRA16 is a novel gene with 139 amino acids in an open reading frame with a molecular mass of 16 kDa, which did not match any published gene sequences. Mammalian two-hybrid system and co-immunoprecipitation assays both demonstrate that TRA16 can interact strongly with TR4. The electrophoretic mobility shift assay suggests that TRA16 may suppress TR4-mediated transactivation via decreased binding between the TR4 protein and the TR4 response element on the target gene(s). Furthermore, TRA16 can also block the interaction between TR4 and TR4 ligand-binding domain through interacting with TR4-DNA-binding and ligand-binding domains. These unique suppression mechanisms suggest that TRA16 may function as a novel repressor to selectively suppress the TR4-mediated transactivation.
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MESH Headings
- Amino Acid Sequence
- Animals
- Base Sequence
- Blotting, Northern
- COS Cells
- Cell Nucleus/metabolism
- Cells, Cultured
- DNA, Complementary/metabolism
- Gene Library
- Genes, Reporter
- Humans
- Immunohistochemistry
- Male
- Mice
- Microscopy, Fluorescence
- Molecular Sequence Data
- Nuclear Proteins/chemistry
- Nuclear Proteins/metabolism
- Protein Binding
- Protein Structure, Tertiary
- RNA, Messenger/metabolism
- Receptors, Androgen/metabolism
- Receptors, Glucocorticoid/metabolism
- Receptors, Progesterone/metabolism
- Receptors, Steroid/metabolism
- Receptors, Thyroid Hormone/metabolism
- Repressor Proteins/chemistry
- Repressor Proteins/metabolism
- Testis/metabolism
- Tissue Distribution
- Transcriptional Activation
- Transfection
- Two-Hybrid System Techniques
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Affiliation(s)
- Yue Yang
- George Whipple Laboratory for Cancer Research Department of Pathology, University of Rochester Medical Center, Rochester, New York 14642, USA
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Lee YF, Lee HJ, Chang C. Recent advances in the TR2 and TR4 orphan receptors of the nuclear receptor superfamily. J Steroid Biochem Mol Biol 2002; 81:291-308. [PMID: 12361719 DOI: 10.1016/s0960-0760(02)00118-8] [Citation(s) in RCA: 79] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
The human testicular receptor 2 (TR2) and TR4 orphan receptors are two evolutionarily related proteins belonging to the nuclear receptor superfamily. Numerous TR2 and TR4 variants and homologs have been identified from different species, including vertebrates (e.g. human, murine, rabbit, fish, and amphibian) and invertebrates (e.g. Drosophila, sea urchin, and nematode) since TR2 was initially isolated over a decade ago. Specific tissue distribution, genomic organization, and chromosomal assignment of both orphan receptors have been investigated. In order to reveal the physiological functions played by both TR2 and TR4, upstream modulators of TR2 and TR4 gene expression, their downstream target gene regulation, feedback mechanisms, and differential modulation mediated by the recruitment of other nuclear receptors and coregulators have been investigated. Studies summarized in the present report have provided unexpected insights into the TR2 and TR4 functions in a variety of biological processes. The essential and difficult tasks of identifying orphan receptor ligands, agonist/antagonist assignment, their physiological functions, and mechanisms of action will continue to challenge nuclear receptor researchers in the future.
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Affiliation(s)
- Yi-Fen Lee
- George Whipple Laboratory for Cancer Research, Department of Urology, University of Rochester, NY 14642, USA
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