Polyubiquitination of the demethylase Jhd2 controls histone methylation and gene expression

The identification of histone methyltransferases and demethylases has uncovered a dynamic methylation system needed to modulate appropriate levels of gene expression. Gene expression levels of various histone demethylases, such as the JARID1 family, show distinct patterns of embryonic and adult expression and respond to different environmental cues, suggesting that histone demethylase protein levels must be tightly regulated for proper development. In our study, we show that the protein level of the yeast histone H3 Lys 4 (H3 K4) demethylase Jhd2/Kdm5 is modulated through polyubiquitination by the E3 ubiquitin ligase Not4 and turnover by the proteasome. We determine that polyubiquitin-mediated degradation of Jhd2 controls in vivo H3 K4 trimethylation and gene expression levels. Finally, we show that human NOT4 can polyubiquitinate human JARID1C/SMCX, a homolog of Jhd2, suggesting that this is likely a conserved mechanism. We propose that Not4 is an E3 ubiquitin ligase that monitors and controls a precise amount of Jhd2 protein so that the proper balance between histone demethylase and histone methyltransferase activities occur in the cell, ensuring appropriate levels of H3 K4 trimethylation and gene expression.     

Histone methyltransferases regulating rRNA gene dose and dosage control in Arabidopsis

Eukaryotes have hundreds of nearly identical 45S ribosomal RNA (rRNA) genes, each encoding the 18S, 5.8S, and 25S catalytic rRNAs. Because cellular demands for ribosomes and protein synthesis vary during development, the number of active rRNA genes is subject to dosage control. In genetic hybrids, one manifestation of dosage control is nucleolar dominance, an epigenetic phenomenon in which the rRNA genes of one progenitor are repressed. For instance, in Arabidopsis suecica, the allotetraploid hybrid of Arabidopsis thaliana and Arabidopsis arenosa, the A. thaliana-derived rRNA genes are selectively silenced. An analogous phenomenon occurs in nonhybrid A. thaliana, in which specific classes of rRNA gene variants are inactivated. An RNA-mediated knockdown screen identified SUVR4 {SUPPRESSOR OF VARIEGATION 3-9 [SU(VAR)3-9]-RELATED 4} as a histone H3 Lys 9 (H3K9) methyltransferase required for nucleolar dominance in A. suecica. H3K9 methyltransferases are also required for variant-specific silencing in A. thaliana, but SUVH5 [SU(VAR)3-9 HOMOLOG 5] and SUVH6, rather than SUVR4, are the key activities in this genomic context. Mutations disrupting the H3K27 methyltransferases ATXR5 or ATXR6 affect which rRNA gene variants are expressed or silenced, and in atxr5 atxr6 double mutants, dominance relationships among variants are reversed relative to wild type. Interestingly, these changes in gene expression are accompanied by changes in the relative abundance of the rRNA gene variants at the DNA level, including overreplication of the normally silenced class and decreased abundance of the normally dominant class. Collectively, our results indicate that histone methylation can affect both the doses of different variants and their differential silencing through the choice mechanisms that achieve dosage control.     

Nucleosome-binding activities within JARID2 and EZH1 regulate the function of PRC2 on chromatin

Polycomb-repressive complex 2 (PRC2) comprises specific members of the Polycomb group of epigenetic modulators. PRC2 catalyzes methylation of histone H3 at Lys 27 (H3K27me3) through its Enhancer of zeste (Ezh) constituent, of which there are two mammalian homologs: Ezh1 and Ezh2. Several ancillary factors, including Jarid2, modulate PRC2 function, with Jarid2 facilitating its recruitment to target genes. Jarid2, like Ezh2, is present in poorly differentiated and actively dividing cells, while Ezh1 associates with PRC2 in all cells, including resting cells. We found that Jarid2 exhibits nucleosome-binding activity that contributes to PRC2 stimulation. Moreover, such nucleosome-binding activity is exhibited by PRC2 comprising Ezh1 (PRC2–Ezh1), in contrast to PRC2–Ezh2. The presence of Ezh1 helps to maintain PRC2 occupancy on its target genes in myoblasts where Jarid2 is not expressed. Our findings allow us to propose a model in which PRC2–Ezh2 is important for the de novo establishment of H3K27me3 in dividing cells, whereas PRC2–Ezh1 is required for its maintenance in resting cells.     

Modifications of chromatin structure in control of gene expression

Repetitive sequences in intron and spacer DNA could be sites for binding of chromosomal proteins which maintain chromatin structure and control gene activity. Methylation of DNA guides the binding of acidic nonhistone proteins and maintains the differentiation state during DNA replication. Differentiation inducers modify repressor proteins permiting unfolding of chromatin. Histone H 1 must be removed for gene activity. Phosphorylation of nonhistone proteins probably induces allosteric modifications which permit unfolding of chromatin. Acetylation of nucleosomal histones is necessary to permit passage of RNA polymerase. Deacetylation quickly returns the gene to a normal histone repressed state. Chromosomal RNA attached to nonhistone proteins aids the binding of RNA polymerase to the DNA template. Carcinogens can disrupt normal gene control leading to circumvention of normal cell cycle controls.     

组蛋白甲基化与lncRNAs的表达调控

LncRNAs是长度超过200 nt非编码蛋白质的RNA分子,并参与细胞内多种调控过程.LncRNA在发育和基因表达中发挥着复杂精确的调控功能,补充解释了基因组复杂性的生物意义,同时也使人们重新认识了生命过程中基因表达调控网络的复杂性.LncRNA参与了X染色体沉默、基因组印记、染色质修饰、转录激活、转录干扰以及核内运输等多种重要的调控过程.组蛋白修饰是基因表达调控的重要形式,尤其是组蛋白甲基化的调控作用.研究发现组蛋白甲基化对基因异常表达的调控参与多种肿瘤的发生.组蛋白去甲基化酶的发现又提出了一种新的理念,即组蛋白甲基化对lncRNA表达调控作用,这将是基因表达调控研究的新领域.     

阿托伐他汀下调大鼠心肌梗死后线粒体融合素2表达和细胞凋亡简

 目的 研究阿托伐他汀通过下调线粒体融合素2表达抑制大鼠心肌梗死后细胞凋亡。 方法 雄性SD大鼠48只,随机分为假手术组（Sham）,心肌梗死组（MI）,阿托伐他汀1组（Statin 1）和2组（Statin 2）,每组12只。分别于术前7天开始每日灌胃,给予阿托伐他汀10和40mg/kg,MI组以蒸馏水灌胃,随后结扎大鼠前降支建立心肌梗死模型。术后24h用TUNEL法检测心肌细胞凋亡,免疫组化法检测线粒体融合素2表达,免疫印迹法检测磷酸化蛋白激酶B（p-Akt）表达。 结果 与Sham组相比,MI组心肌细胞凋亡显著增加,线粒体融合素2表达显著增加,p-Akt表达显著下降（p＜0.01）;与MI组相比,Statin 1和2组心肌细胞凋亡和线粒体融合素2表达均显著降低（p＜0.05）,而p-Akt表达显著增高（p＜0.05）,且Statin 2组较1组变化更显著（p＜0.05）。结论 阿托伐他汀可抑制大鼠心肌梗死后的细胞凋亡,其作用可能与下调线粒体融合素2表达有关。     

丁酸钠对卵巢癌细胞株SKOV-3表达H3K9Ac、H3K14Ac、H4K20TriMe蛋白的影响

目的观察不同浓度丁酸钠处理SKOV-3细胞后对H3K9Ac、H3K14Ac、H4K20TriMe蛋白表达的影响。方法在上皮性卵巢癌细胞株SKOV-3培养过程中给予不同浓度的丁酸钠处理，观察细胞生长情况，用western blot方法测定处理前后SKOV-3细胞中H3K9Ac、H3K14Ac、H4K20TriMe蛋白表达情况。结果①当丁酸钠浓度为4～16mmol／L时，对SKOV-3细胞体外生长均有明显的抑制作用。②丁酸钠可影响SKOV-3细胞的形态。③丁酸钠处理48h后H3K9Ac的表达明显增高，而H3K14Ac、H4K20TriMe蛋白表达无明显变化。结论①组蛋白去乙酰化酶抑制剂具有抗肿瘤作用。②去乙酰化酶抑制剂可通过逆转特异位点的组蛋白乙酰化而发挥抗肿瘤的作用，这可能与其上调一系列关键基因的表达有关。     

Distinct expression patterns of polycomb oncoproteins and their binding partners during the germinal center reaction

Polycomb group (PcG) genes encode two chromatin-binding protein complexes, the PRC1 and the PRC2 PcG complexes, which are essential for the maintenance of cell identity and play a role in oncogenesis. PcG complexes were recently identified as novel regulators of hematopoiesis, and appear to be expressed in a non-overlapping pattern in resting and mature follicular B cells. Using highly specific antisera in combination with immunohistochemistry and triple immunofluorescence, we investigated the expression pattern of nine human PcG genes in germinal center (GC) B cells and highly purified germinal center B cell subpopulations. PcG proteins were detected in characteristic binding patterns that were not necessarily related to mutually exclusive expression of the two PcG complexes. We conclude that the two PcG complexes are expressed throughout GC development, and that the fine composition of each complex is determined by the differentiation status of the cell. In addition, a subset of dividing cells with a centrocyte CD marker profile was identified that co-expresses core components of the PRC1 and PRC2 complex. We propose that these cells reflect a transitional stage between resting and dividing follicular B lymphocytes, and that they possibly represent the healthy precursors of nodal large B cell lymphomas.     

Pituitary tumor transforming gene PTTG2 induces psoriasis by regulating vimentin and E-cadherin expression

Psoriasis is a common and intractable skin disease affecting the physical and mental health of patients. This study focused on the roles of pituitary tumor transforming gene 2 (PTTG2) in psoriasis. Using real-time quantitative PCR and western blot, the expression patterns of PTTG2 were compared in psoriatic epidermis cells and normal cells, from both mRNA levels and protein levels. Knockdown of PTTG2 by siRNA was conducted in HaCaT cells to investigate the changes in cell viability and migration in vitro. Expression changes of vimentin and E-cadherin were also detected in the transfected cells. Results showed PTTG2 was significantly overexpressed in the psoriatic epidermis cells (P < 0.05). The cell viability and migration were inhibited by the knockdown of PTTG2. Besides, knockdown of PTTG2 resulted in down-regulation of vimentin and up-regulation of E-cadherin, with significant differences compared to the siRNA control group (P < 0.05). This study indicated the involvement of PTTG2 in mediating epidermis cell viability and migration and in pathogenesis of psoriasis. PTTG2 might be a potential therapeutic target for psoriasis through inducing epithelial-to-mesenchymal transition (EMT) via regulating the expression of vimentin and E-cadherin.     

启动子DNA甲基化调节胃癌中PDX1基因表达

目的明确PDXl启动子DNA甲基化,探讨启动子甲基化对PDXl在胃癌中表达的调节作用。方法收集3例胃癌活检组织,免疫组化检测PDXl蛋白表达：吉西他滨处理3株胃癌细胞,RT—PCR检测不同药物剂量和作用时间下PDXlmRNA表达;构建PDXl报告基因,检测启动子活性及吉西他滨处理前后启动子活性的变化;甲基化特异性PCR（MSP）检测3株胃癌细胞和8对配对胃癌组织中PDXl启动子甲基化状态。结果免疫组化结果显示胃癌中PDXl表达低于正常胃黏膜;RT—PCR显示吉西他滨使PDXlmRNA重获表达,且随剂量和时间依赖性。F383有最强启动子活性,吉西他滨显著增加了PDXl启动子活性（P〈0．05）。F383在AGS、BCG823、SGC7901中呈DNA完全甲基化状态;87．5％的胃癌组织出现F383部分甲基化,12．5％出现完全甲基化。癌旁正常组织仅有37．5％出现F383部分甲基化,未出现完全甲基化,两者比较有显著性差异（P〈0．05）。结论PDXl启动子存在DNA高甲基化,抑制了胃癌中PDXl的表达。     

A PGE2-MEF2A axis enables context-dependent control of inflammatory gene expression

1. Download : Download high-res image (209KB)
2. Download : Download full-size image