The many faces of telomerase: emerging extratelomeric effects

MNS16A polymorphism of the TERT gene on cancer risk: a systematic review and meta-analysis

Some studies have suggested that MNS16A polymorphism in telomerase reverse transcriptase (TERT) gene is associated with cancer risk in various populations and types of cancer. However, the results of previous studies exploring this link have been inconclusive. To be able to accurately assess the association between TERT MNS16A polymorphism and cancer risk, we performed a meta-analysis based on 17 studies described in 12 articles, including 13,764 controls and 7,132 cases. Combined odds ratios (ORs) and their corresponding 95% confidence intervals (CIs) were estimated to assess the strength of the association in either a fixed-effects model or, if applicable, a random-effects model. Heterogeneity between articles and their publication bias were also tested. Overall, pooled results showed that no significant association between this polymorphism and cancer was found in the five gene models tested.Considering that there may be too many negative studies in the included studies, diluting the results of the total sample size, we stratified these studies according to ethnicity, source of controls and cancer type. In the stratified analysis, a statistically significant association was observed between Asians and population-based studies. We also analyzed by cancer type and found a significantly increased risk of brain cancer in five genetic models. Our results suggest that TERT MNS16A polymorphism is likely to contribute to increased cancer risk.

The competing endogenous RNA network of CYP4Z1 and pseudogene CYP4Z2P exerts an anti-apoptotic function in breast cancer

The competing endogenous RNA network (ceRNET) is involved in tumorigenesis and has become a hot spot of research. The ceRNET between CYP4Z1 and the pseudogene CYP4Z2P promotes angiogenesis and mediates tamoxifen resistance in breast cancer. Nevertheless, the effects of this ceRNET on cell apoptosis and related mechanisms remain unclear. In the present study, we found that downregulation of CYP4Z1 or the CYP4Z2P 3'-UTR promotes cell apoptosis, mirroring the functions of human telomerase reverse transcriptase (hTERT). Furthermore, the ceRNET between CYP4Z1 and pseudogene CYP4Z2P modulates hTERT expression by operating as a sub-ceRNET for hTERT. Our data demonstrate that the ceRNET between CYP4Z1 and pseudogene CYP4Z2P acts as a sub-ceRNET for hTERT and, thus, inhibits breast cancer apoptosis.

hTERT genetic variation in depression

BACKGROUND

Telomeres are protective DNA-protein complexes forming the chromosome ends. TL differs between tissues. Shorter telomere length (TL) in blood leukocytes (LTL) has been associated with major depression, and with previous exposure to childhood adversity. TL studies on non-invasively sampled salivary DNA are less common. Telomerase, with its catalytic subunit hTERT, counteracts telomere shortening. Reduced telomerase activity associates with depression-like behavior in mice. Recently, the minor allele of the hTERT polymorphism rs2736100 was associated with shorter LTL among primarily healthy individuals. We hypothesized that (i) TL in saliva DNA is shortened in adults with a history of depression, and that (ii) rs2736100 is implicated in depression and depressive episodes in bipolar disorder type 1 (BD1).

METHODS

Individuals with a history of depression and those without (controls) were identified using self-reported questionnaires from a well-characterized population-based cohort. Clinical BD1 patients were diagnosed by specialized psychiatrists. Saliva TL was measured in age-matched depressed individuals and controls (n=662) using qRT-PCR. rs2736100 was genotyped in 436 depressed individuals, 1590 controls, and 368 BD1 patients.

RESULTS

Saliva TL was shorter in depressed individuals compared to controls. The rs2736100 minor allele was associated with depression among those without experience of childhood adversity, and with number of depressive episodes in BD1 patients responding well to lithium.

LIMITATION

Psychopathological symptoms were recorded at two time points only, 3 and 6 years prior to DNA sampling.

CONCLUSIONS

This is the first report on hTERT genetic variation in mood disorder. It proposes that genetic variation in hTERT may influence the susceptibility to depression.

Tissue formation and tissue engineering through host cell recruitment or a potential injectable cell-based biocomposite with replicative potential: Molecular mechanisms controlling cellular senescence and the involvement of controlled transient telomerase activation therapies

Accumulated data indicate that wound-care products should have a composition equivalent to that of the skin: a combination of particular growth factors and extracellular matrix (ECM) proteins endogenous to the skin, together with viable epithelial cells, fibroblasts, and mesenchymal stem cells (MSCs). Strategies consisting of bioengineered dressings and cell-based products have emerged for widespread clinical use; however, their performance is not optimal because chronic wounds persist as a serious unmet medical need. Telomerase, the ribonucleoprotein complex that adds telomeric repeats to the ends of chromosomes, is responsible for telomere maintenance, and its expression is associated with cell immortalization and, in certain cases, cancerogenesis. Telomerase contains a catalytic subunit, the telomerase reverse transcriptase (hTERT). Introduction of TERT into human cells extends both their lifespan and their telomeres to lengths typical of young cells. The regulation of TERT involves transcriptional and posttranscriptional molecular biology mechanisms. The manipulation, regulation of telomerase is multifactorial in mammalian cells, involving overall telomerase gene expression, post-translational protein-protein interactions, and protein phosphorylation. Reactive oxygen species (ROS) have been implicated in aging, apoptosis, and necrosis of cells in numerous diseases. Upon production of high levels of ROS from exogenous or endogenous generators, the redox balance is perturbed and cells are shifted into a state of oxidative stress, which subsequently leads to modifications of intracellular proteins and membrane lipid peroxidation and to direct DNA damage. When the oxidative stress is severe, survival of the cell is dependent on the repair or replacement of damaged molecules, which can result in induction of apoptosis in the injured with ROS cells. ROS-mediated oxidative stress induces the depletion of hTERT from the nucleus via export through the nuclear pores. Nuclear export is initiated by ROS-induced phosphorylation of tyrosine 707 within hTERT by the Src kinase family. It might be presumed that protection of mitochondria against oxidative stress is an important telomere length-independent function for telomerase in cell survival. Biotechnology companies are focused on development of therapeutic telomerase vaccines, telomerase inhibitors, and telomerase promoter-driven cell killing in oncology, have a telomerase antagonist in late preclinical studies. Anti-aging medicine-oriented groups have intervened on the market with products working on telomerase activation for a broad range of degenerative diseases in which replicative senescence or telomere dysfunction may play an important role. Since oxidative damage has been shown to shorten telomeres in tissue culture models, the adequate topical, transdermal, or systemic administration of antioxidants (such as, patented ocular administration of 1% N-acetylcarnosine lubricant eye drops in the treatment of cataracts) may be beneficial at preserving telomere lengths and delaying the onset or in treatment of disease in susceptible individuals. Therapeutic strategies toward controlled transient activation of telomerase are targeted to cells and replicative potential in cell-based therapies, tissue engineering and regenerative medicine.

Different Effects of BORIS/CTCFL on Stemness Gene Expression, Sphere Formation and Cell Survival in Epithelial Cancer Stem Cells

Cancer stem cells are cancer cells characterized by stem cell properties and represent a small population of tumor cells that drives tumor development, progression, metastasis and drug resistance. To date, the molecular mechanisms that generate and regulate cancer stem cells are not well defined. BORIS (Brother of Regulator of Imprinted Sites) or CTCFL (CTCF-like) is a DNA-binding protein that is expressed in normal tissues only in germ cells and is re-activated in tumors. Recent evidences have highlighted the correlation of BORIS/CTCFL expression with poor overall survival of different cancer patients. We have previously shown an association of BORIS-expressing cells with stemness gene expression in embryonic cancer cells. Here, we studied the role of BORIS in epithelial tumor cells. Using BORIS-molecular beacon that was already validated, we were able to show the presence of BORIS mRNA in cancer stem cell-enriched populations (side population and spheres) of cervical, colon and breast tumor cells. BORIS silencing studies showed a decrease of sphere formation capacity in breast and colon tumor cells. Importantly, BORIS-silencing led to down-regulation of hTERT, stem cell (NANOG, OCT4, SOX2 and BMI1) and cancer stem cell markers (ABCG2, CD44 and ALDH1) genes. Conversely, BORIS-induction led to up-regulation of the same genes. These phenotypes were observed in cervical, colon and invasive breast tumor cells. However, a completely different behavior was observed in the non-invasive breast tumor cells (MCF7). Indeed, these cells acquired an epithelial mesenchymal transition phenotype after BORIS silencing. Our results demonstrate that BORIS is associated with cancer stem cell-enriched populations of several epithelial tumor cells and the different phenotypes depend on the origin of tumor cells.

Telomerase dysregulation in the hippocampus of a rat model of depression: normalization by lithium

BACKGROUND

Telomeres are protective DNA-protein complexes at the ends of each chromosome, maintained primarily by the enzyme telomerase. Shortening of the blood leukocyte telomeres is associated with aging, several chronic diseases, and stress, eg, major depression. Hippocampus is pivotal in the regulation of cognition and mood and the main brain region of telomerase activity. Whether there is telomere dysfunction in the hippocampus of depressed subjects is unknown. Lithium, used in the treatment and relapse prevention of mood disorders, was found to protect against leukocyte telomere shortening in humans, but the mechanism has not been elucidated. To answer the questions whether telomeres are shortened and the telomerase activity changed in the hippocampus and whether lithium could reverse the process, we used a genetic model of depression, the Flinders Sensitive Line rat, and treated the animals with lithium.

METHODS

Telomere length, telomerase reverse transcriptase (Tert) expression, telomerase activity, and putative mediators of telomerase activity were investigated in the hippocampus of these animals.

RESULTS

The naïve Flinders Sensitive Line had shorter telomere length, downregulated Tert expression, reduced brain-derived neurotrophic factor levels, and reduced telomerase activity compared with the Flinders Resistant Line controls. Lithium treatment normalized the Tert expression and telomerase activity in the Flinders Sensitive Line and upregulated β-catenin.

CONCLUSION

This is the first report showing telomere dysregulation in hippocampus of a well-defined depression model and restorative effects of lithium treatment. If replicated in other models of mood disorder, the findings will contribute to understanding both the telomere function and the mechanism of lithium action in hippocampus of depressed patients.

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).

Nuclear translocation of telomerase reverse transcriptase is a critical process in lymphatic metastasis of nasopharyngeal carcinoma

Telomerase reverse transcriptase (TERT) is the predominant functional unit of telomerase and maintains the telomere length and the stability of chromosomes. Recently, TERT has been shown to be a critical factor in a number of other biological processes, including cell proliferation and cancer metastasis. In addition, although numerous studies have been conducted, the subcellular localization of the TERT protein and the association of such with cancer metastasis remains unclear. To investigate the involvement of TERT in in vivo metastasis, quantum dots-based immunofluorescence and western blot analysis were conducted to detect changes in the subcellular localization of TERT in human nasopharyngeal carcinoma (NPC) tissues and metastatic lymph nodes. To further investigate, metastatic and non-metastatic models of NPC were generated using 5-8F (high metastasis capability) and 6-10B (low metastasis capability) cell lines, respectively. It was found that TERT protein was overexpressed in NPC tissue samples and metastatic lymph nodes and TERT was predominantly located in the cytoplasm of primary NPC tissues, while TERT was predominantly located in the nucleus of the metastatic lymph nodes. The ratio of cytoplasmic TERT/nuclear TERT for the primary tumor of the 6-10B cell line was almost six-fold higher than that of the metastatic lymph nodes of the 5-8F cell line. TERT translocation from the cytoplasm to nucleus may present a critical step in the lymphatic metastasis of NPC. Thus, TERT translocation may be more useful than TERT expression level and telomerase activity for predicting the metastasis of NPC.

The MNS16A polymorphism in the TERT gene in peri-centenarians from the Han Chinese population

MNS16A, a variable number of tandem repeats polymorphism in the TERT gene, has been suggested to regulate telomerase activity. As telomerase activity has been reported to be related to life-span, we hypothesized that this polymorphism might affect human longevity by controlling the length of the telomere. To test this hypothesis, we collected 446 unrelated pericentenarian individuals (age[Symbol: see text]90, mean 94.45±3.45 years) and 332 normal controls (age 22-53, mean 35.0±12.0 years) from Dujiangyan, Sichuan, China. We typed the MNS16A polymorphism in both groups, and compared the allele and genotype frequencies between the peri-centenarian and control groups using the chi-squared test. There was no significant difference between the peri-centenarian and control groups. Thus, the MNS16A polymorphism in TERT might not influence human life-span, at least in the Han Chinese population studied here.

Multiple tumor suppressor microRNAs regulate telomerase and TCF7, an important transcriptional regulator of the Wnt pathway

The human TERT (hTERT) gene encodes the telomerase catalytic subunit which plays a role in telomerase regulation. Telomerase is activated in more than 90% of all human malignancies and understanding how telomerase is regulated is necessary for implementation of successful anti-cancer therapies. microRNAs (miRNAs) are important regulators of gene expression in eukaryotic cells but evidence of their role in telomerase regulation has not been documented. To determine whether hTERT activity is regulated by multiple miRNAs, eight miRNAs which have putative binding sites in the hTERT 3'UTR together with miR-138-5p were evaluated in luciferase assays with a reporter containing the hTERT 3'UTR. Six miRNAs (let-7g*, miR-133a, miR-138-5p, miR-342-5p, miR-491-5p, and miR-541-3p) specifically inhibited the expression of the reporter luciferase-driven constructs and let-7g*, miR-133a, miR-138-5p, and miR-491-5p also downregulated endogenous telomerase activity in cells. Moreover, all six miRNAs significantly inhibited cell proliferation. miRNAs (miR-133a, miR-138-5p, 342-5p, 491-5p, 541-3p) also have predicted binding sites within the 3'UTR of three genes involved in Wnt signaling (TCF7, MSI1, and PAX5). These miRNAs inhibited the expression of the luciferase reporter constructs containing 3'UTRs of these genes and downregulated protein expression of the TCF7 transcription factor, which mediates the canonical Wnt pathway. Together, these results suggest the existence of a miRNA regulatory network involving the hTERT and Wnt pathway.

Aging and HIV/AIDS: pathogenetic role of therapeutic side effects

The intersection of aging and HIV/AIDS is a looming 'epidemic within an epidemic.' This paper reviews how HIV/AIDS and its therapy cause premature aging or contribute mechanistically to HIV-associated non-AIDS illnesses (HANA). Survival with HIV/AIDS has markedly improved by therapy combinations containing nucleoside reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors, and protease inhibitors (PIs) called HAART (highly active antiretroviral therapy). Because NRTIs and PIs together prevent or attenuate HIV-1 replication, and prolong life, the population of aging patients with HIV/AIDS increases accordingly. However, illnesses frequently associated with aging in the absence of HIV/AIDS appear to occur prematurely in HIV/AIDS patients. Theories that help to explain biological aging include oxidative stress (where mitochondrial oxidative injury exceeds antioxidant defense), chromosome telomere shortening with associated cellular senescence, and accumulation of lamin A precursors (a nuclear envelop protein). Each of these has the potential to be enhanced or caused by HIV/AIDS, antiretroviral therapy, or both. Antiretroviral therapy has been shown to enhance events seen in biological aging. Specifically, antiretroviral NRTIs cause mitochondrial dysfunction, oxidative stress, and mitochondrial DNA defects that resemble features of both HANA and aging. More recent clinical evidence points to telomere shortening caused by NRTI triphosphate-induced inhibition of telomerase, suggesting telomerase reverse transcriptase (TERT) inhibition as being a pathogenetic contributor to premature aging in HIV/AIDS. PIs may also have a role in premature aging in HIV/AIDS as they cause prelamin A accumulation. Overall, toxic side effects of HAART may both resemble and promote events of aging and are worthy of mechanistic studies.

Telomere-independent ageing in the longest-lived non-colonial animal, Arctica islandica

The shortening of telomeres as a causative factor in ageing is a widely discussed hypothesis in ageing research. The study of telomere length and its regenerating enzyme telomerase in the longest-lived non-colonial animal on earth, Arctica islandica, should inform whether the maintenance of telomere length plays a role in reaching the extreme maximum lifespan (MLSP) of >500years in this species. Since longitudinal measurements on living animals cannot be achieved, a cross-sectional analysis of a short-lived (MLSP 40years from the Baltic Sea) and a long-lived population (MLSP 226years Northeast of Iceland) and in different tissues of young and old animals from the Irish Sea was performed. A high heterogeneity of telomere length was observed in investigated A. islandica over a wide age range (10-36years for the Baltic Sea, 11-194years for Irish Sea, 6-226years for Iceland). Constant telomerase activity and telomere lengths were detected at any age and in different tissues; neither correlated with age or population habitat. Stable telomere maintenance might contribute to the long lifespan of A. islandica. Telomere dynamics are no explanation for the distinct MLSPs of the examined populations and thus the cause of it remains to be investigated.

Telomerase contributes to fludarabine resistance in primary human leukemic lymphocytes

We report that Imetelstat, a telomerase inhibitor that binds to the RNA component of telomerase (hTR), can sensitize primary CLL lymphocytes to fludarabine in vitro. This effect was observed in lymphocytes from clinically resistant cases and with cytogenetic abnormalities associated with bad prognosis. Imetelstat mediated-sensitization to fludarabine was not associated with telomerase activity, but with the basal expression of Ku80. Since both Imetelstat and Ku80 bind hTR, we assessed 1) if Ku80 and Imetelstat alter each other's binding to hTR in vitro and 2) the effect of an oligonucleotide complementary to the Ku binding site in hTR (Ku oligo) on the survival of primary CLL lymphocytes exposed to fludarabine. We show that Imetelstat interferes with the binding of Ku70/80 (Ku) to hTR and that the Ku oligo can sensitize CLL lymphocytes to FLU. Our results suggest that Ku binding to hTR may contribute to fludarabine resistance in CLL lmphocytes. This is the first report highlighting the potentially broad effectiveness of Imetelstat in CLL, and the potential biological and clinical implications of a functional interaction between Ku and hTR in primary human cancer cells.

Telomerase inhibition may contribute to accelerated mitochondrial aging induced by anti-retroviral HIV treatment

HIV-infected individuals undergoing long-term anti-retroviral treatment tend to show premature senescence. Accelerated mitochondrial aging induced by nucleoside reverse transcriptase inhibitors (NRTIs) has been implicated as a part of this phenomenon. Traditionally, this has been attributed to inhibition of mtDNA polymerase γ by these drugs, but alternative explanations have been proposed. It is known that NRTIs can not only inhibit viral reverse transcriptase, but also human telomerase. A number of extratelomeric roles of telomerase, including protection of mitochondrial DNA and function, have emerged recently. In this paper, I propose that inhibition of mitochondrial telomerase activity by NRTI drugs contributes to the mitochondrial toxicity and premature aging seen in treated HIV patients, and discuss objections and experimental testing of the hypothesis.

Telomerase reverse transcriptase promotes the proliferation of human laryngeal carcinoma cells through activation of the activator protein 1

TERT is the main functional unit of telomerase, which maintains telomere length and chromosome structure stability. TERT has been shown to act as a key factor in various biological processes, such as cell proliferation, via uncharacterized mechanisms. We transfected HEp-2 laryngeal carcinoma cells with a TERT overexpressing adenovirus (Ad-TERT) and TERT shRNA silencing adenovirus (Ad-sh-TERT), and examined the effect on TERT and the AP-1 transcription factor subunits c-Fos and c-Jun using RT-PCR and western blot analysis. TERT mRNA expression was quantified using RT-PCR in 24 human laryngeal carcinoma samples, and TERT protein co-expression with AP-1 was investigated in a human laryngeal carcinoma tissue microarray using quantum-dot based immunofluorescence. The effect of specific ERK and p38 inhibitors on ERK, p38, c-Jun and c-Fos phosphorylation was investigated in TERT-overexpressing HEp-2 cells. TERT overexpression led to increased TERT, c-Jun and c-Fos mRNA and protein expression and increased cell proliferation, while TERT silencing had the opposite effects. TERT mRNA expression levels were positively correlated with c-Fos and c-Jun mRNA in human laryngeal carcinoma tissue. TERT and AP-1 protein were expressed at high levels and positively correlated in laryngeal carcinoma tissues. Treatment of TERT-overexpressing HEp-2 cells with specific p38 and ERK inhibitors indicated that TERT modulates the expression and phosphorylation of the AP-1 subunits c-Jun and c-Fos through the p38 and ERK signaling pathways. In conclusion, the results of this study indicate that TERT is capable of promoting cell proliferation via activation of the AP-1 subunits, c-Jun and c-Fos, in laryngeal carcinoma cells.

The TERT MNS16A polymorphism contributes to cancer susceptibility: meta-analysis of the current studies

The MNS16A polymorphism in the telomerase reverse transcriptase (TERT) gene has been implicated in cancer risk in multiple populations and various types of cancers; however, the results of previous studies exploring this association were inconclusive. To more precisely evaluate the relationship between the TERT MNS16A polymorphism and cancer risk, we performed a meta-analysis based on 8 studies described in 7 articles comprising 7864 controls and 4355 cases. The summary odds ratios (ORs) and their corresponding 95% confidence intervals (CIs) were estimated to assess the strength of the association in a fixed-effects model or a random-effects model where appropriate. Heterogeneity among articles and their publication bias were also tested. Overall, the pooled results indicated that the MNS16A polymorphism was significantly associated with increased cancer risk in the homozygote comparison model (SS vs. LL: OR=1.280, 95% CI 1.060-1.547) and the recessive model (SS vs. LL+SL: OR=1.201, 95% CI 1.004-1.436). In the stratified analyses, a statistically significant association was observed among Caucasians and in population-based studies. We also performed the analyses by cancer type, and a significantly increased risk of glioma was found in four genetic models. Our results suggest that the TERT MNS16A polymorphism most likely contributes to an increased risk of cancer. Moreover, the same relationship was found when the studies were stratified by cancer type, ethnicity and source of controls.

HTERT MNS16A polymorphism in breast cancer: a case-control study

This case-control study aims to investigate the role of HTERT MNS16A polymorphism as a potential risk factors and/or a prognostic marker for breast cancer. 113 consecutive incident cases of histologically confirmed ductal breast cancer and 124 healthy controls were recruited. HTERT MNS16A polymorphism was genotyped (L: long allele, S: short allele); multivariate logistic regression was performed. No significant association was noted either at the overall analysis (OR = 1.57, 95 % CI 0.84-2.93 for heterozygous LS carriers; OR = 1.02, 95 % CI 0.54-1.95 for homozygous SS carriers) or at the subanalyses in premenopausal and postmenopausal women. With respect to survival analysis, HTERT MNS16A polymorphism was not associated with either disease-free survival or overall survival. HTERT MNS16A polymorphism does not seem to be a risk factor for breast cancer in the Caucasian Greek population. Further, larger studies from other countries and subjects seem to be needed as this novel polymorphism is being examined in depth.

Guaianolide sesquiterpene lactones, a source to discover agents that selectively inhibit acute myelogenous leukemia stem and progenitor cells

Small molecules that can selectively target cancer stem cells (CSCs) remain rare currently and exhibit no common structural features. Here we report a series of guaianolide sesquiterpene lactones (GSLs) and their derivatives that can selectively eradicate acute myelogenous leukemia (AML) stem or progenitor cells. Natural GSL compounds arglabin, an anticancer clinical drug, and micheliolide (MCL), are able to reduce the proportion of AML stem cells (CD34⁺CD38⁻) in primary AML cells. Targeting of AML stem cells is further confirmed by a sharp reduction of colony-forming units of primary AML cells upon MCL treatment. Moreover, DMAMCL, the dimethylamino Michael adduct of MCL, slowly releases MCL in plasma and in vivo and demonstrates remarkable therapeutic efficacy in the nonobese diabetic/severe combined immunodeficiency AML models. These findings indicate that GSL is an ample source for chemical agents against AML stem or progenitor cells and that GSL is potentially highly useful to explore anti-CSC approaches.

Telomere-independent functions of telomerase in nuclei, cytoplasm, and mitochondria

Telomerase canonical activity at telomeres prevents telomere shortening, allowing chromosome stability and cellular proliferation. To perform this task, the catalytic subunit (telomerase reverse transcriptase, TERT) of the enzyme works as a reverse transcriptase together with the telomerase RNA component (TERC), adding telomeric repeats to DNA molecule ends. Growing evidence indicates that, besides the telomeric-DNA synthesis activity, TERT has additional functions in tumor development and is involved in many different biological processes, among which cellular proliferation, gene expression regulation, and mitochondrial functionality. TERT has been shown to act independently of TERC in the Wnt-β-catenin signaling pathway, regulating the expression of Wnt target genes, which play a role in development and tumorigenesis. Moreover, TERT RNA-dependent RNA polymerase activity has been found, leading to the genesis of double-stranded RNAs that act as precursor of silencing RNAs. In mitochondria, a TERT TERC-independent reverse transcriptase activity has been described that could play a role in the protection of mitochondrial integrity. In this review, we will discuss some of the extra-telomeric functions of telomerase.

Alternatively spliced telomerase reverse transcriptase variants lacking telomerase activity stimulate cell proliferation

Eight human and six chicken novel alternatively spliced (AS) variants of telomerase reverse transcriptase (TERT) were identified, including a human variant (Δ4-13) containing an in-frame deletion which removed exons 4 through 13, encoding the catalytic domain of telomerase. This variant was expressed in telomerase-negative normal cells and tissues as well as in transformed telomerase-positive cell lines and cells which employ an alternative method to maintain telomere length. The overexpression of the Δ4-13 variant significantly elevated the proliferation rates of several cell types without enhancing telomerase activity, while decreasing the endogenous expression of this variant by use of small interfering RNA (siRNA) technology reduced cell proliferation. The expression of the Δ4-13 variant stimulated Wnt signaling. In chicken cells, AS TERT variants containing internal deletions or insertions that eliminated or reduced telomerase activity also enhanced cell proliferation. This is the first report that naturally occurring AS TERT variants which lack telomerase activity stimulate cell proliferation.

Insights into the evolution of mammalian telomerase: platypus TERT shares similarities with genes of birds and other reptiles and localizes on sex chromosomes

Background

The TERT gene encodes the catalytic subunit of the telomerase complex and is responsible for maintaining telomere length. Vertebrate telomerase has been studied in eutherian mammals, fish, and the chicken, but less attention has been paid to other vertebrates. The platypus occupies an important evolutionary position, providing unique insight into the evolution of mammalian genes. We report the cloning of a platypus TERT (OanTERT) ortholog, and provide a comparison with genes of other vertebrates.

Results

The OanTERT encodes a protein with a high sequence similarity to marsupial TERT and avian TERT. Like the TERT of sauropsids and marsupials, as well as that of sharks and echinoderms, OanTERT contains extended variable linkers in the N-terminal region suggesting that they were present already in basal vertebrates and lost independently in ray-finned fish and eutherian mammals. Several alternatively spliced OanTERT variants structurally similar to avian TERT variants were identified. Telomerase activity is expressed in all platypus tissues like that of cold-blooded animals and murine rodents. OanTERT was localized on pseudoautosomal regions of sex chromosomes X3/Y2, expanding the homology between human chromosome 5 and platypus sex chromosomes. Synteny analysis suggests that TERT co-localized with sex-linked genes in the last common mammalian ancestor. Interestingly, female platypuses express higher levels of telomerase in heart and liver tissues than do males.

Conclusions

OanTERT shares many features with TERT of the reptilian outgroup, suggesting that OanTERT represents the ancestral mammalian TERT. Features specific to TERT of eutherian mammals have, therefore, evolved more recently after the divergence of monotremes.

Differential expression of senescence and cell death factors in non-small cell lung and colorectal tumors showing telomere attrition

OBJECTIVE

The main aim of this work is to investigate the expression of factors related to senescence and cell death pathways in non-small cell lung cancers (NSCLCs) and colorectal cancers (CRCs) in relation to telomere status.

METHODS

We analyzed 158 tissue samples, 36 NSCLCs, 43 CRCs, and their corresponding control tissues obtained from patients submitted to surgery. Telomere function was evaluated by determining telomerase activity and telomere length. Expression of factors related to senescence, cell death pathways, transformation and tumorigenesis was investigated using arrays. Results were validated by real-time quantitative PCR.

RESULTS

Considering tumors with telomere shortening, expression for BNIP3, DAPK1, NDRG1, EGFR, and CDKN2A was significantly higher in NSCLC than in CRC, whereas TP53 was overexpressed in CRC with respect to NSCLC. Moreover, compared to nontumor samples, DAPK1, GADD45A, SHC1, and TP53 were downregulated in the group of NSCLCs with telomere shortening, and no significant differences were found in CRC.

CONCLUSIONS

In NSCLC, the failure of pathways which involve factors such as DAPK1, GADD45A, SHC1, and TP53, in response to short telomeres, could promote tumor progression. In CRC, the viability of these pathways in response to short telomeres could contribute to limiting tumorigenesis.

Methods of studying telomere damage induced by quadruplex-ligand complexes

The burgeoning knowledge about the structure of telomeres and the roles of various factors involved in telomere maintenance provides several possible targets for pharmacological intervention. To date the area that has received major attention regarding drug discovery is the targeting the telomeric G-quadruplex (G4) structure. G4 ligands were initially designed to counteract telomerase action at telomeres. Surprisingly, their antiproliferative effects can occur in telomerase negative cells and follow kinetics, which cannot be merely explained by telomere shortening, suggesting that these compounds affect other pathways, not necessarily related to telomere biology. Impressively, it has been shown that polyaromatic compounds featuring end-stacking binding properties trigger a strong DNA damage response at telomeres. This is typical of the telomere deprotection occurring during cellular senescence or upon telomere injury. It emerged that the G4-interacting agents are more than simple telomerase inhibitors and that their direct target is rather telomere than telomerase. This review summarizes the most valid experimental approaches for studying the pharmacological telomere damage induced by G4-ligand complexes.

Cellular transformation by human papillomaviruses: lessons learned by comparing high- and low-risk viruses

The oncogenic potential of papillomaviruses (PVs) has been appreciated since the 1930s yet the mechanisms of virally-mediated cellular transformation are still being revealed. Reasons for this include: a) the oncoproteins are multifunctional, b) there is an ever-growing list of cellular interacting proteins, c) more than one cellular protein may bind to a given region of the oncoprotein, and d) there is only limited information on the proteins encoded by the corresponding non-oncogenic PVs. The perspective of this review will be to contrast the activities of the viral E6 and E7 proteins encoded by the oncogenic human PVs (termed high-risk HPVs) to those encoded by their non-oncogenic counterparts (termed low-risk HPVs) in an attempt to sort out viral life cycle-related functions from oncogenic functions. The review will emphasize lessons learned from the cell culture studies of the HPVs causing mucosal/genital tract cancers.

Separation of telomerase functions by reverse genetics

The canonical function of the human telomerase protein (hTERT) is to synthesize telomeric DNA, but it has other biological activities, including enhancing cell proliferation, decreasing apoptosis, regulating DNA damage responses, and increasing cellular proliferative lifespan. The mechanistic relationships among these activities are not understood. We previously demonstrated that ectopic hTERT expression in primary human mammary epithelial cells diminishes their requirement for exogenous mitogens, thus giving them a proliferative advantage in a mitogen-depleted environment. Here, we show that this phenotype is caused by a combination of increased cell division and decreased apoptosis. In addition, we use a panel of hTERT mutants to demonstrate that this enhanced cell proliferation can be uncoupled not only from telomere elongation, but also from other telomerase activities, including cellular lifespan extension and regulation of DNA damage responses. We also find that the proliferative function of hTERT, which requires hTERT catalytic activity, is not caused by increased Wnt signaling, but is accompanied by alterations in key cell cycle regulators and is linked to an hTERT-catalyzed decrease in the levels of the RNA component of mitochondrial RNA processing endoribonuclease. Thus, enhanced cell proliferation is an independent function of hTERT that could provide a new target for the development of anti-telomerase cancer therapeutic agents.

A functional tandem-repeats polymorphism in the downstream of TERT is associated with the risk of nasopharyngeal carcinoma in Chinese population

BACKGROUND

Increases in human telomerase reverse transcriptase (TERT) expression and telomerase activity are frequently seen in nasopharyngeal carcinoma (NPC). Recently, a variable tandem-repeats polymorphism, MNS16A, located in the downstream region of the TERT gene, was identified and reported to have an effect on TERT expression and telomerase activity. We examined whether the functional MNS16A was related to the risk of occurrence or progression of NPC in the Chinese population.

METHODS

We genotyped the MNS16A polymorphism in a case-control study of 855 patients with NPC and 1036 cancer-free controls using PCR, and determined genotype by classifying the DNA band of 243 or 272 base pairs (bp) as the short (S) allele and 302 or 333 bp as the long (L) allele. The genetic associations with the risk of NPC were analyzed by logistic regression.

RESULTS

The MNS16A genotype was not associated with the progression of NPC. However, individuals carrying the S alleles (SL + SS genotype) had a significantly reduced risk of NPC occurrence compared with those carrying the LL genotype (odds ratio (OR) = 0. 71, 95% confidence interval (CI) = 0. 52 to 0. 96, P = 0. 025). Using a immunohistochemical assay on the NPC tissues, the SL genotype carriers were found to have lower TERT expression than the LL genotype carriers (P = 0. 035).

CONCLUSIONS

Our study indicates that the TERT MNS16A polymorphism may contribute to the risk of NPC onset in Chinese population.

Human telomerase reverse transcriptase regulates cyclin D1 and G1/S phase transition in laryngeal squamous carcinoma

CONCLUSION

Down-regulating human telomerase reverse transcriptase (hTERT) expression will significantly suppress the cell viability of laryngeal squamous cell carcinoma Hep-2, which was mainly due to the inhibition of cyclin D1 and thus G1/S phase transition.

OBJECTIVE

Small-interfering RNA (siRNA) targeting hTERT can arrest the cell cycle of cancer cells, as well as inhibit telomerase activity and cell viability. However, the precise mechanisms still remain unclear. Here, we investigate the regulatory role of hTERT in cyclin D1 in laryngeal squamous carcinoma.

METHODS

Short hairpin RNAs (shRNAs) specifically targeting hTERT were constructed and expressed in Hep-2 cells. Cell proliferation was measured by CCK-8 assay. Expression of hTERT, cyclin D1, cyclin E, c-myc, and GAPDH was detected by RT-PCR and Western blot; cyclin D1 and hTERT proteins in laryngeal squamous carcinoma tissue microarray were analyzed by quantum dots immunofluorescence.

RESULTS

hTERT silence by shRNAs decreased the proliferation of Hep-2 cells by 76.8% at day 4 (96 h). Furthermore, transfection with hTERT shRNA for 48 h also significantly reduced expression of hTERT, cyclin D1, and c-Myc, but not cyclin E. Quantum dots immunofluorescence analysis of 36 laryngeal squamous carcinoma tissue samples found that hTERT expression was highly correlated with cyclin D1 expression.

The p53/p21(WAF/CIP) pathway mediates oxidative stress and senescence in dyskeratosis congenita cells with telomerase insufficiency

Telomere attrition is a natural process that occurs due to inadequate telomere maintenance. Once at a critically short threshold, telomeres signal growth arrest, leading to senescence. Telomeres can be elongated by the enzyme telomerase, which adds de novo telomere repeats to the ends of chromosomes. Mutations in genes for telomere binding proteins or components of telomerase give rise to the premature aging disorder dyskeratosis congenita (DC), which is characterized by extremely short telomeres and an aging phenotype. The current study demonstrates that DC cells signal a DNA damage response through p53 and its downstream mediator, p21(WAF/CIP), which is accompanied by an elevation in steady-state levels of superoxide and percent glutathione disulfide, both indicators of oxidative stress. Poor proliferation of DC cells can be partially overcome by reducing O(2) tension from 21% to 4%. Further, restoring telomerase activity or inhibiting p53 or p21(WAF/CIP) significantly mitigated growth inhibition as well as caused a significant decrease in steady-state levels of superoxide. Our results support a model in which telomerase insufficiency in DC leads to p21(WAF/CIP) signaling, via p53, to cause increased steady-state levels of superoxide, metabolic oxidative stress, and senescence.

hTERT overexpression alleviates intracellular ROS production, improves mitochondrial function, and inhibits ROS-mediated apoptosis in cancer cells

The human telomerase reverse transcriptase (hTERT) is the catalytic subunit of the telomerase holoenzyme. Evidence is accumulating to link hTERT to activities other than telomere maintenance and immortalization. Here, we show that hTERT overexpression not only reduces the basal cellular reactive oxygen species (ROS) levels but also inhibits endogenous ROS production in response to stimuli that induce intracellular ROS generation. Conversely, siRNA-mediated gene silencing of hTERT potentiated the increase in cellular ROS levels following exposure to oxidative stress. This antioxidant effect of hTERT is mediated via a significant increase in the ratio of reduced to oxidized glutathione (GSH:GSSG) as well as efficient recovery of the oxidized peroxiredoxin to its nonoxidized form. Our data also provide evidence for mitochondrial localization of hTERT, and a significantly higher activity of cytochrome C oxidase, the rate-limiting enzyme in the mitochondrial electron transport chain, in hTERT overexpressing cells. To ascertain whether the improved mitochondrial function and antioxidant effect of hTERT could provide cancer cells with a survival advantage, the effect of oxidative stress on mitochondrial apoptosis was evaluated. Indeed, hTERT overexpressing cells inhibited cytosolic acidification, translocation of Bax, the drop in mitochondrial transmembrane potential, the release of cytochrome C to the cytosol, and cell death. Taken together, these data demonstrate a hitherto undefined role of hTERT in alleviating cellular ROS levels by way of potentiating the cellular antioxidant defense systems, and in doing so endowing cancer cells with the ability to evade death stimuli.

The emerging role of telomerase reverse transcriptase in mitochondrial DNA metabolism

Telomerase is a reverse transcriptase specialized in telomere synthesis. The enzyme is primarily nuclear where it elongates telomeres but recent reports have shown that it also localizes to mitochondria. The function of TERT in mitochondria is largely unknown but the available findings point to a role in mitochondrial DNA metabolism. This paper discusses the available data on mitochondrial telomerase with particular emphasis on its effects upon the organellar DNA.

Telomere/telomerase interplay in virus-driven and virus-independent lymphomagenesis: pathogenic and clinical implications

Telomerase is a ribonucleoprotein complex critically involved in extending and maintaining telomeres. Unlike the majority of somatic cells, in which hTERT and telomerase activity are generally silent, normal lymphocytes show transient physiological hTERT expression and telomerase activity according to their differentiation/activation status. During lymphomagenesis, induction of persistent telomerase expression and activity may occur before or after telomere shortening, as a consequence of the different mechanisms through which transforming factors/agents may activate telomerase. Available data indicate that the timing of telomerase activation may allow the distinction of two different lymphomagenetic models: (i) an early activation of telomerase via exogenous regulators of hTERT, along with an increased lymphocyte growth and a subsequent selection of cells with increased transforming potential may characterize several virus-related lymphoid malignancies; (ii) a progressive shortening of telomeres, leading to genetic instability which favors a subsequent activation of telomerase via endogenous regulators may occur in most virus-unrelated lymphoid tumors. These models may have clinically relevant implications, particularly for the tailoring of therapeutic strategies targeting telomerase.

The load of short telomeres, estimated by a new method, Universal STELA, correlates with number of senescent cells

Short telomeres are thought to trigger senescence, most likely through a single - or a group of few - critically shortened telomeres. Such short telomeres are thought to result from a combination of gradual linear shortening resulting from the end replication problem, reflecting the division history of the cell, superimposed by a more stochastic mechanism, suddenly causing a significant shortening of a single telomere. Previously, studies that have tried to explore the role of critically shortened telomeres have been hampered by methodological problems. With the method presented here, Universal STELA, we have a tool that can directly investigate the relationship between senescence and the load of short telomeres. The method is a variant of the chromosome-specific STELA method but has the advantage that it can demonstrate short telomeres regardless of chromosome. With Universal STELA, we find a strong correlation between the load of short telomeres and cellular senescence. Further we show that the load of short telomeres is higher in senescent cells compared to proliferating cells at the same passage, offering an explanation of premature cell senescence. This new method, Universal STELA, offers some advantages compared to existing methods and can be used to explore many of the unanswered questions in telomere biology including the role that telomeres play in cancer and aging.

The telomere story or the triumph of an open-minded research

The Nobel Assembly at Karolinska Institute has decided to award The Nobel Prize in Physiology or Medicine 2009 jointly to Elizabeth H. Blackburn, Carol W. Greider and Jack W. Szostak for the discovery of "how chromosomes are protected by telomeres and the enzyme telomerase". This discovery had major impacts within the scientific community and led to intense research in this field. All the studies performed are now the bases for future investigations and stimulate the development of potential new therapies.

Telomeres and immunological diseases of aging

A defining feature of the eukaryotic genome is the presence of linear chromosomes. This arrangement, however, poses several challenges with regard to chromosomal replication and maintenance. To prevent the loss of coding sequences and to suppress gross chromosomal rearrangements, linear chromosomes are capped by repetitive nucleoprotein structures, called telomeres. Each cell division results in a progressive shortening of telomeres that, below a certain threshold, promotes genome instability, senescence, and apoptosis. Telomeric erosion, maintenance, and repair take center stage in determining cell fate. Cells of the immune system are under enormous proliferative demand, stressing telomeric intactness. Lymphocytes are capable of upregulating telomerase, an enzyme that can elongate telomeric sequences and, thus, prolong cellular lifespan. Therefore, telomere dynamics are critical in preserving immune function and have become a focus for studies of immunosenescence and autoimmunity. In this review, we describe the role of telomeric nucleoproteins in shaping telomere architecture and in suppressing DNA damage responses. We summarize new insights into the regulation of telomerase activity, hereditary disorders associated with telomere dysfunction, the role of telomere loss in immune aging, and the impact of telomere dysfunction in chronic inflammatory disease.

Necdin: a multi functional protein with potential tumor suppressor role?

Necdin (NDN), a member of the melanoma-associated antigen (MAGE) family of proteins was first identified in mouse stem cells of embryonal carcinoma origin induced to differentiate by treatment with retinoic acid. The human gene maps to chromosome 15q11. This imprinted region is implicated in the pathogenesis of Prader-Willi syndrome (PWS), a neurodevelopmental disorder, where NDN is one of multiple genes silenced by deletion, maternal uniparental disomy or translocation. Due to this association, much interest has focused on the role of NDN in neuronal development and differentiation. However, a considerable number of studies have identified additional functions of NDN. Taken together these studies suggest a pleiotropic protein with diverse functions some of which may be relevant to tumorigenesis. Downregulation of NDN occurs in carcinoma cell lines and primary tumors, suggesting a tumor suppressor role. Our working hypothesis is that NDN is a worthy candidate for further studies with regard to a potential tumor suppressor role. In this article we outline the considerable evidence supporting the hypothesis that NDN has multiple functions, some of which indicate that it could be a tumor suppressor. The roles of NDN in key processes such as interaction with p53 and E2F-1, hematopoietic stem cell quiescence, transcriptional repression, angiogenesis, differentiation and interaction with the polycomb group gene BMI1 are discussed. Confirmation of NDN as a tumor suppressor may have implications for monitoring of PWS patients and could present a novel cancer therapeutic target.

4-hydroxynonenal, a lipid peroxidation product of dietary polyunsaturated fatty acids, has anticarcinogenic properties in colon carcinoma cell lines through the inhibition of telomerase activity

The effects of polyunsaturated fatty acids (PUFAs) obtained from the diet on colorectal cancer have been widely explored. However, controversial results have been obtained about the role played by the lipid peroxidation products of PUFAs, such as 4-hydroxy-nonenal (HNE), in the control of colon cancer growth. This aldehyde, indeed, showed both procarcinogenic and protective effects. In an attempt to verify the action of HNE, we studied the effects of a low dose of HNE (1 microM), similar to those "physiologically" found in normal cells and plasma, on telomerase activity, a key parameter of malignant transformation. Caco-2 cells were exposed to HNE and, paralleling cell growth inhibition, we observed the down-regulation of telomerase activity and hTERT expression. Similar effects have also been observed in HT-29 cells, in which HNE inhibited cell proliferation, telomerase activity and hTERT expression, suggesting that the inhibition of telomerase activity could be a general mechanism involved in the antiproliferative effect exerted by this aldehyde. Finally, we elucidated the mechanism of hTERT inhibition by HNE. A reduction of GSH content preceded the decrease of telomerase activity, but this only partially explained the telomerase activity inhibition. The major mechanism of HNE action seems to be the modulation of expression and activity of transcription factors belonging to the Myc/Mad/Max network. Since the presence of PUFAs in the diet exposes epithelial colon cells to HNE, this aldehyde could contribute to cell growth control through the inhibitory action on telomerase activity and hTERT expression, suggesting a protective effect on colon mucosa.

Structures of telomerase subunits provide functional insights

BACKGROUND

Telomerase continues to generate substantial attention both because of its pivotal roles in cellular proliferation and aging and because of its unusual structure and mechanism. By replenishing telomeric DNA lost during the cell cycle, telomerase overcomes one of the many hurdles facing cellular immortalization. Functionally, telomerase is a reverse transcriptase, and it shares structural and mechanistic features with this class of nucleotide polymerases. Telomerase is a very unusual reverse transcriptase because it remains stably associated with its template and because it reverse transcribes multiple copies of its template onto a single primer in one reaction cycle.

SCOPE OF REVIEW

Here, we review recent findings that illuminate our understanding of telomerase. Even though the specific emphasis is on structure and mechanism, we also highlight new insights into the roles of telomerase in human biology.

GENERAL SIGNIFICANCE

Recent advances in the structural biology of telomerase, including high resolution structures of the catalytic subunit of a beetle telomerase and two domains of a ciliate telomerase catalytic subunit, provide new perspectives into telomerase biochemistry and reveal new puzzles.

The promise of telomere length, telomerase activity and its regulation in the translocation-dependent cancer ESFT; clinical challenges and utility

The Ewing's sarcoma family of tumours (ESFT) are diagnosed by EWS-ETS gene translocations. The resulting fusion proteins play a role in both the initiation and maintenance of these solid aggressive malignant tumours, suppressing cellular senescence and increasing cell proliferation and survival. EWS-ETS fusion proteins have altered transcriptional activity, inducing expression of a number of different target genes including telomerase. Up-regulation of hTERT is most likely responsible for the high levels of telomerase activity in primary ESFT, although telomerase activity and expression of hTERT are not predictive of outcome. However levels of telomerase activity in peripheral blood may be useful to monitor response to some therapeutics. Despite high levels of telomerase activity, telomeres in ESFT are frequently shorter than those of matched normal cells. Uncertainty about the role that telomerase and regulators of its activity play in the maintenance of telomere length in normal and cancer cells, and lack of studies examining the relationship between telomerase activity, regulators of its activity and their clinical significance in patient samples have limited their introduction into clinical practice. Studies in clinical samples using standardised assays are critical to establish how telomerase and regulators of its activity might best be exploited for patient benefit.