Valproate induces replication-independent active DNA demethylation

In this report, we demonstrate that valproic acid (VPA), a drug that has been used for decades in the treatment of epilepsy and as a mood stabilizer, triggers replication-independent active demethylation of DNA. Thus, this drug can potentially reverse DNA methylation patterns and erase stable methylation imprints on DNA in non-dividing cells. Recent discoveries support a role for VPA in the regulation of methylated genes; however, the mechanism has been unclear because it is difficult to dissociate active demethylation from the absence of DNA methylation during DNA synthesis. We therefore took advantage of an assay that measures active DNA demethylation independently from other DNA methylation and DNA replication activities in human embryonal kidney 293 cells. We show that VPA induces histone acetylation, DNA demethylation, and expression of an ectopically methylated CMV-GFP plasmid in a dose-dependent manner. In contrast, valpromide, an analogue of VPA that does not induce histone acetylation, does not induce demethylation or expression of CMV-GFP. Furthermore, we illustrate that methylated DNA-binding protein 2/DNA demethylase (MBD2/dMTase) participates in this reaction since antisense knockdown of MBD2/dMTase attenuates VPA-induced demethylation. Taken together, our data support a new mechanism of action for VPA as enhancing intracellular demethylase activity through its effects on histone acetylation and raises the possibility that DNA methylation is reversible independent of DNA replication by commonly prescribed drugs.     

Virion protein 16 induces demethylation of DNA integrated within chromatin in a novel mammalian cell model

DNA methylation and demethylation play important roles in mediating epigenetic regulation. So far, the mechanism of DNA demethylation remains elusive and controversial. Here, we constructed a plasmid, named with pCBS-luc, that contained an artificial CpG island, eight Gal4 DNA-binding domain binding site, an SV40 promoter, and a firefly luciferase reporter gene. The linearized pCBS-luc plasmid was methylated in vitro by DNA methyltransferase, and transfected into the HEK293 cells. The stable HEK293 transfectants with methylated pCBS-luc (me-pCBS-luc) were selected and obtained. The methylation status of the selected stable cell lines were confirmed by bisulfite sequencing polymerase chain reaction amplification. The methylation status could be maintained even after 15 passages. The virion protein 16 (VP16) was reported to enhance DNA demethylation around its binding sites of the promoter region in Xenopus fertilized eggs. Using our me-pCBS-luc model, we found that VP16 also had the ability to activate the expression of methylated luciferase reporter gene and induce DNA demethylation in chromatin DNA in mammalian cells. Altogether, we constructed a cell model stably integrated with the me-pCBS-luc reporter plasmid, and in this model we found that VP16 could lead to DNA demethylation. We believe that this cell model will have many potential applications in the future research on DNA demethylation and dynamic process of chromatin modification.     

The methyl donor S-Adenosylmethionine inhibits active demethylation of DNA: a candidate novel mechanism for the pharmacological effects of S-Adenosylmethionine

S-Adenosylmethionine (AdoMet) is the methyl donor of numerous methylation reactions. The current model is that an increased concentration of AdoMet stimulates DNA methyltransferase reactions, triggering hypermethylation and protecting the genome against global hypomethylation, a hallmark of cancer. Using an assay of active demethylation in HEK 293 cells, we show that AdoMet inhibits active demethylation and expression of an ectopically methylated CMV-GFP (green fluorescent protein) plasmid in a dose-dependent manner. The inhibition of GFP expression is specific to methylated GFP; AdoMet does not inhibit an identical but unmethylated CMV-GFP plasmid. S-Adenosylhomocysteine (AdoHcy), the product of methyltransferase reactions utilizing AdoMet does not inhibit demethylation or expression of CMV-GFP. In vitro, AdoMet but not AdoHcy inhibits methylated DNA-binding protein 2/DNA demethylase as well as endogenous demethylase activity extracted from HEK 293, suggesting that AdoMet directly inhibits demethylase activity, and that the methyl residue on AdoMet is required for its interaction with demethylase. Taken together, our data support an alternative mechanism of action for AdoMet as an inhibitor of intracellular demethylase activity, which results in hypermethylation of DNA.     

Role of human ribosomal RNA (rRNA) promoter methylation and of methyl-CpG-binding protein MBD2 in the suppression of rRNA gene expression

The methylation status of the CpG island located within the ribosomal RNA (rRNA) promoter in human hepatocellular carcinomas and pair-matched liver tissues was analyzed by bisulfite genomic sequencing. Significant hypomethylation of methyl-CpGs in the rRNA promoter was observed in the tumor samples compared with matching normal tissues, which was consistent with the relatively high level of rRNA synthesis in rapidly proliferating tumors. To study the effect of CpG methylation on RNA polymerase I (pol I)-transcribed rRNA genes, we constructed pHrD-IRES-Luc (human rRNA promoter-luciferase reporter). In this plasmid, Kozak sequence of the pGL3-basic vector was replaced by the internal ribosome entry site (IRES) of encephalomyocarditis viral genome to optimize pol I-driven reporter gene expression. Transfection of this plasmid into HepG2 (human) cells revealed reduced pol I-driven luciferase activity with an increase in methylation density at the promoter. Markedly reduced luciferase activity in Hepa (mouse) cells compared with HepG2 (human) cells showed that pHrD-IRES-Luc is transcribed by pol I. Site-specific methylation of human rRNA promoter demonstrated that methylation of CpG at the complementary strands located in the promoter (-9, -102, -347 with respect to the +1 site) inhibited luciferase activity, whereas symmetrical methylation of a CpG in the transcribed region (+152) did not affect the promoter activity. Immunofluorescence studies showed that the methyl-CpG-binding proteins, MBD1, MBD2, MBD3, and MeCP2, are localized both in the nuclei and nucleoli of HepG2 cells. Transient overexpression of MBD2 suppressed luciferase activity specifically from the methylated rRNA promoter, whereas MBD1 and MBD3 inhibited rRNA promoter activity irrespective of the methylation status. Chromatin immunoprecipitation analysis confirmed predominant association of MBD2 with the endogenous methylated rRNA promoter, which suggests a selective role for MBD2 in the methylation-mediated inhibition of ribosomal RNA gene expression.     

FLT-1启动子在血管内皮细胞中特异生物活性的检测及其意义

目的:构建带有组织特异性FLT-1启动子的真核表达载体,检测其在转染的人脐静脉内皮细胞（HUVEC）中对荧光素酶报告基因表达的驱动能力。方法：采用PCR扩增FLT-1启动子,插入到pGL3-Basic-luc载体中,构建携带FLT-1启动子的真核表达载体pGL3-FLT-Basic-luc,经脂质体法转染HUVEC、HepG2、NIH3T3和HEK293细胞,于转染48h后采用双荧光报告系统检测荧光素酶表达活性。 结果:酶切及测序证实构建的pGL3-FLT-Basic-luc载体中含有序列正确的FLT-1基因启动子,双荧光报告系统检测显示,转染的HUVEC细胞其荧光素酶活性明显高于HEK293细胞（P<0.01）,而转染的HepG2和NIH3T3细胞中未检测出荧光素酶表达。结论:克隆的FLT-1启动子具有较高的血管内皮特异性转录活性,可作为血管疾病靶向基因治疗的启动子来源。图     

Intracellular mechanisms of gonadotropin-stimulated gene expression in granulosa cells.

Previous studies have shown that the gonadotropins follicle-stimulating hormone and luteinizing hormone stimulate proopiomelanocortin (POMC) promoter activity and mRNA levels in ovarian granulosa cells. The objective of these studies was to determine the role of cAMP-dependent protein kinases (pKA) in gonadotropin-stimulated gene expression. Primary cultures of rat granulosa cells were transfected with a gene construct consisting of the POMC promoter (-150 to +63; designated pOMC-CAT) fused to the chloramphenicol acetyltransferase (CAT) reporter gene either alone or cotransfected with an expression plasmid (designated mutant RI), which overexpresses a mutant form of the murine RI subunit incapable of binding cAMP and serving as an irreversible inhibitor of the catalytic subunit of pKA. Follicle-stimulating hormone or isoproterenol caused a significant stimulation of pOMC-CAT activity in transfected cells. Cotransfection of pOMC-CAT with mutant RI caused a significant inhibition of basal pOMC-CAT activity and abolished the gonadotropin stimulation. As a control, transfection of the SV-40 viral enhancer-promoter fused to CAT (pSV2-CAT) was unresponsive to follicle-stimulating hormone stimulation and cotransfection with mutant RI had no significant effect on pSV2-CAT activity. These studies suggest that gonadotropin regulation of the POMC promoter is mediated by pKA and that promoter activity is stringently controlled by pKA.     

解偶联蛋白UCP1启动子荧光素酶报告基因载体的构建

目的:构建解偶联蛋白UCP1启动子荧光素酶报告基因载体,为寻找调控UCP1表达的小分子化合物提供有效工具。方法:从小鼠基因组DNA中PCR扩增小鼠UCP1启动子上游2000 bp序列,并将该序列连接到荧光素酶报告基因载体p GL3-basic中,构建p GL3-UCP1启动子。测序正确后,提取质粒,然后将上述载体与p RL-TK载体共转染至HEK293细胞、小鼠白色脂肪前体细胞和小鼠棕色脂肪前体细胞,48 h后裂解细胞检测荧光素酶的活性。结果:通过PCR成功扩增获得了目的片段,并将其克隆至p GL3-basic中。与细胞内源UCP1表达水平相似,荧光素酶报告系统表明构建的p GL3-UCP1在棕色脂肪细胞中启动子活性最高,在白色脂肪细胞中活性较低,在HEK293细胞中基本没有活性。同时β3肾上腺素受体激动剂CL 316,243同样能够上调p GL3-UCP1的启动子活性。结论:成功构建了小鼠UCP1启动子荧光素酶报告基因载体,并证明在棕色脂肪细胞中,该启动子具有很强的启动子活性,而在白色脂肪和HEK293细胞中,启动子活性很低。该启动子报告系统有望为寻找激活UCP1的小分子化合物提供重要平台。     

Stable Expression of a Foreign Gene, Delivered by Gene Gun, in the Muscle of Rainbow Trout Oncorhynchus mykiss

We report the efficient delivery of a foreign gene into muscle of rainbow trout Oncorhynchus mykiss with a gene gun. The foreign gene was a reporter gene, chloramphenicol acetyltransferase (CAT). Two CAT-containing plasmids were used: pCMV-CAT, which contains cytomegalovirus immediate early promoter, and pSV2-CAT, which contains the simian virus 40 early promoter. All plasmids were introduced by particle bombardment using a gene gun. During the 90-day sampling period following bombardment, CAT was strongly and stably expressed in the muscle of all the fish bombarded with pCMV-CAT and pSV2-CAT. No CAT expression was detected in the blood samples until 90 days after introduction, when it was found in only one fish from the pCMV-CAT group and one from the pSV2-CAT group. The stable and long-term expression of plasmid DNA in muscle makes muscle an attractive target tissue for the introduction of viral DNA for the purpose of DNA vaccination. Received June 5, 1999; accepted November 2, 1999.     

Use of the human elongation factor 1 alpha promoter as a versatile and efficient expression system

We have characterized the promoter region of the human elongation factor 1 alpha-encoding gene (EF-1 alpha) and developed a versatile expression system which has a wide host range and a high efficiency of gene expression. To identify the promoter region of the EF-1 alpha gene necessary for efficient gene expression, we constructed four pEF-CAT plasmids that have the bacterial cat gene fused to four different sites of the human EF-1 alpha gene: (i) ligated to the end of the TATA box (pEF220-CAT); (ii) ligated in exon 1 (pEF204-CAT and pEF233-CAT), and (iii) ligated in exon 2 (pEF321-CAT). All the pEF-CAT plasmids were highly expressed in all the cell types tested, including fibroblasts and lymphoid cells. Plasmid pEF321-CAT, which contains the first exon and the first intron, gave the highest level of cat expression. Plasmids pEF204- and pEF233-CAT, which contain part of the first exon but do not contain the first intron, were less efficient in cat expression than was pEF321-CAT. Plasmid pEF220-CAT, which lacks both the first exon and the first intron, was the least efficient. Plasmid pEF321-CAT was several- to 100-fold more efficient in cat expression than plasmid pSV2-CAT depending on the recipient cell types. The promoter of pEF321 plasmid also directed the stable expression of the bacterial neo gene more efficiently than the promoter of the simian virus 40 (SV40) early gene or the long terminal repeat of Rous sarcoma virus. Using this system, the SV40 early gene and the cDNA encoding human CD4 were also expressed efficiently.     

On the mechanism of demethylation of 5-methylcytosine in DNA

DNA methylation is an important biological process that programmes gene expression in vertebrates. The methylation pattern is generated by a combination of methylation and demethylation reactions catalyzed by DNA methyltransferases and putative demethylases. MBD2 binds methylated DNA and possesses DNA demethylase activity. We use here direct analysis of the reaction mixture by GC-MS using a water-tolerant gas chromatographic column to avoid the loss of potential volatile products and identify the leaving residue of the demethylation reaction. We show that the DNA demethylase reaction catalyzed by a recombinant human MBD2 purified from SF9 insect cells releases dideuteroformaldehyde from [Me-(2)H(3)]-5-methylcytosine in DNA. A mechanism of the DNA demethylation reaction is proposed based on this observation.     

牙鲆甲状腺激素受体TRαA介导甲状腺激素调控的靶基因鉴定

为了鉴定牙鲆甲状腺激素受体TRs介导甲状腺激素调控的靶基因, 研究采用RT-PCR克隆了TRαA基因的CDS区, 并构建了p3×Flag-TRαA重组真核表达载体;该重组质粒转染HEK293T细胞后, RT-PCR、实时定量PCR与Western blot检测均表明牙鲆TRαA在哺乳动物蛋白表达系统中成功转录并翻译;且重组质粒转染的细胞裂解液通过G1亲和层析柱纯化、过滤除菌可得到纯的融合蛋白3×Flag-TRαA, 然后双荧光素酶报告实验通过在HEK293T细胞中共转染p3×Flag-TRαA和含候选靶启动子的报告基因表达载体pGL3-Pro-atoh8-1517/1333/708, 表明TRαA受体结合在atoh8基因启动子区–1497— –688特异的2个TRE识别序列来调控该基因的转录, 即atoh8是TRαA介导甲状腺激素直接调控的靶基因。研究为深入探究甲状腺激素受体TRαA介导甲状腺激素调控的信号通路提供了基础依据。     

小鼠组蛋白H3K4甲基化酶Smyd3启动子荧光素酶报告载体的构建与活性分析

为研究小鼠(Mus musculus)组蛋白H3 K4甲基化酶基因Smyd3转录调控的分子机制,本研究首先通过PCR的方法克隆了5条不同长度的Smyd3启动子5’端缺失片段,与pMD19-T载体连接后,双酶切克隆入pGL3-Basic荧光素酶报告基因载体,构建Smyd3启动子-pGL3-Basic报告基因重组质粒,瞬时转染HEK293细胞48 h后采用双报告基因检测试剂盒检测Smyd3启动子各缺失片段的相对荧光活性.结果表明,本研究成功构建Smyd3启动子5’端缺失片段-pGL3-Basic荧光报告基因重组质粒,所构建的启动子重组子转染组与阳性对照组相比表现出荧光活性,并且pGL3-Smyd3-4的荧光活性最强,是其他的2至4倍左右,pGL3-Smyd3-5的荧光活性最弱.本研究初步确定Smyd3基因的启动子核心区域可能位于-533～-42bp之间,在-2026～-533 bp之间可能存在启动子负调控序列.