Hypermethylation of the human telomerase catalytic subunit (hTERT) gene correlates with telomerase activity

DNA methylation is an epigenetic process involved in embryonic development, differentiation and aging. It is 1 of the mechanisms resulting in gene silencing in carcinogenesis, especially in tumor suppressor genes (e.g., p16, Rb). Telomerase, the DNA polymerase adding TTAGGG repeats to the chromosome end, is involved in the regulation of the replicative life span by maintaining telomere length. This enzyme is activated in germ and stem cells, repressed in normal somatic cells and reactivated in a large majority of tumor cells. The promoter region of the hTERT gene, encoding for the catalytic subunit of human telomerase, has been located in a CpG island and may therefore be regulated at least in part by DNA methylation. We analyzed the methylation status of 27 CpG sites within the hTERT promoter core region by methylation-sensitive single-strand conformation analysis (MS-SSCA) and direct sequencing using bisulfite-modified DNA in 56 human tumor cell lines, as well as tumor and normal tissues from different organs. A positive correlation was observed among hypermethylation of the hTERT promoter, hTERT mRNA expression and telomerase activity (p < 0.00001). Furthermore, this correlation was confirmed in normal tissues where hypermethylation of the hTERT promoter was found exclusively in hTERT-expressing telomerase-positive samples and was absent in telomerase-negative samples (p < 0.00002). Since tumor tissues contain also nonneoplastic stromal elements, we performed microdissection to allow confirmation that the hTERT promoter methylation truly occurred in tumor cells. Our results suggest that methylation may be involved in the regulation of hTERT gene expression. To our knowledge, this is the first gene in which methylation of its promoter sequence has been found to be positively correlated with gene expression.     

人端粒酶催化亚单位(hTERT)启动子的克隆及其在人肺癌细胞中转录活性的研究

目的克隆人端粒酶催化亚单位(hTERT)的启动子，并检测它在多种人肺癌细胞株和人胚肺成纤维细胞株中的转录活性，为肺癌靶向性基因治疗的研究奠定基础。方法以人胚肾293细胞基因组DNA为模板，应用PCR方法克隆hTERT 5’端上游旁侧序列长约1．1kh的启动子片段，经DNA测序无误后克隆人荧光素酶报告质粒pGL3-Basic的荧光素酶基因上游，构建pGL3-hTER Tp重组质粒，用脂质体法瞬时转染人肺癌细胞株A549、SPC-A-1、LTEPa-2、NCI—H446、YTMLC、GLC-82、A2，以及人胚肺成纤维细胞株MRC5，转染48h后检测hTERT启动子在各细胞株中的转录活性。结果琼脂糖凝胶电泳显示PCR克隆的hTERT启动子片段长约1.1kb。DNA测序结果与GenBank中hTERT启动子DNA序列完全一致，其5’端和3’端分别位于hTERT基因转录起始位点上游1126bp和43bp，片段长度为1084bp。采用双酶切和PCR两种方法鉴定pGL3-hTERTp重组质粒，均显示构建成功。瞬时转染及荧光素酶活性检测实验显示，hTERT启动子在所检测的肺癌细胞株中均有高低不同的转录活性，而在MRC-5细胞株中无转录活性。结论该实验克隆的1084bp大小的hTERT启动子在多种肺癌细胞株中均有转录活性，在人胚肺成纤维细胞中无转录活性。hTERT启动子有可能作为调控元件用于肿瘤靶向性基因治疗。     

Repression of the telomerase catalytic subunit by a gene on human chromosome 3 that induces cellular senescence

The cellular senescence program is controlled by multiple genetic pathways, one of which involves the regulation of telomerase and telomere shortening. The introduction of a normal human chromosome 3 into the human renal cell carcinoma cell line RCC23 caused repression of telomerase activity, progressive shortening of telomeres, and restoration of the cellular senescence program. We attributed the repression of telomerase activity to the marked downregulation of the gene encoding the catalytic subunit of telomerase (hEST2/hTRT) but not another protein component (TP1/TLP1) or the RNA component of telomerase. These results suggest that a senescence-inducing gene on chromosome 3 controls hEST2/hTRT gene expression either directly or indirectly and support the notion that hEST2/hTRT is the major determinant of telomerase enzymatic activity in human cells. Mol. Carcinog. 22:65–72, 1998. © 1998 Wiley-Liss, Inc. This article is a US Government work and, as such, is in the public domain in the United States of America.     

非小细胞肺癌端粒酶活性与端粒酶RNA、端粒酶催化亚单位的相关性及其意义

目的 探讨非小细胞肺癌（non-small cell lung cancer,NSCLC)端粒酶活性（telomerase activity,TA)、端粒酶RNA（telomerase RNA,hTR）端粒酶催化亚单位（telomerase catalytic subunit,hTRT/hEST2)编码基因的表达,彼此间相关性及其预后意义。方法 TRAP－PCR（telomerase repeat amplification protocol PCR)检测TA,RT－PCR检测hTR,原位杂交检测hTRT/hEST2mRNA表达。结果 非小细胞肺癌TA、hTR和hTRT/hEST2 mRNA阳性率分别为68．4％（26／38）、55．2％（32／58）和74．1％（43／58）,TA与hTRT/hEST2阳性相关（r=0.84,P=0.01),与hTR无相关性（r=0.16,P=0.23）。低分化以及有淋巴结或远处转移者TA及hTRT/hEST2阳性率增高;TA及hTRT/hEST2阳性组中位生存期（10．4个月,7．5个月）低于阴性组（13．5个月,14．7个月;P＝0．074,P＝0．01）,hTR阳性组中位生存期（12．9个月）略高于阴性组（11．5个月,P＝0．23）,仅hTRT/hEST2阳性与阴性组生存期差异有显著性;多因素Cox回归分析hTRT/hEST2对生存期有影响。结论 非小细胞肺癌TA、hEST2阳性与阴性组生存期差异有显著性;多因素Cox回归分析hTRT/hEST2对生存期有影响。结论 非小细胞肺癌TA、hTR和hTRT/hEST2表达高于癌旁正常组织;hTRT/hEST2够能反应TA活性,二者为NSCLC恶性表型增高的标志;hTRT/hEST2具有独立的预后意义。     

榄香烯对HeLa细胞端粒酶催化亚单位基因表达的作用

目的：探讨HeLa细胞凋亡过程中,人乳头瘤病毒18型E6（HPV18-E6）基因、p53基因、端粒酶催化亚单位（hTERT)基因表达的变化,为临床诊断、治疗宫颈癌提供新思路,方法：用流式细胞仪、电镜证实细胞凋亡;用PCR和RT-PCR方法研究HPV18-E6基因、p53基因、hTERT基因表达在细胞凋亡过程中的变化及相互关系。结果：在榄香烯作用下,HPV18-E6基因、p53基因表达没有变化,但hTERT基因表达受到抑制。结论：榄香烯诱导HeLa细胞凋亡过程中的变化及相互关系。结果：在榄香烯作用下,HPV18-E6基因、p53基因表达没有变化,但hTERT基因表达受到抑制。结论：榄香烯诱导HeLa细胞凋亡过程中,hTERT基因表达受到明显抑制,对于耐药性肿瘤,端粒酶活的抑制可能代表一种新的化疗策略。     

A nuclear factor is required for transactivation of HTLV-I gene expression

Human T-cell Leukemia Virus (HTLV-I) gene expression is regulated by both constitutive and inducible enhancer elements. The viral components required for inducible enhancer function include the cis-acting 21 bp motifs present three times within the U3 region of the LTR and the trans-acting 40 kD tax protein encoded by the 3' end of the viral genome. A gel electrophoresis protein-DNA binding assay was used to identify the interaction of nuclear factors with the 21 bp element. Specific protein-DNA interactions were observed in all cell lines examined. No differences were observed in the binding patterns obtained from extracts prepared from those cell lines that did or did not express the tax protein. Dimethyl sulfate (DMS) interference analysis indicates that a nuclear factor contacts the guanine residues in the sequence GGCGTTGAGG. Oligonucleotide directed mutagenesis of the putative protein-DNA contact points markedly reduces the transactivation response. These results support an indirect mechanism for transactivation whereby regulation of HTLV-I gene expression is controlled by the interaction of the cis-acting 21 bp motif with a nuclear factor activated by tax.     

Interaction between Sp1 and cell cycle regulatory proteins is important in transactivation of a differentiation-related gene

The stratified squamous epithelium is a model system in which to define molecular mechanisms underlying the switch from proliferation to differentiation. This can be achieved through the functional dissection of keratin gene promoters. Having previously established the importance of keratin 4 in maintaining the differentiated phenotype in corneal epithelial cells, we investigated the role of Sp1-mediated transactivation of the keratin 4 promoter given the role of Sp1 in differentiation and cell cycle progression. Sp1 transactivation of the keratin 4 promoter was diminished in cyclin D1-overexpressing cells, which may be mediated through a newly described direct interaction between Sp1 and cyclin D1 and opposed by a complex between Sp1 and pRB.     

Vitamin D3 receptor/Sp1 complex is required for the induction of p27Kip1 expression by vitamin D3

1alpha,25-dihydroxyvitamin D3 (vitamin D3) has been shown to upregulate p27Kip1 expression via Sp1 and NF-Y binding sites in the p27Kip1 promoter. However, whether vitamin D3 receptor (VDR) involves in this process is unclear. In this study, we demonstrated that expression of VDR in SW620 cells, which exhibited low level of endogenous VDR, increased vitamin D3-stimulated p27Kip1 promoter activity. On the contrary, suppression of Sp1 expression by small interference RNA reduced the stimulation of p27Kip1 promoter activity by vitamin D3 in LNCaP cells. DNA affinity precipitation assay and chromatin immunoprecipitation assay showed that VDR bound to the p27Kip1 promoter in vitro and in vivo. In addition, we also demonstrated that VDR interacted with Sp1 in vitro and in cells. Collectively, our results suggest that VDR is involved in the induction of p27Kip1 by vitamin D3 and may interact with Sp1 to modulate the expression of target genes that lack VDR response element (VDRE) in their promoters.