Functional interaction of transcriptional coactivator ASC-2 and C/EBPalpha in granulocyte differentiation of HL-60 promyelocytic cell

The Landscape of AhR Regulators and Coregulators to Fine-Tune AhR Functions

The aryl-hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates numerous cellular responses. Originally investigated in toxicology because of its ability to bind environmental contaminants, AhR has attracted enormous attention in the field of immunology in the last 20 years. In addition, the discovery of endogenous and plant-derived ligands points to AhR also having a crucial role in normal cell physiology. Thus, AhR is emerging as a promiscuous receptor that can mediate either toxic or physiologic effects upon sensing multiple exogenous and endogenous molecules. Within this scenario, several factors appear to contribute to the outcome of gene transcriptional regulation by AhR, including the nature of the ligand as such and its further metabolism by AhR-induced enzymes, the local tissue microenvironment, and the presence of coregulators or specific transcription factors in the cell. Here, we review the current knowledge on the array of transcription factors and coregulators that, by interacting with AhR, tune its transcriptional activity in response to endogenous and exogenous ligands.

The transcriptional co-activator NCOA6 promotes estrogen-induced GREB1 transcription by recruiting ERα and enhancing enhancer-promoter interactions

Estrogen and its cognate receptor, ERα, regulate cell proliferation, differentiation, and carcinogenesis in the endometrium by controlling gene transcription. ERα requires co-activators to mediate transcription via mechanisms that are largely uncharacterized. Herein, using growth-regulating estrogen receptor binding 1 (GREB1) as an ERα target gene in Ishikawa cells, we demonstrate that nuclear receptor co-activator 6 (NCOA6) is essential for estradiol (E2)/ERα-activated GREB1 transcription. We found that NCOA6 associates with the GREB1 promoter and enhancer in an E2-independent manner and that NCOA6 knockout reduces chromatin looping, enhancer-promoter interactions, and basal GREB1 expression in the absence of E2. In the presence of E2, ERα bound the GREB1 enhancer and also associated with its promoter, and p300, myeloid/lymphoid or mixed-lineage leukemia protein 4 (MLL4), and RNA polymerase II were recruited to the GREB1 enhancer and promoter. Consequently, the levels of the histone modifications H3K4me1/3, H3K9ac, and H3K27ac were significantly increased; enhancer and promoter regions were transcribed; and GREB1 mRNA was robustly transcribed. NCOA6 knockout reduced ERα recruitment and abolished all of the aforementioned E2-induced events, making GREB1 completely insensitive to E2 induction. We also found that GREB1-deficient Ishikawa cells are much more resistant to chemotherapy and that human endometrial cancers with low GREB1 expression predict poor overall survival. These results indicate that NCOA6 has an essential role in ERα-mediated transcription by increasing enhancer-promoter interactions through chromatin looping and by recruiting RNA polymerase II and the histone-code modifiers p300 and MLL4. Moreover, GREB1 loss may predict chemoresistance of endometrial cancer.

ATXN8 -62 G/A promoter polymorphism and risk of Taiwanese Parkinson's disease

BACKGROUND AND PURPOSE

We recently reported a novel -62 G/A polymorphism within ataxin 8 (ATXN8) gene promoter region, with -62 G displaying significantly higher luciferase activity compared with -62 A. Phenotypic variability in spinocerebellar ataxia type 8 (SCA8) has been suggested, and large SCA8 repeats were found in patients with Parkinson's disease (PD). We aimed to investigate the association of ATXN8 -62 G/A polymorphism with the risk of Taiwanese PD, and identify the trans-acting factor modulating the ATXN8 promoter activity.

METHODS

A case-control study in a cohort of 569 PD cases and 547 ethnically matched controls was conducted by polymerase chain reaction (PCR) and restriction enzyme analysis. The trans-acting factor binding to the ATXN8 promoter was examined by chromatin immunoprecipitation (ChIP)-PCR assay, cDNA co-transfection and luciferase reporter assay.

RESULTS

When genotype distribution was calculated by comparing the rare AA genotype with the GG + GA genotypes (recessive model), a significant difference was found (P = 0.035, 1 df). Individuals carrying AA genotype exhibited a decreased risk of developing PD (odds ratio: 0.73; 95% CI: 0.55-0.98, P = 0.035). After stratification by age, individuals over 60 years of age carrying AA genotype demonstrated a further decrease in the risk of developing PD (odds ratio: 0.64; 95% CI: 0.43-0.96, P = 0.030). ChIP-PCR and cDNA over-expression revealed that CCAAT/enhancer-binding protein alpha binds to the ATXN8 proximal promoter to upregulate ATXN8 expression in neuroblastoma SK-N-SH cells.

CONCLUSIONS

Our data suggest that ATXN8 -62 G/A polymorphism plays a role in Taiwanese PD susceptibility.

Role of histone methylation and demethylation in adipogenesis and obesity

Adipocyte differentiation is a complex developmental process that involves the coordinated interplay of numerous transcription factors. PPARγ has emerged as a master regulator of adipogenesis and recent global target gene analysis demonstrated that PPARγ targets many genes encoding chromatin modification enzymes as well as genes of lipid metabolism and storage. Among such modification enzymes are histone lysine methyltransferases, which play important roles in transcriptional regulation. Histone methyltransferases are involved in PPARγ gene expression and subsequent adipogenesis. In addition, recent studies revealed that demethylation of histone H3 at lys9 is associated with resistance to obesity. We here review the role of histone methylation and demethylation in adipogenesis, metabolism and obesity.

Targeted inactivation of MLL3 histone H3-Lys-4 methyltransferase activity in the mouse reveals vital roles for MLL3 in adipogenesis

Activating signal cointegrator-2 (ASC-2), a transcriptional coactivator of multiple transcription factors that include the adipogenic factors peroxisome proliferator-activated receptor gamma (PPARgamma) and C/EBPalpha, is associated with histone H3-Lys-4-methyltransferase (H3K4MT) MLL3 or its paralogue MLL4 in a complex named ASCOM (ASC-2 complex). Indeed, ASC-2-null mouse embryonic fibroblasts (MEFs) have been demonstrated to be refractory to PPARgamma-stimulated adipogenesis and fail to express the PPARgamma-responsive adipogenic marker gene aP2. However, the specific roles for MLL3 and MLL4 in adipogenesis remain undefined. Here, we provide evidence that MLL3 plays crucial roles in adipogenesis. First, MLL3(Delta/Delta) mice expressing a H3K4MT-inactivated mutant of MLL3 have significantly less white fat. Second, MLL3(Delta/Delta) MEFs are mildly but consistently less responsive to inducers of adipogenesis than WT MEFs. Third, ASC-2, MLL3, and MLL4 are recruited to the PPARgamma-activated aP2 gene during adipogenesis, and PPARgamma is shown to interact directly with the purified ASCOM. Moreover, although H3K4 methylation of aP2 is readily induced in WT MEFs, it is not induced in ASC-2(-/-) MEFs and only partially induced in MLL3(Delta/Delta) MEFs. These results suggest that ASCOM-MLL3 and ASCOM-MLL4 likely function as crucial but redundant H3K4MT complexes for PPARgamma-dependent adipogenesis.

JWA is required for the antiproliferative and pro-apoptotic effects of all-trans retinoic acid in Hela cells

1. All-trans retinoic acid (ATRA) is known to inhibit cellular proliferation and induce differentiation and apoptosis. It usually activates gene expression by binding to a nuclear receptor that interacts with retinoic acid-response elements (RARE) and then activates the mitogen-activated protein kinase signal pathway. JWA, a newly identified ATRA-responsive gene, has recently been proposed as an important molecule for cellular differentiation induced by some chemicals, including ATRA. 2. To investigate the possible involvement of JWA in the inhibition of cellular proliferation and induction of apoptosis by ATRA, HeLa cells were stably transfected with sense or antisense JWA to establish cell lines that overexpressed or were deficient in JWA; ATRA (0.05-10 micromol/L) was used to induce cellular differentiation and apoptosis. 3. Western blot analysis revealed that ATRA caused increased expression of JWA in HeLa cells in a dose- and time-dependent manner, accompanied by activation of extracellular signal-regulated kinase (ERK) 1/2 phosphorylation. However, ERK1/2 phosphorylation induced by ATRA was inhibited in JWA-deficient HeLa cells. In JWA-overexpressing HeLa cells, ATRA showed more significant antiproliferative effects and induced more apoptosis. 4. The reporter gene assay showed that ATRA (5 mmol/L) enhanced the transcriptional activity of JWA by interacting with its promoter in the region from -194 to +107 bp (P < 0.01). Bioinformatic analysis indicated that the JWA promoter did not contain RARE, but did contain two CCAAT boxes in this fragment spanning -194 to +107 bp, which may be responsive to the ATRA-activated nuclear transcription factor CCAAT/enhancer binding proteins (C/EBP) or interacting proteins. Therefore, ATRA-inhibited cellular proliferation and -induced apoptosis in HeLa cells may be dependent on JWA transactivation via its C/EBP-binding motifs. 5. These data indicate that the inhibition of proliferation and the induction of apoptosis by ATRA are dependent on JWA expression in HeLa cells. The findings may represent a novel mechanism by which the effects of ATRA in regulating cellular proliferation and apoptosis are mediated, at least in part, by JWA expression.

Relationship of expression of aquaglyceroporin 9 with arsenic uptake and sensitivity in leukemia cells

Arsenic trioxide (As2O3) is highly efficacious in acute promyelocytic leukemia (APL). Aquaglyceroporin 9 (AQP9) is a transmembrane protein that may be involved in arsenic uptake. In 10 of 11 myeloid and lymphoid leukemia lines, quantitative polymerase chain reaction (Q-PCR) and Western blotting showed that AQP9 expression correlated positively with As2O3-induced cytotoxicity. As a proof-of-principle, transfection of EGFP-tagged AQP9 to the hepatoma line Hep3B, not expressing AQP9 and As2O3 insensitive, led to membrane AQP9 expression and increased As2O3-induced cytotoxicity. Similarly, the chronic myeloid leukemia line K562 expressed low levels of AQP9 and was As2O3 insensitive. The K562EGFP-AQP9 transfectant accumulated significantly higher levels of intracellular arsenic than control K562EGFP when incubated with As2O3, resulting in significantly increased As2O3-induced cytotoxicity. Pretreatment of the myeloid leukemia line HL-60 with all-trans retinoic acid (ATRA) up-regulated AQP9, leading to a significantly increased arsenic uptake and As2O3-induced cytotoxicity on incubation with As2O3, which might explain the synergism between ATRA and As2O3. Therefore, AQP9 controlled arsenic transport and might determine As2O3 sensitivity. Q-PCR showed that primary APL cells expressed AQP9 significantly (2-3 logs) higher than other acute myeloid leukemias (AMLs), which might explain their exquisite As2O3 sensitivity. However, APL and AML with maturation expressed comparable AQP9 levels, suggesting that AQP9 expression was related to granulocytic maturation.

The CCAAT Enhancer-Binding Protein-α Negatively Regulates the Transactivation of Androgen Receptor in Prostate Cancer Cells

The basic leucine zipper transcription factor, CCAAT enhancer-binding protein-alpha (C/EBPalpha), negatively regulates cell proliferation and induces terminal differentiation of various cell types. C/EBPalpha is expressed in the prostate, but its potential role in the tissue is unknown. Herein, we show that C/EBPalpha is highly expressed at the stage of growth arrest during prostate development. Furthermore, overexpression of C/EBPalpha decreases the rate of DNA synthesis in LNCaP prostate cancer cells. Investigation of the potential cross-talk between C/EBPalpha and androgen receptor (AR) that is responsible for androgen-dependent prostate proliferation demonstrates that androgen-dependent transactivation of AR is strongly repressed by C/EBPalpha. C/EBPalpha directly binds AR in vitro and forms a complex with AR in vivo. C/EBPalpha neither prevents the nuclear translocation of AR nor disrupts the N/C-terminal interaction of AR, which are both necessary for its proper transactivation activity upon ligand binding. To modulate AR transactivation, however, C/EBPalpha does compete with AR coactivators for AR binding. Additionally, C/EBPalpha is recruited onto AR-target promoters with AR and is further able to inhibit the expression of endogenous prostate-specific antigen in prostate cancer cells. Our results suggest C/EBPalpha as a potent AR corepressor and provide insight into the role of C/EBPalpha in prostate development and cancer.

Haploid inactivation of the amplified-in-breast cancer 3 coactivator reduces the inhibitory effect of peroxisome proliferator-activated receptor gamma and retinoid X receptor on cell proliferation and accelerates polyoma middle-T antigen-induced mammary tumorigenesis in mice

The amplified-in-breast cancer 3 (AIB3) is a nuclear receptor coactivator amplified and overexpressed in human breast cancers. AIB3(-/-) mice die during gestation, whereas AIB3(+/-) mice exhibit normal development. Here, we demonstrate that AIB3 protein is mainly located in the nuclei of mammary epithelial cells and tumor cells and its levels are elevated in mammary epithelial cells at middle pregnant stage and in mammary tumor cells. To examine whether AIB3 reduction affects mammary tumorigenesis, we generated wild-type mouse mammary tumor virus/polyoma middle-T (WT/PyMT) and AIB3(+/-)/PyMT mice. Mammary tumor development in AIB3(+/-)/PyMT female and male mice was substantially accelerated compared with that in WT/PyMT mice, because of increased cell proliferation in early tumorigenic lesions, including ductal hyperplasia and mammary intraepithelial neoplasia. Tumor formation in nude mice that received premalignant AIB3(+/-)/PyMT mammary tissue was much faster than in nude mice that received transplants of premalignant WT/PyMT mammary tissue, which indicated that the accelerated tumorigenesis in AIB3(+/-)/PyMT mammary glands is due to a mammary epithelial autonomous defect. Expression of PyMT, estrogen receptor alpha and estrogen receptor alpha-regulated genes was unaffected in AIB3(+/-)/PyMT mammary glands, which suggests that the acceleration of mammary tumor formation in AIB3(+/-)/PyMT mice was not a consequence of changes in PyMT expression or in estrogen receptor function. Importantly, the inhibitory effects of peroxisome proliferator-activated receptor gamma (PPARgamma) and retinoid-X receptor (RXR) ligands on AIB3(+/-)/PyMT cell proliferation and the transcriptional function of PPARgamma in AIB3(+/-)/PyMT cells were reduced. Thus, AIB3 haplodeficiency may facilitate PyMT-induced tumorigenesis through a partial impairment of PPARgamma and RXR function. These results suggest that AIB3 may be a tumor suppressor that is required for the inhibition of cell proliferation by PPARgamma and RXR.

Differential recruitment of nuclear receptor coactivators may determine alternative RNA splice site choice in target genes

The biological consequences of steroid hormone-mediated transcriptional activation of target genes might be difficult to predict because alternative splicing of a single neosynthesized precursor RNA can result in production of different protein isoforms with opposite biological activities. Therefore, an important question to address is the manner in which steroid hormones affect the splicing of their target gene transcripts. In this report, we demonstrate that individual steroid hormones had different and opposite effects on alternative splicing decisions, stimulating the production of different spliced variants produced from genes driven by steroid hormone-dependent promoters. Steroid hormone transcriptional effects are mediated by steroid hormone receptor coregulators that also modify alternative splicing decisions. Our data suggest that activated steroid hormone receptors recruit coregulators to the target promoter that participate in both the production and the splicing of the target gene transcripts. Because different coregulators activating transcription can have opposite effects on alternative splicing decisions, we conclude that the precise nature of the transcriptional coregulators recruited by activated steroid receptors, depending on the promoter and cellular contexts, may play a major role in regulating the nature of the spliced variants produced from certain target genes in response to steroid hormones.

Activation and Interaction of ATF2 with the Coactivator ASC-2 Are Responsive for Granulocytic Differentiation by Retinoic Acid

Terminal differentiation of hematopoietic cells follows a precisely orchestrated program of transcriptional regulatory events at the promoters of both lineage-specific and ubiquitous genes. Here we show that the transcription factor ATF2 is associated with the induction of granulocytic differentiation, and the molecular interaction of ATF2 with a tissue-specific coactivator activating signal cointegator-2 (ASC-2) potentiates the differentiation procedure. All-trans retinoic acid (RA) induced the phosphorylation and expression of ATF2 in the early and middle phase of granulocyte differentiation, respectively. The activation of granulocyte-specific gene expression is increased with the concerted action of another basic regionleucine zipper factor, CCAAT/enhancer-binding protein (C/EBPalpha), and ASC-2, which function in a cooperative manner. The interaction between ATF2 and C/EBPalpha in RA-treated cells was enhanced by the ectopic expression of ASC-2. ATF2-mediated transactivation was also increased by co-transfection of ASC-2. This resulted from the direct protein interaction that the N-terminal transactivation domain of ATF2 interacts with the central region of ASC-2. Furthermore, the molecular interaction of ATF2 and ASC-2 was stimulated by RA treatment and inhibited by p38beta kinase inhibitor. Taking these results together, these results suggest that the differentiation-dependent expression and phosphorylation of ATF2 protein physically and functionally interacts with C/EBPalpha and coativator ASC-2 and synergizes to induce target gene transcription during granulocytic differentiation.

Deletion of the cancer-amplified coactivator AIB3 results in defective placentation and embryonic lethality

The amplified in breast cancer-3 (AIB3, ASC-2, RAP250, PRIP, TRBP, NRC, or NcoA6) gene is characterized as a cancer-amplified transcriptional coactivator for nuclear receptors, which include the peroxisome proliferator-activated receptor gamma (PPARgamma). To assess its biological function, we deleted the AIB3 gene in mice by homologous recombination. AIB3(+/-) mice are developmentally normal and fertile. AIB3(-/-) embryos exhibit growth restriction and lethality during 9.75-11.5 days postconception. The embryonic lethality is probably attributed to defects in the development of the placental vascular network and cardiac hypoplasia. These defects include the failure of labyrinthine development, the dilation of maternal blood sinuses, the massive erythrophagocytosis by trophoblasts, the alteration of trophoblast populations, and the lower proliferation of myocardium, which are similar to those encountered in mice lacking PPARgamma or the PPARgamma-binding protein (PBP, TRAP220, or DRIP205). In addition, the transcriptional activities of PPARgamma are significantly affected in mouse embryonic fibroblasts lacking AIB3. These results suggest that AIB3 is required for PPARgamma function in placental development and for normal heart development. These results also indicate that the biological function of AIB3 is not redundant with other classes of nuclear receptor coactivators such as PBP and members of the steroid receptor coactivator family.

Biomic study of human myeloid leukemia cells differentiation to macrophages using DNA array, proteomic, and bioinformatic analytical methods

A biomic approach by integrating three independent methods, DNA microarray, proteomics and bioinformatics, is used to study the differentiation of human myeloid leukemia cell line HL-60 into macrophages when induced by 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Analysis of gene expression changes at the RNA level using cDNA against an array of 6033 human genes showed that 5950 (98.6%) of the genes were expressed in the HL-60 cells. A total of 624 genes (10.5%) were found to be regulated during HL-60 cell differentiation. Most of these genes have not been previously associated with HL-60 cells and include genes encoded for secreted proteins as well as genes involved in cell adhesion, signaling transduction, and metabolism. Protein analysis using two-dimensional gel electrophoresis showed a total of 682 distinct protein spots; 136 spots (19.9%) exhibited quantitative changes between HL-60 control and macrophages. These differentially expressed proteins were identified by mass spectrometry. We developed a bioinformatics program, the Bulk Gene Search System (BGSS, http://www.sinica.edu.tw:8900/perl/genequery.pl) to search for the functions of genes and proteins identified by cDNA microarrays and proteomics. The identified regulated proteins and genes were classified into seven groups according to subcellular locations and functions. This powerful holistic biomic approach using cDNA microarray, proteomics coupled to bioinformatics can provide in-depth information on the impact and importance of the regulated genes and proteins for HL-60 differentiation.