Downstream E-box-mediated regulation of the human telomerase reverse transcriptase (hTERT) gene transcription: evidence for an endogenous mechanism of transcriptional repression

Fabricating niosomal-PEG nanoparticles co-loaded with metformin and silibinin for effective treatment of human lung cancer cells

Background

Despite current therapies, lung cancer remains a global issue and requires the creation of novel treatment methods. Recent research has shown that biguanides such as metformin (MET) and silibinin (SIL) have a potential anticancer effect. As a consequence, the effectiveness of MET and SIL in combination against lung cancer cells was investigated in this study to develop an effective and novel treatment method.

Methods

Niosomal nanoparticles were synthesized via the thin-film hydration method, and field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR), atomic force microscopy (AFM), and dynamic light scattering (DLS) techniques were used to evaluate their physico-chemical characteristics. The cytotoxic effects of free and drug-loaded nanoparticles (NPs), as well as their combination, on A549 cells were assessed using the MTT assay. An apoptosis test was used while under the influence of medication to identify the molecular mechanisms behind programmed cell death. With the use of a cell cycle test, it was determined whether pharmaceutical effects caused the cell cycle to stop progressing. Additionally, the qRT-PCR technique was used to evaluate the levels of hTERT, BAX, and BCL-2 gene expression after 48-h medication treatment.

Results

In the cytotoxicity assay, the growth of A549 lung cancer cells was inhibited by both MET and SIL. Compared to the individual therapies, the combination of MET and SIL dramatically and synergistically decreased the IC50 values of MET and SIL in lung cancer cells. Furthermore, the combination of MET and SIL produced lower IC50 values and a better anti-proliferative effect on A549 lung cancer cells. Real-time PCR results showed that the expression levels of hTERT and BCL-2 were significantly reduced in lung cancer cell lines treated with MET and SIL compared to single treatments (p< 0.001).

Conclusion

It is anticipated that the use of nano-niosomal-formed MET and SIL would improve lung cancer treatment outcomes and improve the therapeutic efficiency of lung cancer cells.

TERT promoter mutations and telomeres during tumorigenesis

Telomerase regulation and telomere shortening act as a strong tumor suppressor mechanism in human somatic cells. Point mutations in the promoter of telomerase reverse transcriptase (TERT) are the most frequent non-coding mutation in cancer. These TERT promoter mutations (TPMs) create de novo ETS factor binding sites upstream of the start codon of the gene, which can be bound by different ETS factors. TPMs can occur early during tumorigenesis and are thought to be among the first mutations in melanoma, glioblastoma and hepatocellular carcinoma. Despite their association with increased TERT levels, TPMs do not prohibit telomere shortening and TPM-harboring cancers present with short telomeres. Their short telomere length combined with their high prevalence and specificity for cancer makes TPMs an attractive target for future therapeutic exploitation of telomerase inhibition and telomere deprotection-induced cell death.

Timing the Landmark Events in the Evolution of Clear Cell Renal Cell Cancer: TRACERx Renal

Clear cell renal cell carcinoma (ccRCC) is characterized by near-universal loss of the short arm of chromosome 3, deleting several tumor suppressor genes. We analyzed whole genomes from 95 biopsies across 33 patients with clear cell renal cell carcinoma. We find hotspots of point mutations in the 5' UTR of TERT, targeting a MYC-MAX-MAD1 repressor associated with telomere lengthening. The most common structural abnormality generates simultaneous 3p loss and 5q gain (36% patients), typically through chromothripsis. This event occurs in childhood or adolescence, generally as the initiating event that precedes emergence of the tumor's most recent common ancestor by years to decades. Similar genomic changes drive inherited ccRCC. Modeling differences in age incidence between inherited and sporadic cancers suggests that the number of cells with 3p loss capable of initiating sporadic tumors is no more than a few hundred. Early development of ccRCC follows well-defined evolutionary trajectories, offering opportunity for early intervention.

TIP60 represses telomerase expression by inhibiting Sp1 binding to the TERT promoter

HIV1-TAT interactive protein (TIP60) is a haploinsufficient tumor suppressor. However, the potential mechanisms endowing its tumor suppressor ability remain incompletely understood. It plays a vital role in virus-induced cancers where TIP60 down-regulates the expression of human papillomavirus (HPV) oncoprotein E6 which in turn destabilizes TIP60. This intrigued us to identify the role of TIP60, in the context of a viral infection, where it is targeted by oncoproteins. Through an array of molecular biology techniques such as Chromatin immunoprecipitation, expression analysis and mass spectrometry, we establish the hitherto unknown role of TIP60 in repressing the expression of the catalytic subunit of the human telomerase complex, TERT, a key driver for immortalization. TIP60 acetylates Sp1 at K639, thus inhibiting Sp1 binding to the TERT promoter. We identified that TIP60-mediated growth suppression of HPV-induced cervical cancer is mediated in part due to TERT repression through Sp1 acetylation. In summary, our study has identified a novel substrate for TIP60 catalytic activity and a unique repressive mechanism acting at the TERT promoter in virus-induced malignancies.

Telomerase Induction in HPV Infection and Oncogenesis

Telomerase extends the repetitive DNA at the ends of linear chromosomes, and it is normally active in stem cells. When expressed in somatic diploid cells, it can lead to cellular immortalization. Human papillomaviruses (HPVs) are associated with and high-risk for cancer activate telomerase through the catalytic subunit of telomerase, human telomerase reverse transcriptase (hTERT). The expression of hTERT is affected by both high-risk HPVs, E6 and E7. Seminal studies over the last two decades have identified the transcriptional, epigenetic, and post-transcriptional roles high-risk E6 and E7 have in telomerase induction. This review will summarize these findings during infection and highlight the importance of telomerase activation as an oncogenic pathway in HPV-associated cancer development and progression.

Interplay between cancer cell cycle and metabolism: Challenges, targets and therapeutic opportunities

A growing interest has emerged in the field of studying the cross-talk between cancer cell cycle and metabolism. In this review, we aimed to present how metabolism and cell cycle are correlated and how cancer cells get energy to drive cell cycle. Cell proliferation and cell death largely depend on the metabolic activity of the cell. Cell cycle proteins, e.g. cyclin D, cyclin dependent kinase (CDK), some pro-apoptotic and anti-apoptotic proteins, and P53 have been shown to be regulated by metabolic crosstalk. Dysregulation of this cross-talk between metabolism and cell cycle leads to degenerative disorder(s) and cancer. It is not fully understood the actual reason of aberration between metabolism and cell cycle, but it is a hallmark of cancer research. Herein, we discussed the role of some regulatory molecules relative of cell cycle and metabolism and highlight how they control the function of each other. We also pointed out, current therapeutic opportunities and some additional crucial therapeutic targets on these fields that could be a breakthrough in cancer research.

MR molecular imaging of tumors based on an optimal hTERT promoter tyrosinase expression system

The early diagnosis and treatment of tumors is of vital significance to increase patient survival. Therefore, we constructed a lentiviral vector expressing tyrosinase (TYR) driven by an optimized human telomerase reverse transcriptase (hTERT) promoter or a cytomegalovirus(CMV) promoter in the hopes of performing noninvasive and real-time tumor-specific imaging. First, hTERT-TYR and CMV-TYR were constructed to infect cancer cell lines (telomerase-negative cell line: U2OS; telomerase-positive cell lines: SGC-7901, SW480 and HepG2). Subsequently, stable tyrosinase-expressing cell lines were sorted by flow cytometry out of these infected cancer cell lines. Then, the mRNA and protein levels of tyrosinase were analyzed. Thetyrosinase activity, melanin production and ferric ion adsorption were measured followed by an MR scan. Consequently the results showed that tyrosinase was only expressed in telomerase-positive tumor cells infected by hTERT-TYR, whereas tyrosinase was expressed in both telomerase-negative and telomerase-positive tumor cells infected by CMV-TYR. Finally, we performed in vivo tumor MR using a clinical 3T MR scanner and found increased signals at T1W1 from telomerase-positive cells infected by hTERT-TYR, which revealed that MR scanning could distinguish cells with hTERT -positive cells from hTERT-negative cells infected with the optimized lentivirus. The mechanism underlying this effect is that tyrosinase promotes melanin production and ferric ion adsorption only in hTERT-expressing cells. Taken together, these data show that this optimized hTERT promoter-driving tyrosinase expression system might be a useful diagnostic tool for the detection of tumors using MR imaging.

Variations in telomere maintenance and the role of telomerase inhibition in gastrointestinal cancer

Immortalization is an important step toward the malignant transformation of human cells and is critically dependent upon telomere maintenance. There are two known mechanisms to maintain human telomeres. The process of telomere maintenance is either mediated through activation of the enzyme telomerase or through an alternative mechanism of telomere lengthening called ALT. While 85% of all human tumors show reactivation of telomerase, the remaining 15% are able to maintain telomeres via ALT. The therapeutic potential of telomerase inhibitors is currently investigated in a variety of human cancers. Gastrointestinal tumors are highly dependent on telomerase as a mechanism of telomere maintenance, rendering telomeres as well as telomerase potential targets for cancer therapy. This article focuses on the molecular mechanisms of telomere biology and telomerase activation in gastrointestinal cancers and reviews strategies of telomerase inhibition and their potential therapeutic use in these tumor entities.

Human Telomerase Reverse Transcriptase (hTERT) Transcription Requires Sp1/Sp3 Binding to the Promoter and a Permissive Chromatin Environment

The transcription of human telomerase gene hTERT is regulated by transcription factors (TFs), including Sp1 family proteins, and its chromatin environment. To understand its regulation in a relevant chromatin context, we employed bacterial artificial chromosome reporters containing 160 kb of human genomic sequence containing the hTERT gene. Upon chromosomal integration, the bacterial artificial chromosomes recapitulated endogenous hTERT expression, contrary to transient reporters. Sp1/Sp3 expression did not correlate with hTERT promoter activity, and these TFs bound to the hTERT promoters in both telomerase-positive and telomerase-negative cells. Mutation of the proximal GC-box resulted in a dramatic decrease of hTERT promoter activity, and mutations of all five GC-boxes eliminated its transcriptional activity. Neither mutations of GC-boxes nor knockdown of endogenous Sp1 impacted promoter binding by other TFs, including E-box-binding proteins, and histone acetylation and trimethylation of histone H3K9 at the hTERT promoter in telomerase-positive and -negative cells. The result indicated that promoter binding by Sp1/Sp3 was essential, but not a limiting step, for hTERT transcription. hTERT transcription required a permissive chromatin environment. Importantly, our data also revealed different functions of GC-boxes and E-boxes in hTERT regulation; although GC-boxes were essential for promoter activity, factors bound to the E-boxes functioned to de-repress hTERT promoter.

Genetic mapping and comparative expression analysis of transcription factors in cotton

Transcription factors (TFs) play an important role in the regulation of plant growth and development. The study of the structure and function of TFs represents a research frontier in plant molecular biology. The findings of these studies will provide significant information regarding genetic improvement traits in crops. Currently, a large number of TFs have been cloned, and their function has been verified. However, relatively few studies that genetically map TFs in cotton are available. To genetically map TFs in cotton in this study, specific primers were designed for TF genes that were published in the Plant Transcription Factor Database. A total of 977 TF primers were obtained, and 31 TF polymorphic loci were mapped on 15 cotton chromosomes. These polymorphic loci were clearly preferentially distributed on chromosomes 5, 11, 19 and 20; and TFs from the same family mapped to homologous cotton chromosomes. In-silico mapping verified that many mapped TFs were mapped on their corresponding chromosomes or their homologous chromosomes’ corresponding chromosomes in the diploid genomes. QTL mapping for fiber quality revealed that TF-Ghi005602-2 mapped on Chr19 was associated with fiber length. Eighty-five TF genes were selected for RT-PCR analysis, and 4 TFs were selected for qRT-PCR analysis, revealing unique expression patterns across different stages of fiber development between the mapping parents. Our data offer an overview of the chromosomal distribution of TFs in cotton, and the comparative expression analysis between Gossypium hirsutum and G. barbadense provides a rough understanding of the regulation of TFs during cotton fiber development.

A Combinatory Strategy for Detection of Live CTCs Using Microfiltration and a New Telomerase-Selective Adenovirus

Circulating tumor cells (CTC) have become an important biomarker for early cancer diagnosis, prognosis, and treatment monitoring. Recently, a replication-competent recombinant adenovirus driven by a human telomerase gene (hTERT) promoter was shown to detect live CTCs in blood samples of patients with cancer. Here, we report a new class of adenoviruses containing regulatory elements that repress the hTERT gene in normal cells. Compared with the virus with only the hTERT core promoter, the new viruses showed better selectivity for replication in cancer cells than in normal cells. In particular, Ad5GTSe, containing three extra copies of a repressor element, displayed a superior tropism for cancer cells among leukocytes and was thus selected for CTC detection in blood samples. To further improve the efficiency and specificity of CTC identification, we tested a combinatory strategy of microfiltration enrichment using flexible micro spring arrays and Ad5GTSe imaging. Our experiments showed that this method efficiently detected both cancer cells spiked into healthy blood and potential CTCs in blood samples of patients with breast and pancreatic cancer, demonstrating its potential as a highly sensitive and reliable system for detection and capture of CTCs of different tumor types.

A co-culture genome-wide RNAi screen with mammary epithelial cells reveals transmembrane signals required for growth and differentiation

INTRODUCTION

The extracellular signals regulating mammary epithelial cell growth are of relevance to understanding the pathophysiology of mammary epithelia, yet they remain poorly characterized. In this study, we applied an unbiased approach to understanding the functional role of signalling molecules in several models of normal physiological growth and translated these results to the biological understanding of breast cancer subtypes.

METHODS

We developed and utilized a cytogenetically normal clonal line of hTERT immortalized human mammary epithelial cells in a fibroblast-enhanced co-culture assay to conduct a genome-wide small interfering RNA (siRNA) screen for evaluation of the functional effect of silencing each gene. Our selected endpoint was inhibition of growth. In rigorous postscreen validation processes, including quantitative RT-PCR, to ensure on-target silencing, deconvolution of pooled siRNAs and independent confirmation of effects with lentiviral short-hairpin RNA constructs, we identified a subset of genes required for mammary epithelial cell growth. Using three-dimensional Matrigel growth and differentiation assays and primary human mammary epithelial cell colony assays, we confirmed that these growth effects were not limited to the 184-hTERT cell line. We utilized the METABRIC dataset of 1,998 breast cancer patients to evaluate both the differential expression of these genes across breast cancer subtypes and their prognostic significance.

RESULTS

We identified 47 genes that are critically important for fibroblast-enhanced mammary epithelial cell growth. This group was enriched for several axonal guidance molecules and G protein-coupled receptors, as well as for the endothelin receptor PROCR. The majority of genes (43 of 47) identified in two dimensions were also required for three-dimensional growth, with HSD17B2, SNN and PROCR showing greater than tenfold reductions in acinar formation. Several genes, including PROCR and the neuronal pathfinding molecules EFNA4 and NTN1, were also required for proper differentiation and polarization in three-dimensional cultures. The 47 genes identified showed a significant nonrandom enrichment for differential expression among 10 molecular subtypes of breast cancer sampled from 1,998 patients. CD79A, SERPINH1, KCNJ5 and TMEM14C exhibited breast cancer subtype-independent overall survival differences.

CONCLUSION

Diverse transmembrane signals are required for mammary epithelial cell growth in two-dimensional and three-dimensional conditions. Strikingly, we define novel roles for axonal pathfinding receptors and ligands and the endothelin receptor in both growth and differentiation.

Insights into epigenetic regulation of microRNA-155 expression in multiple myeloma

CONTEXT

MiR-155 plays a critical role in the development of B-cell malignancies. Previous studies have shown a deregulation of miR-155 in specific cytogenetic subtypes of multiple myeloma (MM). However, the mechanisms that regulate miR-155 expression in MM are not fully understood.

OBJECTIVE

In the present study, we explored the regulation of miRNA-155 in MM by DNA methylation mechanisms and the impact of miR-155 expression in survival of MM patients.

METHOD

Primary samples were obtained from 95 patients with newly diagnosed myeloma. Methylation was analyzed by Methylation Specific PCR, sequencing of bisulfite treated DNA and luciferase assay.

RESULTS

qRT-PCR analysis revealed that miR-155 was differentially expressed in MM and its upregulation was associated with longer survival. DNA methylation of CpG island present in the first exon of miR-155 host gene was associated with its low expression in MM cell lines and patient samples. Our results showed for the first time that in vitro methylation of part of the promoter and first exon abrogated the miR-155 expression. We further showed that miR-155 expression in MM cell lines was increased by demethylating 5-aza-dC treatment and decreased by RNA-directed DNA methylation. Additionally, we found that LPS "immunological challenge" was insufficient to induce miR-155 expression in MM cell lines with methylated DNA around transcription start site (TSS).

CONCLUSION

This study provides evidence that DNA methylation contributes to miR-155 expression in myeloma cells. Interestingly, the survival data showed an association between miR-155 expression and outcome of MM.

Dual roles of c-Myc in the regulation of hTERT gene

Human telomerase gene hTERT is important for cancer and aging. hTERT promoter is regulated by multiple transcription factors (TFs) and its activity is dependent on the chromatin environment. However, it remains unsolved how the interplay between TFs and chromatin environment controls hTERT transcription. In this study, we employed the recombinase-mediated BAC targeting and BAC recombineering techniques to dissect the functions of two proximal E-box sites at -165 and +44 nt in regulating the hTERT promoter in the native genomic contexts. Our data showed that mutations of these sites abolished promoter binding by c-Myc/Max, USF1 and USF2, decreased hTERT promoter activity, and prevented its activation by overexpressed c-Myc. Upon inhibition of histone deacetylases, mutant and wildtype promoters were induced to the same level, indicating that the E-boxes functioned to de-repress the hTERT promoter and allowed its transcription in a repressive chromatin environment. Unexpectedly, knockdown of endogenous c-Myc/Max proteins activated hTERT promoter. This activation did not require the proximal E-boxes but was accompanied by increased promoter accessibility, as indicated by augmented active histone marks and binding of multiple TFs at the promoter. Our studies demonstrated that c-Myc/Max functioned in maintaining chromatin-dependent repression of the hTERT gene in addition to activating its promoter.

Poly (ADP-ribose) polymerase 3 (PARP3), a potential repressor of telomerase activity

BACKGROUND

Considering previous result in Non-Small Cell Lung Cancer (NSCLC), we investigated in human cancer cells the role of PARP3 in the regulation of telomerase activity.

METHODS

We selected A549 (lung adenocarcinoma cell line) and Saos-2 (osteosarcoma cell line), with high and low telomerase activity levels, respectively. The first one was transfected using a plasmid construction containing a PARP3 sequence, whereas the Saos-2 cells were submitted to shRNA transfection to get PARP3 depletion. PARP3 expression on both cell systems was evaluated by real-time quantitative PCR and PARP3 protein levels, by Western-blot. Telomerase activity was determined by TRAP assay.

RESULTS

In A549 cells, after PARP3 transient transfection, data obtained indicated that twenty-four hours after transfection, up to 100-fold increased gene expression levels were found in the transfected cells with pcDNA/GW-53/PARP3 in comparison to transfected cells with the empty vector. Moreover, 48 hours post-transfection, telomerase activity decreased around 33%, and around 27%, 96 hours post-transfection. Telomerase activity average ratio was 0.67 ± 0.05, and 0.73 ± 0.06, respectively, with significant differences. In Saos-2 cells, after shRNA-mediated PARP3 silencing, a 2.3-fold increase in telomerase activity was detected in relation to the control.

CONCLUSION

Our data indicated that, at least in some cancer cells, repression of PARP3 could be responsible for an increased telomerase activity, this fact contributing to telomere maintenance and, therefore, avoiding genome instability.

Chromatin dynamics at the hTERT promoter during transcriptional activation and repression by c-Myc and Mnt in Xenopus leavis oocytes

The transcription factors c-Myc and Mnt regulate gene expression through dimerization with Max and binding to E-boxes in target genes. While c-Myc activates gene expression via recruitment of histone modifying complexes, Mnt acts as a transcriptional repressor. Here, we used the Xenopus leavis oocyte system to address the effect of c-Myc and Mnt on transcription and chromatin remodeling over the E-box region in the human telomerase reverse transcriptase (hTERT) promoter. As expected we found elevated and decreased levels of hTERT transcription upon exogenously expressed c-Myc/Max and Mnt/Max, respectively. In addition, we confirmed binding of these heterodimers to both E-boxes already enriched with H3K9ac and H4K16ac. These chromatin marks were further enhanced upon c-Myc/Max binding followed by increased DNA accessibility in the E-box region. In contrast, Mnt/Max inhibited Myc-induced transcription and mediated repression through complete chromatin condensation and deacetylation of H3K9 and H4K16 across the E-box region. Importantly, Mnt was able to counteract c-Myc mediated activation even when expressed at low levels, suggesting Mnt to act as a strong repressor by closing the chromatin structure. Collectively our data demonstrate that the balance between c-Myc and Mnt activity determines the transcriptional outcome of the hTERT promoter by modulation of the chromatin architecture.

Upstream stimulatory factor 2 and hypoxia-inducible factor 2α (HIF2α) cooperatively activate HIF2 target genes during hypoxia

While the functions of hypoxia-inducible factor 1α (HIF1α)/aryl hydrocarbon receptor nuclear translocator (ARNT) and HIF2α/ARNT (HIF2) proteins in activating hypoxia-inducible genes are well established, the role of other transcription factors in the hypoxic transcriptional response is less clear. We report here for the first time that the basic helix-loop-helix-leucine-zip transcription factor upstream stimulatory factor 2 (USF2) is required for the hypoxic transcriptional response, specifically, for hypoxic activation of HIF2 target genes. We show that inhibiting USF2 activity greatly reduces hypoxic induction of HIF2 target genes in cell lines that have USF2 activity, while inducing USF2 activity in cells lacking USF2 activity restores hypoxic induction of HIF2 target genes. Mechanistically, USF2 activates HIF2 target genes by binding to HIF2 target gene promoters, interacting with HIF2α protein, and recruiting coactivators CBP and p300 to form enhanceosome complexes that contain HIF2α, USF2, CBP, p300, and RNA polymerase II on HIF2 target gene promoters. Functionally, the effect of USF2 knockdown on proliferation, motility, and clonogenic survival of HIF2-dependent tumor cells in vitro is phenocopied by HIF2α knockdown, indicating that USF2 works with HIF2 to activate HIF2 target genes and to drive HIF2-depedent tumorigenesis.

Human telomerase reverse transcriptase (hTERT) is a novel target of the Wnt/β-catenin pathway in human cancer

Telomerase activation plays a critical role in human carcinogenesis through the maintenance of telomeres, but the activation mechanism during carcinogenesis remains unclear. The human telomerase reverse transcriptase (hTERT) promoter has been shown to promote hTERT gene expression selectively in tumor cells but not in normal cells. Deregulation of the Wnt/β-catenin signaling pathway is reported to be associated with human carcinogenesis. However, little is known about whether the Wnt/β-catenin pathway is involved in activating hTERT transcription and inducing telomerase activity (TA). In this study, we report that hTERT is a novel target of the Wnt/β-catenin pathway. Transient activation of the Wnt/β-catenin pathway either by transfection of a constitutively active form of β-catenin or by LiCl or Wnt-3a conditioned medium treatment induced hTERT mRNA expression and elevated TA in different cell lines. Furthermore, we found that silencing endogenous β-catenin expression by β-catenin gene-specific shRNA effectively decreased hTERT expression, suppressed TA, and accelerated telomere shortening. Of the four members of the lymphoid-enhancing factor (LEF)/T-cell factor (TCF) family, only TCF4 showed more effective stimulation on the hTERT promoter. Ectopic expression of a dominant negative form of TCF4 inhibited hTERT expression in cancer cells. Through promoter mapping, electrophoretic mobility shift assay, and chromatin immunoprecipitation assay, we found that hTERT is a direct target of β-catenin·TCF4-mediated transcription and that the TCF4 binding site at the hTERT promoter is critical for β-catenin·TCF4-dependent expression regulation. Given the pivotal role of telomerase in carcinogenesis, these results may offer insight into the regulation of telomerase in human cancer.

Tumor suppressors p53, p63TAα, p63TAy, p73α, and p73β use distinct pathways to repress telomerase expression

The promoter of the telomerase catalytic subunit (TERT) is subject to tight regulation and remains repressed in somatic cells to ensure their limited life span and to prevent tumor initiation. Here we report that the hTERT promoter is strongly repressed by p53 and the related family members p63 and p73. We found that p53-mediated repression was different in human and mouse cells and occurred through p53-dependent transcription inhibition of c-Myc or through E-box/E2F pathways, respectively. Although p63TAα-mediated repression occurred through SP1, p63TAy-mediated repression occurred through E2F signaling. Finally, p73α- and p73β-mediated repression occurred through NF-YB2. Our results show a complex multifactorial mechanism used by p53 and its family members to keep hTERT expression under tight control.

A common polymorphism in the 5' region of the human protein c gene binds USF1

INTRODUCTION

Genetic variation in the Protein C gene (PROC) is associated with altered risk of adverse outcome for a number of diseases. Common single nucleotide polymorphisms (SNPs) in the promoter region and the adjacent 5' region of PROC are associated with Protein C expression. We tested the hypothesis that common SNPs (minor allele frequency >10%) between the frequently studied promoter SNPs -1654 (rs1799808) and -1641 (rs1799809), and the end of PROC intron 2 alter nuclear transcription factor binding.

MATERIALS AND METHODS

We used electrophoretic mobility shift assays with 25-mer oligonucleotides centered on each of the 10 SNPs assessed in this potential regulatory region of the Protein C gene to test for differential binding to nuclear factors isolated from Hep-G2 cells.

RESULTS

We found that the G-allele oligo of the intron 2 SNP rs2069915[G/A] bound nuclear factors more avidly than the A-allele (p=1.9 × 10(-9), n=24). Similarly, we found that the C-allele oligo of the intron 2 SNP rs2069916[C/T] bound nuclear factors more avidly than the T-allele, (p=3.7 × 10(-6), n=19). Cold competition and supershift assays suggested that the protein differentially binding to the C-allele of rs2069916 was USF1. Notably, we observed minimal nuclear factor binding to oligos containing haplotypes of the previously reported -1654 and -1641 SNPs. Luciferase reporter assays that showed the A-T haplotype of rs2069915 and rs2069916 drives transcription significantly more than the C-G haplotype (t-test, P=0.015, n=12).

CONCLUSION

Differential transcription factor binding occurs for common SNPs in the 5' intronic regions of PROC which may contribute to PROC regulation and reported PROC SNP - phenotype associations.

Regulation of the human catalytic subunit of telomerase (hTERT)

Over the past decade, there has been much interest in the regulation of telomerase, the enzyme responsible for maintaining the integrity of chromosomal ends, and its crucial role in cellular immortalization, tumorigenesis, and the progression of cancer. Telomerase activity is characterized by the expression of the telomerase reverse transcriptase (TERT) gene, suggesting that TERT serves as the major limiting agent for telomerase activity. Recent discoveries have led to characterization of various interactants that aid in the regulation of human TERT (hTERT), including numerous transcription factors; further supporting the pivotal role that transcription plays in both the expression and repression of telomerase. Several studies have suggested that epigenetic modulation of the hTERT core promoter region may provide an additional level of regulation. Although these studies have provided essential information on the regulation of hTERT, there has been ambiguity of the role of methylation within the core promoter region and the subsequent binding of various activating and repressive agents. As a result, we found it necessary to consolidate and summarize these recent developments and elucidate these discrepancies. In this review, we focus on the co-regulation of hTERT via transcriptional regulation, the presence or absence of various activators and repressors, as well as the epigenetic pathways of DNA methylation and histone modifications.

Regulation of hTERT by BCR-ABL at multiple levels in K562 cells

BACKGROUND

The cytogenetic characteristic of Chronic Myeloid Leukemia (CML) is the formation of the Philadelphia chromosome gene product, BCR-ABL. Given that BCR-ABL is the specific target of Gleevec in CML treatment, we investigated the regulation of the catalytic component of telomerase, hTERT, by BCR-ABL at multiple levels in K562 cells.

METHODS

Molecular techniques such as over expression, knockdown, real-time PCR, immunoprecipitation, western blotting, reporter assay, confocal microscopy, telomerase assays and microarray were used to suggest that hTERT expression and activity is modulated by BCR-ABL at multiple levels.

RESULTS

Our results suggest that BCR-ABL plays an important role in regulating hTERT in K562 (BCR-ABL positive human leukemia) cells. When Gleevec inhibited the tyrosine kinase activity of BCR-ABL, phosphorylation of hTERT was downregulated, therefore suggesting a positive correlation between BCR-ABL and hTERT. Gleevec treatment inhibited hTERT at mRNA level and significantly reduced telomerase activity (TA) in K562 cells, but not in HL60 or Jurkat cells (BCR-ABL negative cells). We also demonstrated that the transcription factor STAT5a plays a critical role in hTERT gene regulation in K562 cells. Knockdown of STAT5a, but not STAT5b, resulted in a marked downregulation of hTERT mRNA level, TA and hTERT protein level in K562 cells. Furthermore, translocation of hTERT from nucleoli to nucleoplasm was observed in K562 cells induced by Gleevec.

CONCLUSIONS

Our data reveal that BCR-ABL can regulate TA at multiple levels, including transcription, post-translational level, and proper localization. Thus, suppression of cell growth and induction of apoptosis by Gleevec treatment may be partially due to TA inhibition. Additionally, we have identified STAT5a as critical mediator of the hTERT gene expression in BCR-ABL positive CML cells, suggesting that targeting STAT5a may be a promising therapeutic strategy for BCR-ABL positive CML patients.

Human telomerase expression regulation

Since telomerase has been recognized as a relevant factor distinguishing cancer cells from normal cells, it has become a very promising target for anti-cancer therapy. A correlation between short telomere length and increased mortality was revealed in many studies. The telomerase expression/activity appears to be one of the most crucial factors to study to improve cancer therapy and prevention. However, this multisubunit enzymatic complex can be regulated at various levels. Thus, several strategies have been proposed to control telomerase in cancer cells such as anti-sense technology against TR and TERT, ribozymes against TERT, anti-estrogens, progesterone, vitamin D, retinoic acid, quadruplex stabilizers, telomere and telomerase targeting agents, modulation of interaction with other proteins involved in the regulation of telomerase and telomeres, etc. However, the transcription control of key telomerase subunits seems to play the crucial role in whole complexes activity and cancer cells immortality. Thus, the research of telomerase regulation can bring significant insight into the knowledge concerning stem cells metabolism but also ageing. This review summarizes the current state of knowledge of numerous telomerase regulation mechanisms at the transcription level in human that might become attractive anti-cancer therapy targets.

Regulation of hTERT transcription: a target of cellular and viral mechanisms for immortalization and carcinogenesis

A hallmark of human cancer cells is immortal cell growth, which is associated with telomere maintenance by telomerase. The transcriptional regulation of the human telomerase reverse transcriptase (hTERT) gene is a major mechanism that negatively and positively controls telomerase activity in normal and cancer cells, respectively. A growing body of data suggests that various cellular and viral factors and pathways involved in cell senescence, immortalization and carcinogenesis act on the hTERT promoter. The activity of the hTERT promoter is regulated, either directly or through signaling pathways, by oncogene products (e.g., Myc and Ets families) and tumor suppressor proteins (e.g., BRCA1). Endogenous factors involved in the physiological repression of the hTERT gene have also been revealed by chromosome transfer experiments. The integration of viral genomes in the hTERT locus can lead to hTERT activation and telomerase induction. Here, we summarize these findings and pay special attention to recent findings with relevance to the endogenous regulatory mechanisms of hTERT transcription.

Novel, chimeric, cancer-specific, and radiation-inducible gene promoters for suicide gene therapy of cancer

BACKGROUND

Although the promoter of the human telomerase reverse transcriptase (hTERT) gene has been widely used in gene therapy for targeted cancer cells, it has some limitations for clinical use because of its low activity and potential toxicity to certain normal cells. To overcome these defects, the authors generated novel chimeric hTERT promoters that contained the radiation-inducible sequence CC(A/T)(6) GG (known as CArG elements).

METHODS

Chimeric hTERT promoters were synthesized that contained different numbers of CArG elements, and the activity of chimeric promoters was assessed in different cancer cell lines and normal cells. The potential of selected promoters to successfully control horseradish peroxidase (HRP) and prodrug indole-3-acetic acid (IAA) suicide gene therapy was tested in vitro and in vivo.

RESULTS

The promoter activity assays indicated that the synthetic promoter that contained 6 repeating CArG units had the best radiation inducibility than any other promoters that contained different numbers of CArG units, and the chimeric promoters retained their cancer-specific characteristics. The chimeric promoter was better at driving radiation-inducible gene therapy than the control promoters. The sensitizer enhancement ratio of the chimeric promoter system determined by clonogenic assay was higher, and the chimeric promoter system resulted in a significantly higher apoptotic level compared with other promoter systems. The combination of chimeric/promoter-mediated gene therapy and radiotherapy significantly inhibited tumor volume in a xenograft mouse model and resulted in a significant prolongation of survival in mice.

CONCLUSIONS

The current results indicated that a combinational cancer-specific promoter system that is responsive to irradiation has great potential for improving the efficacy of cancer treatment.

High-mobility group A2 protein modulates hTERT transcription to promote tumorigenesis

The high-mobility group A2 gene (HMGA2) is one of the most frequently amplified genes in human cancers. However, functions of HMGA2 in tumorigenesis are not fully understood due to limited knowledge of its targets in tumor cells. Our study reveals a novel link between HMGA2 and the regulation of human telomerase reverse transcriptase (hTERT), the catalytic subunit of telomerase, which offers critical insight into how HMGA2 contributes to tumorigenesis. The expression of HMGA2 modulates the expression of hTERT, resulting in cells with enhanced telomerase activities and increased telomere length. Treatment with suberoylanilide hydroxamide (SAHA), a histone deacetylase (HDAC) inhibitor, causes dose-dependent hTERT reporter activation, mimicking HMGA2 overexpression. By interacting with Sp1, HMGA2 interferes with the recruitment of HDAC2 to the hTERT proximal promoter, enhancing localized histone H3-K9 acetylation and thereby stimulating hTERT expression and telomerase activity. Moreover, HMGA2 knockdown by short hairpin HMGA2 in HepG2 cells leads to progressive telomere shortening and a concurrent decrease of steady-state hTERT mRNA levels, attenuating their ability to form colonies in soft agar. Importantly, HMGA2 partially replaces the function of hTERT during the tumorigenic transformation of normal human fibroblasts. These findings are potentially clinically relevant, because HMGA2 expression is reported to be upregulated in a number of human cancers as telomere maintenance is essential for tumorigenesis.

ZNF703 is a common Luminal B breast cancer oncogene that differentially regulates luminal and basal progenitors in human mammary epithelium

The telomeric amplicon at 8p12 is common in oestrogen receptor-positive (ER+) breast cancers. Array-CGH and expression analyses of 1172 primary breast tumours revealed that ZNF703 was the single gene within the minimal amplicon and was amplified predominantly in the Luminal B subtype. Amplification was shown to correlate with increased gene and protein expression and was associated with a distinct expression signature and poor clinical outcome. ZNF703 transformed NIH 3T3 fibroblasts, behaving as a classical oncogene, and regulated proliferation in human luminal breast cancer cell lines and immortalized human mammary epithelial cells. Manipulation of ZNF703 expression in the luminal MCF7 cell line modified the effects of TGFβ on proliferation. Overexpression of ZNF703 in normal human breast epithelial cells enhanced the frequency of in vitro colony-forming cells from luminal progenitors. Taken together, these data strongly point to ZNF703 as a novel oncogene in Luminal B breast cancer.

Short rare hTERT-VNTR2-2nd alleles are associated with prostate cancer susceptibility and influence gene expression

Background

The hTERT (human telomerase reverse transcriptase) gene contains five variable number tandem repeats (VNTR) and previous studies have described polymorphisms for hTERT-VNTR2-2^nd. We investigated how allelic variation in hTERT-VNTR2-2^nd may affect susceptibility to prostate cancer.

Methods

A case-control study was performed using DNA from 421 cancer-free male controls and 329 patients with prostate cancer. In addition, to determine whether the VNTR polymorphisms have a functional consequence, we examined the transcriptional levels of a reporter gene linked to these VNTRs and driven by the hTERT promoter in cell lines.

Results

Three new rare alleles were detected from this study, two of which were identified only in cancer subjects. A statistically significant association between rare hTERT-VNTR2-2^nd alleles and risk of prostate cancer was observed [OR, 5.17; 95% confidence interval (CI), 1.09-24.43; P = 0.021]. Furthermore, the results indicated that these VNTRs inserted in the enhancer region could influence the expression of hTERT in prostate cancer cell lines.

Conclusions

This is the first study to report that rare hTERT VNTRs are associated with prostate cancer predisposition and that the VNTRs can induce enhanced levels of hTERT promoter activity in prostate cancer cell lines. Thus, the hTERT-VNTR2-2^nd locus may function as a modifier of prostate cancer risk by affecting gene expression.

Human chromosome 5 carries a transcriptional regulator of human telomerase reverse transcriptase (hTERT)

Telomerase activation is crucial for cells that tend to be immortalized. Increased telomerase activity is correlated with upregulation of telomerase reverse transcriptase (TERT) expression. In most human somatic cells, hTERT expression is suppressed by multiple factors. We have previously shown that human chromosome 5 carries a possible suppressor of mouse tert mtert expression in a mouse melanoma cell line, B16-F10 cells. However, the function of the transcriptional regulator of TERT on this chromosome remains unclear. To examine the functional role of a putative hTERT regulator(s) on this chromosome, we transferred human chromosome 5 in a human melanoma cell line, A2058 cells by microcell-mediated chromosome transfer (MMCT). Microcell hybrid clones with an introduced chromosome 5, but not chromosome 10, showed a remarkable decrease in the growth rate with an obvious cellular morphological alteration and eventually cellular senescence. Moreover, this phenomenon was accompanied by a reduction of hTERT expression and telomerase activity. Most importantly, we found that transcriptional suppression of hTERT by the introduction of chromosome 5 is largely mediated by regulating hTERT promoter activity. Furthermore, the hTERT promoter region between -1623 and -1047 was responsible for this function. These results provide evidence that transcriptional regulator(s) of the hTERT is carried on human chromosome 5 as an endogenous mechanism of hTERT suppression.

Telomerase as an important target of androgen signaling blockade for prostate cancer treatment

As the mainstay treatment for advanced prostate cancer, androgen deprivation therapy (ADT) targets the action of androgen receptor (AR) by reducing androgen level and/or by using anti-androgen to compete with androgens for binding to AR. Albeit effective in extending survival, ADT is associated with dose-limiting toxicity and the development of castration-resistant prostate cancer (CRPC) after prolonged use. Because CRPC is lethal and incurable, developing effective strategies to enhance the efficacy of ADT and circumvent resistance becomes an urgent task. Continuous AR signaling constitutes one major mechanism underlying the development of CRPC. The present study showed that methylseleninic acid (MSA), an agent that effectively reduces AR abundance, could enhance the cancer-killing efficacy of the anti-androgen bicalutamide in androgen-dependent and CRPC cells. We found that the combination of MSA and bicalutamide produced a robust downregulation of prostate-specific antigen and a recently identified AR target, telomerase, and its catalytic subunit, human telomerase reverse transcriptase. The downregulation of hTERT occurs mainly at the transcriptional level, and reduced AR occupancy of the promoter contributes to downregulation. Furthermore, apoptosis induction by the two agents is significantly mitigated by the restoration of hTERT. Our findings thus indicate that MSA in combination with anti-androgen could represent a viable approach to improve the therapeutic outcome of ADT. Given the critical role of hTERT/telomerase downregulation in mediating the combination effect and the fact that hTERT/telomerase could be measured in blood and urine, hTERT/telomerase could serve as an ideal tumor-specific biomarker to monitor the efficacy of the combination therapy noninvasively.

Localization of an hTERT repressor region on human chromosome 3p21.3 using chromosome engineering

Telomerase is a ribonucleoprotein enzyme that synthesizes telomeric DNA. The reactivation of telomerase activity by aberrant upregulation/expression of its catalytic subunit hTERT is a major pathway in human tumorigenesis. However, regulatory mechanisms that control hTERT expression are largely unknown. Previously, we and others have demonstrated that the introduction of human chromosome 3, via microcell-mediated chromosome transfer (MMCT), repressed transcription of the hTERT gene. These results suggested that human chromosome 3 contains a regulatory factor(s) involved in the repression of hTERT. To further localize this putative hTERT repressor(s), we have developed a unique experimental approach by introducing various truncated chromosome 3 regions produced by a novel chromosomal engineering technology into the renal cell carcinoma cell line (RCC23 cells). These cells autonomously express ectopic hTERT (exohTERT) promoted by a retroviral LTR promoter in order to permit cellular division after repression of endogenous hTERT. We found a telomerase repressor region located within a 7-Mb interval on chromosome 3p21.3. These results provide important information regarding hTERT regulation and a unique method to identify hTERT repressor elements.

An optimized telomerase-specific lentivirus for optical imaging of tumors

Advances in medical imaging techniques, such as ultrasound, computed tomography, magnetic resonance imaging, and positron emission tomography, have made great progress in detecting tumors. However, these imaging techniques are unable to differentiate malignant tumors from benign ones. Recently developed optical imaging of tumors in small animals provides a useful method to distinguish malignant tumors from their surrounding normal tissues. Human telomerase reverse transcriptase (hTERT) is normally inactivated in most somatic cells, whereas it is commonly reactivated in many cancer cells. In this study, we constructed a lentiviral vector that expresses green fluorescent protein (GFP) driven by an optimized hTERT promoter to create a noninvasive tumor-specific imaging methodology. The activity of this optimized hTERT promoter was found to be equal to the activity of SV40 and cytomegalovirus promoters. In vitro experiments showed that GFP was only expressed in telomerase-positive tumor cells infected with this lentivirus, whereas there was no GFP expression in telomerase-negative tumor cells or normal somatic cells. We also found that subcutaneous telomerase-positive tumors could be visualized 24 hours after an intratumoral injection with this lentivirus by using a charge-coupled device (CCD) camera. In contrast, telomerase-negative tumors could not be imaged after an intratumoral injection even for 30 days. These results suggest that infection with lentivirus containing this optimized hTERT promoter might be a useful diagnostic tool for the real-time visualization of macroscopically invisible tumor tissues using a highly sensitive CCD imaging system.

Chromatin and epigenetic regulation of the telomerase reverse transcriptase gene

Telomerase expression and telomere maintenance are critical for long-term cell proliferation and survival, and they play important roles in development, aging, and cancer. Cumulating evidence has indicated that regulation of the rate-limiting subunit of human telomerase reverse transcriptase gene (hTERT) is a complex process in normal cells and many cancer cells. In addition to a number of transcriptional activators and repressors, the chromatin environment and epigenetic status of the endogenous hTERT locus are also pivotal for its regulation in normal human somatic cells and in tumorigenesis.

Interferon-inducible IFI16, a negative regulator of cell growth, down-regulates expression of human telomerase reverse transcriptase (hTERT) gene

Background

Increased levels of interferon (IFN)-inducible IFI16 protein (encoded by the IFI16 gene located at 1q22) in human normal prostate epithelial cells and diploid fibroblasts (HDFs) are associated with the onset of cellular senescence. However, the molecular mechanisms by which the IFI16 protein contributes to cellular senescence-associated cell growth arrest remain to be elucidated. Here, we report that increased levels of IFI16 protein in normal HDFs and in HeLa cells negatively regulate the expression of human telomerase reverse transcriptase (hTERT) gene.

Methodology/Principal Findings

We optimized conditions for real-time PCR, immunoblotting, and telomere repeat amplification protocol (TRAP) assays to detect relatively low levels of hTERT mRNA, protein, and telomerase activity that are found in HDFs. Using the optimized conditions, we report that treatment of HDFs with inhibitors of cell cycle progression, such as aphidicolin or CGK1026, which resulted in reduced steady-state levels of IFI16 mRNA and protein, was associated with increases in hTERT mRNA and protein levels and telomerase activity. In contrast, knockdown of IFI16 expression in cells increased the expression of c-Myc, a positive regulator of hTERT expression. Additionally, over-expression of IFI16 protein in cells inhibited the c-Myc-mediated stimulation of the activity of hTERT-luc-reporter and reduced the steady-state levels of c-Myc and hTERT.

Conclusions/Significance

These data demonstrated that increased levels of IFI16 protein in HDFs down-regulate the expression of hTERT gene. Our observations will serve basis to understand how increased cellular levels of the IFI16 protein may contribute to certain aging-dependent diseases.

Human telomerase reverse transcriptase promoter-driven oncolytic adenovirus with E1B-19 kDa and E1B-55 kDa gene deletions

We constructed an oncolytic adenovirus, Adeno-hTERT-E1A, with deletions of the viral E1B, E3A, and E3B regions and insertion of a human telomerase reverse transcriptase (hTERT) promoter-driven early viral 1A (E1A) cassette that confers high transcriptional activity in multiple human tumor cell lines. The oncolytic potential of Adeno-hTERT-E1A was characterized in comparison with that of the E1B-55 kDa- and E3B-region-deleted oncolytic adenovirus ONYX-015. Tumor cells infected with Adeno-hTERT-E1A expressed dramatically higher levels of E1A oncoprotein, underwent enhanced lysis, and displayed an earlier and higher apoptotic index than cells infected with ONYX-015. Despite the increase in virus-induced apoptotic death, Adeno-hTERT-E1A replicated and produced functional progeny leading to viral spread, but with reduced efficiency compared with ONYX-015, in particular in A549 cells. Virus-induced E1A expression, host cell apoptosis, viral hexon protein production, and DNA synthesis were markedly reduced in primary human hepatocytes after infection with Adeno-hTERT-E1A as compared with ONYX-015. The strong oncolytic activity of Adeno-hTERT-E1A in tumor cell culture translated into superior antitumor activity in vivo in an MDA-MB-231 solid tumor xenograft model. Adeno-hTERT-E1A thus has strong therapeutic potential and an improved safety profile compared with ONYX-015, which may lead to reduced toxicity in the clinic.

Radiation enhances suicide gene therapy in radioresistant laryngeal squamous cell carcinoma via activation of a tumor-specific promoter

Radioresistant cells have been shown to correlate with poor outcome after radiotherapy. Here, we found that human telomerase reverse transcriptase promoter (hTERTp) had lower activity in laryngeal squamous carcinomas cells than in radioresistant variant cells. Combining radiotherapy with plasmid phTERTp-HRP, in which expression of enzyme horseradish peroxidase (HRP) controlled by hTERTp, resulted in increased apoptosis and necrosis of tumor cells after prodrug indole-3-acetic acid (IAA; converted by HRP into a cytotoxin) incubation. Volume and wet weight of xenograft tumor were reduced more in the combination groups. These data suggest that hTERTp has potential use in targeted cancer gene therapy, especially for radioresistant tumors. Combining radiotherapy with hTERTp-HRP/IAA may overcome radioresistance in laryngeal squamous carcinomas cells and amplify the killing effect in targeted cancer cells.

Differential repression of human and mouse TERT genes during cell differentiation

Differential regulation of telomerase reverse transcriptase (TERT) contributes to the distinct aging and tumorigenic processes in humans and mice. Here, we report that the hTERT gene was strongly repressed during differentiation of human cells, whereas modest mTERT expression was detected in terminally differentiated and post-mitotic cells. The stringent hTERT repression depended on the native chromatin environment because transiently transfected hTERT promoters were not repressed in differentiated cells. Conversely, the transiently transfected mTERT core promoter was repressed during cell differentiation, suggesting that the repression of mTERT promoter did not require its endogenous chromatin structures. To understand the mechanisms of this differential regulation, we examined chromatin structures of the endogenous TERT loci during cell differentiation. In both human and mouse cells, repression was accompanied by the loss of multiple DNase I hypersensitive sites at the TERT promoters and their upstream regions, revealing positions of potential regulatory elements. Interestingly, the hTERT locus was located within a nuclease-resistant chromatin domain in human cells, whereas a corresponding chromatin domain was not detected for the mTERT locus. Taken together, our study indicated that, unlike the repression of mTERT gene, the condensed native chromatin environment of hTERT locus was central to its silencing during cell differentiation.

Estrogen deficiency leads to telomerase inhibition, telomere shortening and reduced cell proliferation in the adrenal gland of mice

Estrogen deficiency mediates aging, but the underlying mechanism remains to be fully determined. We report here that estrogen deficiency caused by targeted disruption of aromatase in mice results in significant inhibition of telomerase activity in the adrenal gland in vivo. Gene expression analysis showed that, in the absence of estrogen, telomerase reverse transcriptase (TERT) gene expression is reduced in association with compromised cell proliferation in the adrenal gland cortex and adrenal atrophy. Stem cells positive in c-kit are identified to populate in the parenchyma of adrenal cortex. Analysis of telomeres revealed that estrogen deficiency results in significantly shorter telomeres in the adrenal cortex than that in wild-type (WT) control mice. To further establish the causal effects of estrogen, we conducted an estrogen replacement therapy in these estrogen-deficient animals. Administration of estrogen for 3 weeks restores TERT gene expression, telomerase activity and cell proliferation in estrogen-deficient mice. Thus, our data show for the first time that estrogen deficiency causes inhibitions of TERT gene expression, telomerase activity, telomere maintenance, and cell proliferation in the adrenal gland of mice in vivo, suggesting that telomerase inhibition and telomere shortening may mediate cell proliferation arrest in the adrenal gland, thus contributing to estrogen deficiency-induced aging under physiological conditions.

Understanding and exploiting hTERT promoter regulation for diagnosis and treatment of human cancers

Telomerase activation is a critical step for human carcinogenesis through the maintenance of telomeres, but the activation mechanism during carcinogenesis remains unclear. Transcriptional regulation of the human telomerase reverse transcriptase (hTERT) gene is the major mechanism for cancer-specific activation of telomerase, and a number of factors have been identified to directly or indirectly regulate the hTERT promoter, including cellular transcriptional activators (c-Myc, Sp1, HIF-1, AP2, ER, Ets, etc.) as well as the repressors, most of which comprise tumor suppressor gene products, such as p53, WT1, and Menin. Nevertheless, none of them can clearly account for the cancer specificity of hTERT expression. The chromatin structure via the DNA methylation or modulation of nucleosome histones has recently been suggested to be important for regulation of the hTERT promoter. DNA unmethylation or histone methylation around the transcription start site of the hTERT promoter triggers the recruitment of histone acetyltransferase (HAT) activity, allowing hTERT transcription. These facts prompted us to apply these regulatory mechanisms to cancer diagnostics and therapeutics. Telomerase-specific replicative adenovirus (Telomelysin, OBP-301), in which E1A and E1B genes are driven by the hTERT promoter, has been developed as an oncolytic virus that replicates specifically in cancer cells and causes cell death via viral toxicity. Direct administration of Telomelysin was proved to effectively eradicate solid tumors in vivo, without apparent adverse effects. Clinical trials using Telomelysin for cancer patients with progressive stages are currently ongoing. Furthermore, we incorporated green fluorescent protein gene (GFP) into Telomelysin (TelomeScan, OBP-401). Administration of TelomeScan into the primary tumor enabled the visualization of cancer cells under the cooled charged-coupled device (CCD) camera, not only in primary tumors but also the metastatic foci. This technology can be applied to intraoperative imaging of metastatic lymphnodes. Thus, we found novel tools for cancer diagnostics and therapeutics by utilizing the hTERT promoter.

Role of human T-cell leukemia virus type I Tax in expression of the human telomerase reverse transcriptase (hTERT) gene in human T-cells

The viral product Tax encoded by human T-cell leukemia virus type I (HTLV-I) is thought to play a central role in leukemogenesis. Clonal expansion of HTLV-I-infected cells requires the extension of cell division with telomere maintenance, which is regulated by the ribonucleoprotein enzyme telomerase. However, the roles of Tax in the expression of telomerase activity in T-cells remains controversial. Our previous study indicated that expression of the human telomerase reverse transcriptase subunit (hTERT) gene, which determines telomerase activity, is tightly regulated in human T-cells. In the present study, we investigated Tax-mediated regulation of hTERT gene expression by Tax in human T-cells. HTLV-I Tax induced expression of the hTERT gene in human peripheral blood leukocytes. Reporter assays revealed that Tax activated the hTERT promoter in quiescent Kit 225 cells, while the promoter activity was repressed by Tax in proliferating Jurkat cells. Both up-regulation and down-regulation by Tax were mediated through the 43-bp sequences in the promoter, which carried at least two elements that independently functioned as repressors. The two elements bound distinct factors. G1 to S phase transition induced by introduction of either cyclin D2 with cdk4 or p130-specific shRNA also activated the hTERT promoter, implying that activation of the hTERT promoter in quiescent Kit 225 cells is associated with cell cycle progression. Our findings suggest that the cell cycle state critically influences Tax-mediated regulation of hTERT expression.

Functional diversity of human protection of telomeres 1 isoforms in telomere protection and cellular senescence

Protection of telomeres 1 (POT1) proteins in various organisms bind telomeres and regulate their structure and function. In contrast to mice carrying two distinct POT1 genes encoding two POT1 proteins (POT1a and POT1b), humans have the single POT1 gene. In addition to full-length POT1 protein (variant v1), the human POT1 gene encodes four other variants due to alternative RNA splicing (variants v2, v3, v4, and v5), whose functions are poorly understood. The functional analyses of the NH(2)-terminally and COOH-terminally truncated POT1 variants in this study showed that neither the single-stranded telomere-binding ability of the NH(2)-terminal oligonucleotide-binding (OB) folds nor the telomerase-dependent telomere elongation activity mediated by the COOH-terminal TPP1-interacting domain was telomere protective by itself. Importantly, a COOH-terminally truncated variant (v5), which consists of the NH(2)-terminal OB folds and the central region of unknown function, was found to protect telomeres and prevent cellular senescence as efficiently as v1. Our data revealed mechanistic and functional differences between v1 and v5: (a) v1, but not v5, functions through the maintenance of telomeric 3' overhangs; (b) p53 is indispensable to v5 knockdown-induced senescence; and (c) v5 functions at only a fraction of telomeres to prevent DNA damage signaling. Furthermore, v5 was preferentially expressed in mismatch repair (MMR)-deficient cells and tumor tissues, suggesting its role in chromosome stability associated with MMR deficiency. This study highlights a human-specific complexity in telomere protection and damage signaling conferred by functionally distinct isoforms from the single POT1 gene.

TAK1 represses transcription of the human telomerase reverse transcriptase gene

In human cells, telomerase activity is tightly regulated by the expression of its catalytic subunit, namely, the human telomerase reverse transcriptase (hTERT). However, the molecular mechanisms involved in the regulation of hTERT expression have not been completely clarified. We have previously reported that transforming growth factor beta (TGF-beta) represses the expression of the hTERT gene. In the present study, we demonstrated that TGF-beta-activated kinase 1 (TAK1), originally identified as a mitogen-activated kinase kinase kinase, represses the hTERT core promoter activity in an E-box-independent manner, and it also represses the transcription of the hTERT gene in human lung adenocarcinoma cell line, A549 cells. This TAK1-induced repression was found to be caused by the recruitment of histone deacetylase to Sp1 at the hTERT promoter and a consequent reduction in the amount of acetylated histone H4 at the hTERT promoter. Finally, we demonstrated that TAK1 induces cellular senescence programs in normal human diploid cells. Thus, we assume that TAK1 triggers the repression mechanisms of the hTERT gene as a result of evoking cellular senescence programs. Considered together, TAK1 is thought to play a causative role in the determination of a finite replicative lifespan of normal and cancer cells.

Involvement of the oncoprotein c-Myc in viral telomerase RNA gene regulation during Marek's disease virus-induced lymphomagenesis

Marek's disease virus (MDV) is an alphaherpesvirus that induces a highly malignant T-lymphoma in chickens. The viral genome encodes two identical copies of a viral telomerase RNA subunit (vTR) that exhibits 88% sequence identity to its chicken ortholog chTR. The minimal telomerase ribonucleoprotein complex consists of a protein subunit with reverse transcriptase activity (TERT) and an RNA subunit (TR). The active complex compensates for the progressive telomere shortening that occurs during mitosis and is involved in the cell immortalization process. We show here that the upregulation of telomerase activity is associated with an increase in vTR gene expression in chickens infected with the highly oncogenic MDV strain RB-1B. A comparative functional analysis of the viral and chicken TR promoters, based on luciferase reporter assays, revealed that the vTR promoter was up to threefold more efficient than the chTR promoter in avian cells. We demonstrated, by directed mutagenesis of the vTR promoter region, that the stronger transcriptional activity of the vTR promoter resulted largely from an E-box located two nucleotides downstream from the transcriptional start site of the vTR gene. Furthermore, transactivation assays and chromatin immunoprecipitation assays demonstrated the involvement of the c-Myc oncoprotein in the transcriptional regulation of vTR. Finally, an Ets binding site was specifically implicated in the transcriptional regulation of vTR in the MDV-transformed lymphoblastoid cell line MSB-1.

TGF-beta and cancer: is Smad3 a repressor of hTERT gene?

Transforming growth factor beta (TGF-beta) carries out tumor suppressor activity in epithelial and lymphoid cells, whereas telomerase is required for most cancers. Although the molecular mechanisms by which TGF-beta acts as a tumor suppressor are yet to be fully established, a link between TGFb and its tumor suppressor activity by telomerase has been suggested. Recently, we have noted a novel mode of action for TGF-beta through which human telomerase reverse transcriptase (hTERT) gene is repressed in immortal and neoplastic cells, confirming that one of the mechanisms underlying TGF-beta suppression of tumor growth may be through inhibiting hTERT gene transcription. Moreover, the inhibition of hTERT gene by TGF-beta suggests a cis action of the TGF-beta signaling molecule Smad3 on hTERT promoter directly. This article examines our current understanding and investigation of TGF-beta regulation of telomerase activity, and presents a model in which Smad3 participates in regulating hTERT gene transcription by acting as a repressor directly. Engineering the interface between Smad3 and hTERT gene may lead to a new strategy to inhibit telomerase activity in cancer.