Mechanism for the reduction of telomerase expression during muscle cell differentiation

Exercise as a Therapy to Maintain Telomere Function and Prevent Cellular Senescence

Exercise transiently impacts the expression, regulation, and activity of TERT/telomerase to maintain telomeres and protect the genome from insults. By protecting the telomeres (chromosome ends) and the genome, telomerase promotes cellular survival and prevents cellular senescence. By increasing cellular resiliency, via the actions of telomerase and TERT, exercise promotes healthy aging.

Ecological Balance of Oral Microbiota Is Required to Maintain Oral Mesenchymal Stem Cell Homeostasis

Oral microbiome is essential for maintenance of oral cavity health. Imbalanced oral microbiome causes periodontal and other diseases. It is unknown whether oral microbiome affect oral stem cells function. This study used a common clinical antibiotic treatment approach to alter oral microbiome ecology and examine whether oral mesenchymal stem cells (MSCs) are affected. We found that altered oral microbiome resulted gingival MSCs deficiency, leading to a delayed wound healing in male mice. Mechanistically, oral microbiome release lipopolysaccharide (LPS) that stimulates the expression of microRNA-21 (miR-21) and then impair the normal function of gingival MSCs and wound healing process through miR-21/Sp1/telomerase reverse transcriptase pathway. This is the first study indicate that interplay between oral microbiome and MSCs homeostasis in male mice. Stem Cells 2018;36:551-561.

Cloning and molecular characterization of telomerase reverse transcriptase (TERT) and telomere length analysis of Peromyscus leucopus

Telomerase reverse transcriptase (TERT) is the catalytic subunit of telomerase complex that regulates telomerase activity to maintain telomere length for all animals with linear chromosomes. As the Mus musculus (MM) laboratory mouse has very long telomeres compared to humans, a potential alternative animal model for telomere research is the Peromyscus leucopus (PL) mouse that has telomere lengths close to the human range and has the wild counterparts for comparison. We report the full TERT coding sequence (pTERT) from PL mice to use in the telomere research. Comparative analysis with eight other mammalian TERTs revealed a pTERT protein considerably homologous to other TERTs and preserved all TERT specific-sequence signatures, yet with some distinctive features. pTERT displayed the highest nucleotide and amino acid sequence homology with hamster TERT. Unlike human but similar to MM mice, pTERT expression was detected in various adult somatic tissues of PL mice, with the highest expression in testes. Four different captive stocks of PL mice and wild-captured PL mice each displayed group-specific average telomere lengths, with the longest and shortest telomeres in inbred and outbred stock mice, respectively. pTERT showed considerable numbers of synonymous and nonsynonymous mutations. A pTERT proximal promoter region cloned was homologous among PL and MM mice and rat, but with species-specific features. From PL mice, we further cloned and characterized ribosomal protein, large, P0 (pRPLP0) to use as an internal control for various assays. Peromyscus mice have been extensively used for various studies, including human diseases, for which pTERT and pRPLP0 would be useful tools.

Saccharomyces cerevisiae as a model for the study of extranuclear functions of mammalian telomerase

The experimental evidence from the last decade made telomerase a prominent member of a family of moonlighting proteins performing different functions at various cellular loci. However, the study of extratelomeric functions of the catalytic subunit of mammalian telomerase (TERT) is often complicated by the fact that it is sometimes difficult to distinguish them from its role(s) at the chromosomal ends. Here, we present an experimental model for studying the extranuclear function(s) of mammalian telomerase in the yeast Saccharomyces cerevisiae. We demonstrate that the catalytic subunit of mammalian telomerase protects the yeast cells against oxidative stress and affects the stability of the mitochondrial genome. The advantage of using S. cerevisiae to study of mammalian telomerase is that (1) mammalian TERT does not interfere with its yeast counterpart in the maintenance of telomeres, (2) yeast telomerase is not localized in mitochondria and (3) it does not seem to be involved in the protection of cells against oxidative stress and stabilization of mtDNA. Thus, yeast cells can be used as a 'test tube' for reconstitution of mammalian TERT extranuclear function(s).

mRNA decay factor AUF1 maintains normal aging, telomere maintenance, and suppression of senescence by activation of telomerase transcription

Inflammation is associated with DNA damage, cellular senescence, and aging. Cessation of the inflammatory cytokine response is mediated in part through cytokine mRNA degradation facilitated by RNA-binding proteins, including AUF1. We report a major function of AUF1-it activates telomerase expression, suppresses cellular senescence, and maintains normal aging. AUF1-deficient mice undergo striking telomere erosion, markedly increased DNA damage responses at telomere ends, pronounced cellular senescence, and rapid premature aging that increases with successive generations, which can be rescued in AUF1 knockout mice and their cultured cells by resupplying AUF1 expression. AUF1 binds and strongly activates the transcription promoter for telomerase catalytic subunit Tert. In addition to directing inflammatory cytokine mRNA decay, AUF1 destabilizes cell-cycle checkpoint mRNAs, preventing cellular senescence. Thus, a single gene, AUF1, links maintenance of telomere length and normal aging to attenuation of inflammatory cytokine expression and inhibition of cellular senescence.

Chronic exercise modifies age-related telomere dynamics in a tissue-specific fashion

We evaluated the impact of long-term exercise on telomere dynamics in wild-derived short telomere mice (CAST/Ei) over 1 year. We observed significant telomere shortening in liver and cardiac tissues in sedentary 1-year-old mice compared with young (8 weeks) baseline mice that were attenuated in exercised 1-year-old animals. In contrast, skeletal muscle exhibited significant telomere shortening in exercise mice compared with sedentary and young mice. Telomerase enzyme activity was increased in skeletal muscle of exercise compared with sedentary animals but was similar in cardiac and liver tissues. We observed significant age-related decreases in expression of telomere-related genes that were attenuated by exercise in cardiac and skeletal muscle but not liver. Protein content of TRF1 was significantly increased in plantaris muscle with age. In summary, long-term exercise altered telomere dynamics, slowing age-related decreases in telomere length in cardiac and liver tissue but contributing to shortening in exercised skeletal muscle.

BAG2 is a target of the c-Myc gene and is involved in cellular senescence via the p21(CIP1) pathway

Suppression of c-Myc is likely to induce cellular senescence in many tumors with unclear mechanisms. A proteomics survey indicated that high levels of BCL2-associated athanogene 2 (BAG2) were found in response to c-Myc repression in TRE293 cells. This observation led to the investigation into the role of BAG2 in c-Myc-induced senescence. The association of the c-Myc/SP1 complex with the BAG2 promoter verified the role of c-Myc/SP1 in regulating BAG2 transcription. Furthermore, high levels of BAG2 were found to induce p21(CIP1)-dependent senescence and subsequent carcinogenetic arrest, suggesting its possible role as an indirect activator of the p21(CIP1) pathway.

Identification of PITX1 as a TERT suppressor gene located on human chromosome 5

Telomerase, a ribonucleoprotein enzyme that maintains telomere length, is crucial for cellular immortalization and cancer progression. Telomerase activity is attributed primarily to the expression of telomerase reverse transcriptase (TERT). Using microcell-mediated chromosome transfer (MMCT) into the mouse melanoma cell line B16F10, we previously found that human chromosome 5 carries a gene, or genes, that can negatively regulate TERT expression (H. Kugoh, K. Shigenami, K. Funaki, J. Barrett, and M. Oshimura, Genes Chromosome Cancer 36:37-47, 2003). To identify the gene responsible for the regulation of TERT transcription, we performed cDNA microarray analysis using parental B16F10 cells, telomerase-negative B16F10 microcell hybrids with a human chromosome 5 (B16F10MH5), and its revertant clones (MH5R) with reactivated telomerase. Here, we report the identification of PITX1, whose expression leads to the downregulation of mouse tert (mtert) transcription, as a TERT suppressor gene. Additionally, both human TERT (hTERT) and mouse TERT (mtert) promoter activity can be suppressed by PITX1. We show that three and one binding site within the hTERT and mtert promoters, respectively, that express a unique conserved region are responsible for the transcriptional activation of TERT. Furthermore, we showed that PITX1 binds to the TERT promoter both in vitro and in vivo. Thus, PITX1 suppresses TERT transcription through direct binding to the TERT promoter, which ultimately regulates telomerase activity.

NFAT5 regulates transcription of the mouse telomerase reverse transcriptase gene

We aimed to clarify the transcription-regulation mechanisms of the mouse telomerase reverse transcriptase gene (mTERT). First, we searched for the promoter region required for transcriptional activation of mTERT and identified an enhancer cis-element (named mTERT-EE) located between -200 and -179bp of the mouse TERT gene (mTERT). EMSA results suggested that nuclear factor of activated T cells (NFAT) member proteins bind to mTERT-EE. We then identified NFAT5 as the factor binding to mTERT-EE and found that it activates the transcription of the mTERT core promoter. The results that siRNA directed against NFAT5 significantly reduced mTERT expression and mTERT core promoter activity and that the expressions of NFAT5 and mTERT were well correlated in various mouse tissues except liver suggest that NFAT5 dominantly and directly regulates mTERT expression. To clarify their functionality further, we investigated the effect of hypertonic stress, a known stimulus affecting the expression and transcriptional activity of NFAT5, on mTERT expression. The result indicated that hypertonic stress activates mTERT transcription via the activation and recruitment of NFAT5 to the mTERT promoter. These results provide useful information about the transcription-regulation mechanisms of mTERT.

A novel zinc finger protein Zfp637 behaves as a repressive regulator in myogenic cellular differentiation

Zinc finger proteins have been implicated as transcription factors in the differentiation and development of cells and tissues in higher organisms. The classical C2H2 zinc finger motif is one main type of motif of zinc finger proteins. Our previous studies have shown that Zfp637, which comprises six consecutively typical and one atypical C2H2 zinc finger motifs, is highly expressed in undifferentiated or poorly differentiated cell lines, but is moderately or slightly expressed in normal tissues. We have also demonstrated that Zfp637 can promote cell proliferation. However, its role in the regulation of cell differentiation remains unknown. We report here that endogenous Zfp637 as well as mTERT is expressed in proliferating C2C12 myoblasts and that their expression is downregulated during myogenic differentiation. Constitutive expression of Zfp637 in C2C12 myoblasts increased mTERT expression and telomerase activity, and promoted the progression of the cell cycle and cell proliferation. By contrast, endogenous repression of Zfp637 expression by RNA interference downregulated the mTERT gene and the activity of telomerase, and markedly reduced cell proliferation. Overexpression of Zfp637 also inhibited the expression of myogenic differentiation-specific genes such as MyoD and myogenin, and prevented C2C12 myoblast differentiation. Our results suggest that Zfp637 inhibits muscle differentiation through a defect in the cell cycle exit by potentially regulating mTERT expression in C2C12 myoblasts. This may provide a new research line for studying muscle differentiation.

RNAi screen for telomerase reverse transcriptase transcriptional regulators identifies HIF1alpha as critical for telomerase function in murine embryonic stem cells

In various types of stem cells, including embryonic stem (ES) cells and hematopoietic stem cells, telomerase functions to ensure long-term self-renewal capacity via maintenance of telomere reserve. Expression of the catalytic component of telomerase, telomerase reverse transcriptase (Tert), which is essential for telomerase activity, is limiting in many types of cells and therefore plays an important role in establishing telomerase activity levels. However, the mechanisms regulating expression of Tert in cells, including stem cells, are presently poorly understood. In the present study, we sought to identify genes involved in the regulation of Tert expression in stem cells by performing a screen in murine ES (mES) cells using a shRNA expression library targeting murine transcriptional regulators. Of 18 candidate transcriptional regulators of Tert expression identified in this screen, only one candidate, hypoxia inducible factor 1 alpha (Hif1alpha), did not have a significant effect on mES cell morphology, survival, or growth rate. Direct shRNA-mediated knockdown of Hif1alpha expression confirmed that suppression of Hif1alpha levels was accompanied by a reduction in both Tert mRNA and telomerase activity levels. Furthermore, gradual telomere attrition was observed during extensive proliferation of Hif1alpha-targeted mES cells. Switching Hif1alpha-targeted mES cells to a hypoxic environment largely restored Hif1alpha levels, as well as Tert expression, telomerase activity levels, and telomere length. Together, these findings suggest a direct effect of Hif1alpha on telomerase regulation in mES cells, and imply that Hif1alpha may have a physiologically relevant role in maintenance of functional levels of telomerase in stem cells.

Regulatory mechanisms of human and mouse telomerase reverse transcriptase gene transcription: distinct dependency on c-Myc

Telomerase-a complex ribonucleoprotein enzyme-synthesizes telomeric repeats to avoid telomere loss that accompanies cell division and chromosomal replication. Expression of telomerase is detectable in embryonic cells and cancer cells, but not in normal human cells. On the other hand, in mice, substantial expression of telomerase is detected in normal cells and tissues as well as in immortalized cells. These results suggest that the regulatory mechanisms of telomerase activity in humans and mice differ. Considering these results along with the fact that the expression of the telomerase reverse transcriptase (TERT) gene is a rate-limiting step for telomerase activity, we compared transcriptional regulatory mechanisms of both the species. A series of luciferase assays and RT-PCR analyses demonstrated that c-Myc, a dominant transactivator for human TERT (hTERT), is not involved in the regulation of mouse TERT (mTERT). These results suggest that distinct molecules and pathways are involved in the process of immortalization and tumorigenesis in human and mouse cells.

Transformation of human liver L-O2 cells mediated by stable HBx transfection

AIM

To explore the mechanism of hepatocarcinogenesis associated with the hepatitis B virus X protein (HBx), we investigated the role of HBx in transformation using human liver L-O2 cells stably transfected with HBx as a model.

METHODS

Plasmids encoding HBx were stably transfected into immortalized human liver L-O2 cells and rodent fibroblast NIH/3T3 cells. The expression of alfa-fetoprotein (AFP), c-Myc, HBx, and survivin in the engineered cells was examined by Western blotting. The malignant phenotype of the cells was demonstrated by anchorage-independent colony formation and tumor formation in nude mice. RNA interference assays, Western blotting, luciferase reporter gene assays and flow cytometry analysis were performed. The number of centrosomes in the L-O2-X cells was determined by gamma-tubulin immunostaining. The effect of HBx on the transcriptional activity of human telomerase reverse transcriptase (hTERT) and hTERT activity in L-O2-X cells and/or 3T3-X cells was detected by the luciferase reporter gene assay and telomerase repeat amplification protocol (TRAP).

RESULTS

Stable HBx transfection resulted in a malignant phenotype in the engineered cells in vivo and in vitro. Meanwhile, HBx was able to increase the transcription of the NF-kappaB, AP-1, and survivin genes and to upregulate the expression levels of c-Myc and survivin. Abnormal centrosome duplication and activated hTERT were responsible for the transformation.

CONCLUSION

Stable HBx transfection leads to genomic instability of host cells, which is responsible for hepatocarcinogenesis; meanwhile, transactivation by the HBx protein contributes to the development of hepatocellular carcinoma (HCC). The L-O2-X cell line is an ideal model for investigating the mechanism of HBx-mediated transformation.

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.

Telomere neurobiology

The ends of chromosomes consist of a hexanucleotide DNA repeat sequence and specialized DNA-binding and telomere-associated proteins. An enzyme activity called telomerase maintains telomere length by using an RNA template (TR) and a reverse transcriptase (TERT) to add the hexanucleotide sequence to the free chromosome end. The structure of telomeres is maintained and modified by telomere repeat-binding factors (TRF1 and TRF2) and proteins known for their role in DNA damage responses, including poly(ADP-ribose) polymerase-1, Werner, and ATM. Telomerase activity can be quantified using a telomere repeat amplification protocol (TRAP) assay, and levels of TERT and telomere-associated proteins are evaluated by immunoblot and immunocytochemical methods. Levels of TERT and telomere-associated proteins can be overexpressed or knocked down using viral vector-based methods. Using the kinds of approaches described here, evidence has been obtained suggesting that telomeres play important roles in regulating neural stem cell proliferation, neuronal differentiation, senescence of glial cells, and apoptosis and DNA damage responses of neural cells.

Mechanisms of action of TGF-beta in cancer: evidence for Smad3 as a repressor of the hTERT gene

Transforming growth factor-beta (TGF-beta) induces cell differentiation and suppresses cell proliferation, but the mechanisms underlying the actions of TGF-beta remain to be fully elucidated. Recent studies suggest that TGF-beta suppresses neoplastic cell development by employing Smad3 protein to repress the gene of human telomerase reverse transcriptase (hTERT). In human breast cancer cells, TGF-beta induces rapid phosphorylation and subsequent entry of Smad3 into the nucleus. In the nucleus, Smad3 binds to the hTERT gene promoter directly and inhibits hTERT gene transcription activity. By interacting with c-myc, Smad3 also represses the c-myc gene. Thus, TGF-beta prevents continuous cell proliferation by switching off telomerase activity through Smad3 repression of the hTERT gene and the action of c-myc. Modulating the interface between Smad3 and the hTERT gene, and the potential feedback loop from telomeres to Smad3 via Smurf2, may represent a novel approach to regulate cell lifespan of proliferation.

The proximal promoter region of mTert is sufficient to regulate telomerase activity in ES cells and transgenic animals

BACKGROUND

The reverse transcriptase of telomerase (Tert) controls telomerase activity maintaining the end of linear chromosomes in eukaryotic cells. Telomerase function is highly active in undifferentiated multipotent stem cells, decreases with cell differentiation and is generally absent from most somatic cells in the adult. Its absence is responsible of telomeres shortening in such somatic cells. Using an in vivo transgenic model and an in vitro culture differentiation of adult stem cells, we examined the elements of the mouse Tert (mTert) promoter that control telomerase activity.

RESULTS

Three constructs comprising 1, 2 or 5 kb of the mTert promoter sequence coupled to the coding sequence of the green fluorescent protein (EGFP) were electroporated into embryonic stem (ES) cells. Transformed ES cells were able to mimic the expected mTert expression, which was associated to green fluorescence. One and 5 kb promoter produced the higher expression of EGFP, on ES cells. When ES cells were allowed to differentiate to embryoid bodies and to other cell types, they lost gradually the expression of mTert-EGFP as consequence of differentiation. No differences were found among the three constructs analyzed. We then generated transgenic mice with the three constructs. Expression of the reporter gene was monitored by reverse transcription-PCR analysis and EGFP visualization. The mRNA expression of the three constructs was lower than the endogenous mTert, but mimicked the endogenous mTert transcription pattern; however, no fluorescent expression of EGFP was detected in adult tissues. EGFP expression of the three constructs was visualized at the blastocysts stage and in new ES cells generated from them; in the germinal ring of E13 dpc foetuses; in ES-like colonies and in germinal stem cells generated from neonatal and adult testis cells; and in neuroesferes generated from E14 dpc foetuses' brain cells.

CONCLUSION

The 1 kb promoter upstream of the initiating ATG codon of mTert contains all the regulatory elements to control telomerase expression in ES cells during in vitro loss of pluripotency. The transgenic mouse lines generated represent an appropriate system to analyze the expression of mouse Tert gene under physiological condition and during establishment of stem cell lines generated from embryonic or adult tissues.

Transcriptional regulation of early transposon elements, an active family of mouse long terminal repeat retrotransposons

While early transposon (ETn) endogenous retrovirus (ERV)-like elements are known to be active insertional mutagens in the mouse, little is known about their transcriptional regulation. ETns are transcribed during early mouse embryogenesis in embryonic stem (ES) and embryonic carcinoma (EC) cell lines. Despite their lack of coding potential, some ETns remain transposition competent through their use of reverse transcriptase encoded by a related group of ERVs-MusD elements. In this study, we have confirmed high expression levels of ETn and MusD elements in ES and EC cells and have demonstrated an increase in the copy number of ETnII elements in the EC P19 cell line. Using transient transfections, we have shown that ETnII and MusD LTRs are much more active as promoters in P19 cells than in NIH 3T3 cells, indicating that genomic context and methylation are not the only factors determining endogenous transcriptional activity of ETns. Three sites in the 5' part of the long terminal repeat (LTR) were demonstrated to bind Sp1 and Sp3 transcription factors and were found to be important for high LTR promoter activity in P19 cells, suggesting that as yet unidentified Sp binding partners are involved in the regulation of ETn activity in undifferentiated cells. Finally, we found multiple transcription start sites within the ETn LTR and have shown that the LTR retains significant promoter activity in the absence of its noncanonical TATA box. These findings lend insight into the transcriptional regulation of this family of mobile mouse retrotransposons.

Differential cis-regulation of human versus mouse TERT gene expression in vivo: identification of a human-specific repressive element

In vivo expression of human telomerase is significantly different from that of mouse telomerase. To assess the basis for this difference, a bacterial artificial chromosome clone containing the entire hTERT (human telomerase reverse transcriptase) gene was introduced in mice. In these transgenic mice, expression of the hTERT transgene was similar to that of endogenous hTERT in humans, rather than endogenous mTERT (mouse telomerase reverse transcriptase). In tissues and cells showing a striking difference in expression levels between hTERT in humans and mTERT in mice (i.e., liver, kidney, lung, uterus, and fibroblasts), expression of the hTERT transgene in transgenic mice was repressed, mimicking hTERT in humans. The transcriptional activity of the hTERT promoter was much lower than that of the mTERT promoter in mouse embryonic fibroblasts or human fibroblasts. Mutational analysis of the hTERT and mTERT promoters revealed that a nonconserved GC-box within the hTERT promoter was responsible for the human-specific repression. These results reveal that a difference in cis-regulation of transcription, rather than transacting transcription factors, is critical to species differences in tissue-specific TERT expression. Our data also suggest that the GC-box-mediated, human-specific mechanism for TERT repression is impaired in human cancers. This study represents a detailed characterization of the functional difference in a gene promoter of mice versus humans and provides not only important insight into species-specific regulation of telomerase and telomeres but also an experimental basis for generating mice humanized for telomerase enzyme and its pattern of expression.

Function of AP-1 in transcription of the telomerase reverse transcriptase gene (TERT) in human and mouse cells

The transcriptional regulation of the human telomerase catalytic subunit (hTERT) plays a critical role in telomerase activity. Approximately 200 bp of the proximal core promoter is responsible for basic hTERT expression; however, the function of the distal regulatory elements remains unclear. The transcription factor activator protein 1 (AP-1) is involved in cellular proliferation, differentiation, carcinogenesis, and apoptosis and is expressed broadly in both cancer and normal cells. There are several putative AP-1 sites in the hTERT promoter, but their functions are unknown. The present study examined the regulatory role of AP-1 in hTERT gene transcription. Overexpression of AP-1 leads to transcriptional suppression of hTERT in cancer cells. The combination of c-Fos and c-Jun or c-Fos and JunD strongly suppresses hTERT promoter activity in transient-expression analyses. The hTERT promoter region between -2000 and -378 is responsible for this function. Gel shift and supershift analyses, as well as ChIP, show binding of JunD and c-Jun on two putative AP-1 sites within this region. Mutations in the AP-1 binding sites rescued suppressions caused by AP-1, suggesting this is a direct regulation of the hTERT promoter. In contrast, there was no effect on mTERT expression or mTERT promoter activity by AP-1 overexpression in mouse fibroblasts. The species-specific function of AP-1 in TERT expression may in part help explain the difference in telomerase activity between normal human and mouse cells.

Regulation of inhibitor of differentiation gene 3 (Id3) expression by Sp2-motif binding factor in myogenic C2C12 cells: Downregulation of DNA binding activity following skeletal muscle differentiation

Id3 is a member of the Id family of transcriptional regulators that have been implicated in the development of multiple tissues. Altered expression of the Id genes and proteins contribute to carcinogenesis and atherosclerosis. Id3 is highly expressed in proliferating skeletal muscle cells but becomes downregulated upon terminal differentiation. We have identified several DNase I protected footprints within a proximal region of the mouse Id3 promoter that has been shown previously to support high levels of transcriptional activity in proliferating skeletal muscle cells. Two of these sites interacted, respectively, in vitro with Sp2 and Egr-1 proteins present in muscle cell nuclear extracts. Mutation analysis revealed that the Sp2 site accounted for a major part of the Id3 promoter activity in proliferating muscle cells whereas the Egr-1 site was dispensable. Consistent with the previously observed downregulation of the endogenous Id3 gene, protein binding to the Sp2 site was substantially reduced with extracts from differentiated muscle cells. Our results reveal Id3 as a potential target for Sp2 and raise the possibility that acute activation and the chronic and maintained expression of Id3 gene might be regulated by different mechanisms.

Effects of hepatitis B virus X protein on human telomerase reverse transcriptase expression and activity in hepatoma cells

In subjects with hepatitis B, carcinogenesis has been associated with the hepatitis B virus (HBV) X protein (HBX) and human telomerase reverse transcriptase (hTERT). In the experiments reported here, we used immunohistochemical methods to study the expression of hTERT and HBV antigens (HBsAg, HBcAg and HBxAg) in 34 cases of HCC and corresponding paratumor tissues, 30 cases of liver cirrhosis, and 6 normal livers. To examine the effect of HBX on hTERT expression and activity in hepatoma cells, we transiently and stably transfected the pCMV-X plasmid cloned HBx gene into H7402 hepatoma cells, then measured the expression of c-Myc and hTERT in these cells with the use of Western-blot analysis. Telomerase activity was detected with the use of the telomerase repeat amplification protocol (TRAP) in transiently and stably transfected cells. We found that hTERT expression was 67.6%, 73.5%, and 100% in tumor, paratumor, and cirrhosis samples, respectively, but found no hTERT positivity in samples of normal liver. HBsAg, HBcAg, and HBxAg were expressed in 58.8%, 26.5%, and 76.5% of tumor tissues, respectively; in 64.7%, 41.2%, and 85.3% of the corresponding paratumor tissues; and in 76.7%, 66.7%, and 100% of cirrhotic tissues. The chi 2 test revealed no significant difference between the expression of hTERT and HBxAg in these tissues. Western-blot analysis revealed that expression of c-Myc and hTERT in the transiently transfected cells was much greater than that in the control cells. We elicited a similar result when we used the TRAP method to measure telomerase activity. Our data collectively demonstrate that HBX up-regulates the expression and activity of hTERT in hepatoma cells, suggesting that hTERT is associated with tumor development.

Ectopic mTERT expression in mouse embryonic stem cells does not affect differentiation but confers resistance to differentiation- and stress-induced p53-dependent apoptosis

The fundamental role of telomerase is to protect telomere ends and to maintain telomere length during replication; hence, telomerase expression is high in stem cells but reduced upon differentiation. Recent studies indicate that telomerase might play other roles besides telomere maintenance. We have investigated the role of telomerase in cellular differentiation and death. Here, we show that ectopic expression of mouse telomerase catalytic subunit (mTERT) does not affect embryonic stem (ES) cell proliferation or differentiation in vitro, but protects ES cells against cell death during differentiation. Ectopic mTERT expression also confers resistance to apoptosis induced by oxidative stress and other genotoxic insults. This resistance depends on the catalytic activity of mTERT. Stress-signal-induced p53 accumulation and consequent p53-dependent apoptotic target gene expression was not affected by mTERT overexpression. However, although chemical inhibition of p53 by alpha-pifithrin reduced stress-induced apoptosis in vector-expressing cells, it did not significantly affect apoptosis in mTERT-expressing cells. Moreover, overexpression of mTERT in p53-/- ES cells did not confer further resistance to genotoxic insults, suggesting that mTERT might exert its protective effect by antagonizing the p53 pathway. Altogether, our findings indicate that ectopic mTERT expression in ES cells does not affect differentiation but confers resistance to apoptosis, and suggest that this strategy might be used in improving the efficiency of stem-cell therapies.

A role for nucleoprotein Zap3 in the reduction of telomerase activity during embryonic stem cell differentiation

Telomerase, the enzyme which maintains the ends of linear chromosomes in eukaryotic cells is found in murine embryonic stem cells; however, its activity is downregulated during in vitro differentiation. Previous work has indicated that this is due to the transcriptional downregulation of murine reverse transcriptase unit (mTert) of telomerase. To investigate the factors that cause the transcriptional repression of mTert we defined a 300 bp region which is essential for its transcription and performed site directed mutagenesis and electrophoretic mobility shift assays. This analysis indicated that Sp1, Sp3 and c-Myc bind to the GC-boxes and E-boxes, respectively, within the promoter and help activate the transcription of mTert gene. We also identified a novel binding sequence, found repeated within the mTert core region, which when mutated caused increased mTert expression. Yeast one hybrid screening combined with electrophoretic mobility shift assays indicated that the nuclear protein Zap3 binds to this site and its overexpression leads to the downregulation of mTert during differentiation. This suggests that regulation of mTert transcription is a complex process which depends on a quantitative balance between transcription factors that cause activation or repression of this gene. Overexpression of Zap3 in murine embryonic stem cells results in reduction in telomerase activity and telomere length as well as reduced proliferative capacity and limited ability to contribute to the development of haematopoietic cells upon differentiation.

p53 is a regulator of macrophage differentiation

While it is well accepted that p53 plays a role in apoptosis, less is known as to its involvement in cell differentiation. Here we show that wild-type p53 facilitates IL-6-dependent macrophage differentiation. Treatment of M1/2 cells expressing the temperature-sensitive p53 143 (Val to Ala) mutant, at the wild-type conformation, facilitated the appearance of mature macrophages that exhibited phagocytic activity. Enhancement of differentiation by the p53 143 (Val to Ala) in the wild-type conformation was coupled with the inhibition of apoptosis induction by this protein. In agreement with previous studies, we found that p53 levels were reduced during p53-dependent macrophage differentiation. This occurred when p53 levels before IL-6 stimuli were high. Interestingly, the p53 143 (Val to Ala) protein, at the mutant conformation, enhanced macrophage differentiation, as did the wild-type conformation, whereas the p53 273 (Arg to His) core mutant exerted an inhibitory effect on this pathway. The transcription-deficient p53 molecules, p53 (22-23) and p53 22,23,143, could not induce p53-dependent differentiation. Moreover, the p53 (22-23) protein inhibited the p53-independent differentiation pathway. Interestingly, the p53 (22-23) protein not only blocked IL-6-mediated differentiation, but also induced significant apoptotic cell death, upon IL-6 stimulation. Taken together, our data show that wild-type p53 enhances macrophage differentiation, while various p53 mutant types exert different effects on this differentiation pathway.

Antisense oligonucleotide targeting at the initiator of hTERT arrests growth of hepatoma cells

AIM: To evaluate the inhibitory effect of antisense phosphorothioate oligonucleotide (asON) complementary to the initiator of human telomerase catalytic subunit (hTERT) on the growth of hepatoma cells.

METHODS: The as-hTERT was synthesized by using a DNA synthesizer. HepG2.2.15 cells were treated with as-hTERT at the concentration of 10 μmol/L. After 72 h, these cells were obtained for detecting growth inhibition, telomerase activity using the methods of MTT, TRAP-PCR-ELISA, respectively. BALB/c(nu/nu) mice were injected HepG2.2.15 cells and a human-nude mice model was obtained. There were three groups for anti-tumor activity study. Once tumors were established, these animals in the first group were administered as-hTERT and saline. Apoptosis of tumor cells was detected by FCM. In the 2nd group, the animals were injected HepG2.2.15 cells together with as-hTERT. In the third group, the animals were given as-hTERT 24 hours postinjection of HepG2.2.15 cells. The anti-HBV effects were assayed with ELISA in vitro and in vivo.

RESULTS: Growth inhibition was observed in cells treated with as-hTERT in vitro. A significant different in the value of A₅₇₀ - A₆₃₀ was found between cells treated with as-hTERT and control (P < 0.01) by MTT method. The telomerase activity of tumor cells treated with as-hTERT was reduced, the value of A₄₅₀ nm was 0.42 compared to control (1.49) with TRAP-PCR-ELISA. The peak of apoptosis in tumor cells given as-hTERT was 21.12%, but not seen in saline-treated control. A prolonged period of carcinogenesis was observed in the second and third group animals. There was inhibitory effect on the expression of HBsAg and HBeAg in vivo and in vitro.

CONCLUSION: As-hTERT has an anti-tumor activity, which may be useful for gene therapy of tumors.

Cloning of rat telomerase catalytic subunit functional domains, reconstitution of telomerase activity and enzymatic profile of pig and chicken tissues

Telomerase is a ribonucleoprotein polymerase which adds TTAGGG repeats to telomeric ends. Recent studies reported the reverse transcription enzyme activity mostly from the catalytic subunit (TERT) of the enzyme complex. Both human telomerase catalytic subunit (hTERT) and mouse telomerase catalytic subunit (mTERT) had been previously cloned but not rat telomerase catalytic subunit rTERT. In this study, the rTERT functional domains were cloned and was found that its function resemble to mouse and human telomerase. In addition, chicken and pig telomerase activity profile were studied and its enzyme activity is related to its proliferation capability of individual tissues. However, its catalytic subunit does not like mouse, rat and human cases that the telomerase activity could not reconstituted by the in-vitro transfection of mTERT and hTERT cloned vectors. Here we demonstrated that rTERT is similar to mTERT and hTERT but not pig and chicken telomerase. Further studies are needed to verify the malignancy characteristics because nowadays artificial organs/tissues from these animals are used for the transplantation to human body.

Helicobacter pylori infection generated gastric cancer through p53-Rb tumor-suppressor system mutation and telomerase reactivation

AIM

To investigate the relationship between Helicobacter pylori (H.pylori) infection and the expressions of the p53, Rb, c-myc, bcl-2 and hTERT mRNA in a series of diseases from chronic gastritis (CG), intestinal metaplasia type I or II(IMI-II), intestinal metaplasia type III (IMIII), mild or modest dysplasia (DysI-II), severe dysplasia (DysIII) to gastric cancer(GC) and to elucidate the mechanism of gastric carcinogenesis relating to H.pylori infection.

METHODS

272 cases between 1998 and 2001 were available for the study including 42 cases of CG, 46 cases of IMI-II, 25 cases of IMIII, 48 cases of DysI-II, 27 cases of DysIII, 84 cases of GC. H.pylori infection and the expressions of p53, Rb, c-myc, bcl-2 were detected by means of streptavidin-peroxidase (SP) immunohistochemical method. HTERT mRNA was detected by in situ hybridization (ISH).

RESULTS

The expressions of p53, Rb, c-myc, hTERT mRNA and bcl-2 were higher in the GC than in CG, IM, Dys. The expression of c-myc was higher in IMIII with H.pylori infection (10/16) than that without infection (1/9) and the positive rate in DysI-II and DysIII with H.pylori infection was 18/30 and 13/17, respectively, higher than that without infection (4/18 and 3/10, respectively). In our experiment mutated p53 had no association with H.pylori infection, the expression of Rb was associated with H.pylori infection in GC, but the p53-Rb tumor-suppressor system abnormal in DysI-II cases, DysIII and GC cases with H.pylori infection was 21/30, 15/17 and 48/48 respectively, higher than non-infection groups (4/18, 3/10, 28/36). Furthermore the level of hTERT mRNA in GC with H.pylori infection (47/48) was higher than that without infection (30/36), however the relationship between bcl-2 and H.pylori was only in IMIII. C-myc had a close association with hTERT mRNA in DysIII and GC (P=0.0 253,0.0 305 respectively).

CONCLUSION

In the gastric carcinogenesis, H.pylori might cause the severe imbalance of proliferation and apoptosis in the precancerous lesions (IMIII and GysIII) first, leading to p53-Rb tumor-suppressor system mutation and telomerase reactivation, and finally causes gastric cancer.

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

Regulation of the hTERT gene encoding the telomerase catalytic subunit plays an important role in human cell senescence, immortalization, and carcinogenesis. By examining the activity of various deleted or mutated hTERT promoter fragments, we show that an E-box element downstream of the transcription initiation site is critical to differential hTERT transcription between the telomerase/hTERT-positive renal cell carcinoma cell line (RCC23) and its telomerase/hTERT-negative counterpart containing a transferred, normal chromosome 3 (RCC23+3). This E-box element mediated repression of hTERT transcription in RCC23+3 but not in RCC23. A copy number-dependent enhancement of the repression suggested active repression, rather than loss of activation, in RCC23+3. Endogenous expression levels of c-Myc or Mad1, which could activate or repress hTERT transcription when overexpressed, did not account for the differential hTERT transcription. Gel mobility shift assays identified the upstream stimulatory factors (USFs) as a major E-box-binding protein complex in both RCC23 and RCC23+3 and, importantly, detected an RCC23+3-specific, E-box-binding factor that was distinct from the USF and Myc/Mad families. The E-box-mediated repression was also active in normal human fibroblasts and epithelial cells and inactive in some, but not all, telomerase/hTERT-positive cancer cells. These findings provide evidence for an endogenous, repressive mechanism that actively functions in telomerase/hTERT-negative normal cells and becomes defective during carcinogenic processes, e.g., by an inactivation of the telomerase repressor gene on chromosome 3.

Cloning and characterization of rice (Oryza sativa L) telomerase reverse transcriptase, which reveals complex splicing patterns

Plant chromosomes terminate in telomeres as in other eukaryotes. Telomeres are vital to genome stability and their malfunctioning is lethal. One of the core components of the telomere complex is telomerase. The enzyme activity depends on RNA (TER) and reverse transcriptase (TERT) subunits. We describe here the isolation, sequencing and characterization of the telomerase reverse transcriptase catalytic subunit from the monocot plant Oryza sativa L. (OsTERT). A single copy of this gene is present in the rice genome. The protein predicted from the OsTERT sequence has all the signature motifs of the TERT family members. Our data indicate that rice telomerase activity is developmentally regulated and is high in in vitro tissue and cell culture. However, steady-state transcript levels of the TERT gene do not seem to correlate with enzyme activity. Northern and RT-PCR analyses of the OsTERT gene transcript profile show multiple differentially spliced transcripts in both telomerase-positive and telomerase-negative tissues. Based on quantitative analysis of these transcripts, we speculate that the overall balance between the quantities of particular alternatively spliced transcripts may determine whether the TERT protein(s) is active or not. The diversity of splicing variants detected suggests that, as recently discovered for mammalian TERT proteins, rice TERT protein variants may perform functions other than telomere maintenance.