The emerging role of telomerase reverse transcriptase in mitochondrial DNA metabolism

Telomerase inhibitors induce mitochondrial oxidation and DNA damage-dependent cell death rescued by Bcl-2/Bcl-xL

BACKGROUND

Reactivation of telomerase is a hallmark of cancer and the majority of cancers over-express telomerase. Telomerase-dependent telomere length maintenance confers immortality to cancer cells. However, telomere length-independent cell survival functions of telomerase also play a critical role in tumorigenesis. Multiple telomerase inhibitors have been developed as therapeutics and include anti-sense oligonucleotides, telomerase RNA component targeting agents, chemical inhibitors of telomerase, small molecule inhibitors of hTERT, and telomerase vaccine. In general, telomerase inhibitors affect cell proliferation and survival of cells depending on the telomere length reduction, culminating in replicative senescence or cell death by crisis. However, most telomerase inhibitors kill cancer cells prior to significant reduction in telomere length, suggesting telomere length independent role of telomerase in early telomere dysfunction-dependent cell death.

METHODS

In this study, we explored the mechanism of cell death induced by three prominent telomerase inhibitors utilizing a series of genetically encoded sensor cells including redox and DNA damage sensor cells.

RESULTS

We report that telomerase inhibitors induce early cell cycle inhibition, followed by redox alterations at cytosol and mitochondria. Massive mitochondrial oxidation and DNA damage induce classical cell death involving mitochondrial transmembrane potential loss and mitochondrial permeabilization. Real-time imaging of the progression of mitochondrial oxidation revealed that treated cells undergo a biphasic mitochondrial redox alteration during telomerase inhibition, emphasizing the potential role of telomerase in the redox regulation at mitochondria. Additionally, silencing of hTERT confirmed its predominant role in maintaining mitochondrial redox homeostasis. Interestingly, the study also demonstrated that anti-apoptotic Bcl-2 family proteins still confer protection against cell death induced by telomerase inhibitors.

CONCLUSION

The study demonstrates that redox alterations and DNA damage contribute to early cell death by telomerase inhibitors and anti-apoptotic Bcl-2 family proteins confer protection from cell death by their ability to safeguard mitochondria from oxidation damage.

Resveratrol-Mediated Regulation of Mitochondria Biogenesis-associated Pathways in Neurodegenerative Diseases: Molecular Insights and Potential Therapeutic Applications

Neurodegenerative disorders include different neurological conditions that affect nerve cells, causing the progressive loss of their functions and ultimately leading to loss of mobility, coordination, and mental functioning. The molecular mechanisms underpinning neurodegenerative disease pathogenesis are still unclear. Nonetheless, there is experimental evidence to demonstrate that the perturbation of mitochondrial function and dynamics play an essential role. In this context, mitochondrial biogenesis, the growth, and division of preexisting mitochondria, by controlling mitochondria number, plays a vital role in maintaining proper mitochondrial mass and function, thus ensuring efficient synaptic activity and brain function. Mitochondrial biogenesis is tightly associated with the control of cell division and variations in energy demand in response to extracellular stimuli; therefore, it may represent a promising therapeutic target for developing new curative approaches to prevent or counteract neurodegenerative disorders. Accordingly, several inducers of mitochondrial biogenesis have been proposed as pharmacological targets for treating diverse central nervous system conditions. The naturally occurring polyphenol resveratrol has been shown to promote mitochondrial biogenesis in various tissues, including the nervous tissue, and an ever-growing number of studies highlight its neurotherapeutic potential. Besides preventing cognitive impairment and neurodegeneration through its antioxidant and anti-inflammatory properties, resveratrol has been shown to be able to enhance mitochondria biogenesis by acting on its main effectors, including PGC-1α, SIRT1, AMPK, ERRs, TERT, TFAM, NRF-1 and NRF-2. This review aims to present and discuss the current findings concerning the impact of resveratrol on the machinery and main effectors modulating mitochondrial biogenesis in the context of neurodegenerative diseases.

The Role of Telomerase in Breast Cancer's Response to Therapy

Currently, breast cancer appears to be the most widespread cancer in the world and the most common cause of cancer deaths. This specific type of cancer affects women in both developed and developing countries. Prevention and early diagnosis are very important factors for good prognosis. A characteristic feature of cancer cells is the ability of unlimited cell division, which makes them immortal. Telomeres, which are shortened with each cell division in normal cells, are rebuilt in cancer cells by the enzyme telomerase, which is expressed in more than 85% of cancers (up to 100% of adenocarcinomas, including breast cancer). Telomerase may have different functions that are related to telomeres or unrelated. It has been shown that high activity of the enzyme in cancer cells is associated with poor cell sensitivity to therapies. Therefore, telomerase has become a potential target for cancer therapies. The low efficacy of therapies has resulted in the search for new combined and more effective therapeutic methods, including the involvement of telomerase inhibitors and telomerase-targeted immunotherapy.

Noncanonical Role of Telomerase in Regulation of Microvascular Redox Environment With Implications for Coronary Artery Disease

Telomerase reverse transcriptase (TERT) (catalytic subunit of telomerase) is linked to the development of coronary artery disease (CAD); however, whether the role of nuclear vs. mitchondrial actions of TERT is involved is not determined. Dominant-negative TERT splice variants contribute to decreased mitochondrial integrity and promote elevated reactive oxygen species production. We hypothesize that a decrease in mitochondrial TERT would increase _mtDNA damage, promoting a pro-oxidative redox environment. The goal of this study is to define whether mitochondrial TERT is sufficient to maintain nitric oxide as the underlying mechanism of flow-mediated dilation by preserving _mtDNA integrity.Immunoblots and quantitative polymerase chain reaction were used to show elevated levels of splice variants α- and β-deletion TERT tissue from subjects with and without CAD. Genetic, pharmacological, and molecular tools were used to manipulate TERT localization. Isolated vessel preparations and fluorescence-based quantification of _mtH₂O₂ and NO showed that reduction of TERT in the nucleus increased flow induced NO and decreased _mtH₂O₂ levels, while prevention of mitochondrial import of TERT augmented pathological effects. Further elevated _mtDNA damage was observed in tissue from subjects with CAD and initiation of _mtDNA repair mechanisms was sufficient to restore NO-mediated dilation in vessels from patients with CAD. The work presented is the first evidence that catalytically active mitochondrial TERT, independent of its nuclear functions, plays a critical physiological role in preserving NO-mediated vasodilation and the balance of mitochondrial to nuclear TERT is fundamentally altered in states of human disease that are driven by increased expression of dominant negative splice variants.

Leukocyte Telomere Length Variability as a Potential Biomarker in Patients with PolyQ Diseases

SCA1, SCA2, and SCA3 are the most common forms of SCAs among the polyglutamine disorders, which include Huntington’s Disease (HD). We investigated the relationship between leukocyte telomere length (LTL) and the phenotype of SCA1, SCA2, and SCA3, comparing them with HD. The results showed that LTL was significantly reduced in SCA1 and SCA3 patients, while LTL was significantly longer in SCA2 patients. A significant negative relationship between LTL and age was observed in SCA1 but not in SCA2 subjects. LTL of SCA3 patients depend on both patient’s age and disease duration. The number of CAG repeats did not affect LTL in the three SCAs. Since LTL is considered an indirect marker of an inflammatory response and oxidative damage, our data suggest that in SCA1 inflammation is present already at an early stage of disease similar to in HD, while in SCA3 inflammation and impaired antioxidative processes are associated with disease progression. Interestingly, in SCA2, contrary to SCA1 and SCA3, the length of leukocyte telomeres does not reduce with age. We have observed that SCAs and HD show a differing behavior in LTL for each subtype, which could constitute relevant biomarkers if confirmed in larger cohorts and longitudinal studies.

Stress and telomere shortening: Insights from cellular mechanisms

Short telomeres confer risk of degenerative diseases. Chronic psychological stress can lead to disease through many pathways, and research from in vitro studies to human longitudinal studies has pointed to stress-induced telomere damage as an important pathway. However, there has not been a comprehensive model to describe how changes in stress physiology and neuroendocrine pathways can lead to changes in telomere biology. Critically short telomeres or the collapse of the telomere structure caused by displacement of telomere binding protein complex shelterin elicit a DNA damage response and lead to senescence or apoptosis. In this narrative review, we summarize the key roles glucocorticoids, reactive oxygen species (ROS) and mitochondria, and inflammation play in mediating the relationship between psychological stress and telomere maintenance. We emphasis that these mediators are interconnected and reinforce each other in positive feedback loops. Telomere length has not been studied across the lifespan yet, but the initial setting point at birth appears to be the most influential point, as it sets the lifetime trajectory, and is influenced by stress. We describe two types of intergenerational stress effects on telomeres - prenatal stress effects on telomeres during fetal development, and 'telotype transmission" -the directly inherited transmission of short telomeres from parental germline. It is clear that the initial simplistic view of telomere length as a mitotic clock has evolved into a far more complex picture of both transgenerational telomere influences, and of interconnected molecular and cellular pathways and networks, as hallmarks of aging where telomere maintenance is a key player interacting with mitochondria. Further mechanistic investigations testing this comprehensive model of stress mediators shaping telomere biology and the telomere-mitochondrial nexus will lead to better understanding from cell to human lifespan aging, and could lead to anti-aging interventions.

GRPEL2 Knockdown Exerts Redox Regulation in Glioblastoma

Malignant brain tumors are responsible for catastrophic morbidity and mortality globally. Among them, glioblastoma multiforme (GBM) bears the worst prognosis. The GrpE-like 2 homolog (GRPEL2) plays a crucial role in regulating mitochondrial protein import and redox homeostasis. However, the role of GRPEL2 in human glioblastoma has yet to be clarified. In this study, we investigated the function of GRPEL2 in glioma. Based on bioinformatics analyses from the Cancer Gene Atlas (TCGA) and the Chinese Glioma Genome Atlas (CGGA), we inferred that GRPEL2 expression positively correlates with WHO tumor grade (p < 0.001), IDH mutation status (p < 0.001), oligodendroglial differentiation (p < 0.001), and overall survival (p < 0.001) in glioma datasets. Functional validation in LN229 and GBM8401 GBM cells showed that GRPEL2 knockdown efficiently inhibited cellular proliferation. Moreover, GRPEL2 suppression induced cell cycle arrest at the sub-G1 phase. Furthermore, GRPEL2 silencing decreased intracellular reactive oxygen species (ROS) without impending mitochondria membrane potential. The cellular oxidative respiration measured with a Seahorse XFp analyzer exhibited a reduction of the oxygen consumption rate (OCR) in GBM cells by siGRPEL2, which subsequently enhanced autophagy and senescence in glioblastoma cells. Taken together, GRPEL2 is a novel redox regulator of mitochondria bioenergetics and a potential target for treating GBM in the future.

Inhibition of c-Myc using 10058-F4 induces anti-tumor effects in ovarian cancer cells via regulation of FOXO target genes

Ovarian cancer, characterized by rapid growth and asymptomatic development in the early stage, is the fifth common cancer in women. The deregulated expression of c-Myc in more than 50% of human tumors including ovarian cancer makes this oncogenic master transcription factor a potential therapeutic target for cancer treatment. In the present study, we evaluated the anti-tumor effects of 10058-F4, a small molecule c-Myc inhibitor, on ovarian cancer cells. We found that 10058-F4 not only inhibited the proliferation and clonal growth of ovarian cancer cells but also enhanced the cytotoxic effects of chemotherapeutic drugs. Our results also revealed that c-Myc inhibition using 10058-F4 increased the intracellular reactive oxygen species production coupled with suppressed expression of hTERT. RT-qPCR analysis indicated that 10058-F4 enhanced the mRNA levels of the forkhead box O (FOXO) family of transcription factors, including FOXO1, 3, and 4. Moreover, 10058-F4 induced G1 cell cycle arrest in 2008C13 ovarian cancer cells, along with increased expression of some key targets of FOXOs involved in the regulation of cell cycle such as p15, p21, p27, and GADD45A. The results of our study also showed that the 10058-F4-induced apoptosis in 2008C13 cell line was associated with the upregulation of FOXO downstream genes, including PUMA, Bim, and FasL. In conclusion, our results, for the first time, suggest that the anti-tumor effects of 10058-F4 in ovarian cancer cells might be mediated through upregulation of FOXO transcription factors and their key target genes involved in G1 cell cycle arrest, apoptosis, and autophagic cell death.

Analysis of TERT Isoforms across TCGA, GTEx and CCLE Datasets

Reactivation of the multi-subunit ribonucleoprotein telomerase is the primary telomere maintenance mechanism in cancer, but it is rate-limited by the enzymatic component, telomerase reverse transcriptase (TERT). While regulatory in nature, TERT alternative splice variant/isoform regulation and functions are not fully elucidated and are further complicated by their highly diverse expression and nature. Our primary objective was to characterize TERT isoform expression across 7887 neoplastic and 2099 normal tissue samples using The Cancer Genome Atlas (TCGA) and the Genotype-Tissue Expression Project (GTEx), respectively. We confirmed the global overexpression and splicing shift towards full-length TERT in neoplastic tissue. Stratifying by tissue type we found uncharacteristic TERT expression in normal brain tissue subtypes. Stratifying by tumor-specific subtypes, we detailed TERT expression differences potentially regulated by subtype-specific molecular characteristics. Focusing on β-deletion splicing regulation, we found the NOVA1 trans-acting factor to mediate alternative splicing in a cancer-dependent manner. Of relevance to future tissue-specific studies, we clustered cancer cell lines with tumors from related origin based on TERT isoform expression patterns. Taken together, our work has reinforced the need for tissue and tumour-specific TERT investigations, provided avenues to do so, and brought to light the current technical limitations of bioinformatic analyses of TERT isoform expression.

Non-canonical functions of Telomerase Reverse Transcriptase - Impact on redox homeostasis

Telomerase consists of the catalytic subunit Telomerase Reverse Transcriptase (TERT) and the Telomerase RNA Component. Its canonical function is the prevention of telomere erosion. Over the last years it became evident that TERT is also present in tissues with low replicative potential. Important non-canonical functions of TERT are protection against apoptosis and maintenance of the cellular redox homeostasis in cancer as well as in somatic tissues. Intriguingly, TERT and reactive oxygen species (ROS) are interdependent on each other, with TERT being regulated by changes in the redox balance and itself controlling ROS levels in the cytosol and in the mitochondria. The latter is achieved because TERT is present in the mitochondria, where it protects mitochondrial DNA and maintains levels of anti-oxidative enzymes. Since numerous diseases are associated with oxidative stress, increasing the mitochondrial TERT level could be of therapeutic value.

Telomerase: Key regulator of inflammation and cancer

The telomerase holoenzyme, which has a highly conserved role in maintaining telomere length, has long been regarded as a high-profile target in cancer therapy due to the high dependency of the majority of cancer cells on constitutive and elevated telomerase activity for sustained proliferation and immortality. In this review, we present the salient findings in the telomerase field with special focus on the association of telomerase with inflammation and cancer. The elucidation of extra-telomeric roles of telomerase in inflammation, reactive oxygen species (ROS) generation, and cancer development further complicated the design of anti-telomerase therapy. Of note, the discovery of the unique mechanism that underlies reactivation of the dormant telomerase reverse transcriptase TERT promoter in somatic cells not only enhanced our understanding of the critical role of TERT in carcinogenesis but also opens up new intervention ideas that enable the differential targeting of cancer cells only. Despite significant effort invested in developing telomerase-targeted therapeutics, devising efficacious cancer-specific telomerase/TERT inhibitors remains an uphill task. The latest discoveries of the telomere-independent functionalities of telomerase in inflammation and cancer can help illuminate the path of developing specific anti-telomerase/TERT therapeutics against cancer cells.

The non-canonical functions of telomerase: to turn off or not to turn off

Telomerase is perceived as an immortality enzyme that enables passing the Hayflick limit. Its main function is telomere restoration but only in a limited group of cells, including cancer cells. Since it is found in a vast majority of cancer cells, it became a natural target for cancer therapy. However, it has much more functions than just altering the metabolism of telomeres-it also reveals numerous so-called non-canonical functions. Thus, a question arises whether it is always beneficial to turn it off when planning a cancer strategy and considering potential side effects? The purpose of this review is to discuss some of the recent discoveries about telomere-independent functions of telomerase in the context of cancer therapy and potential side effects.

TERT biology and function in cancer: beyond immortalisation

Evasion of replicative senescence and proliferation without restriction, sometimes designated as immortalisation, is one of the hallmarks of cancer that may be attained through reactivation of telomerase in somatic cells. In contrast to most normal cells in which there is lack of telomerase activity, upregulation of TERT transcription/activity is detected in 80-90% of malignant tumours. In several types of cancer, there is a relationship between the presence of TERT promoter mutations, TERT mRNA expression and clinicopathological features, but the biological bridge between the occurrence of TERT promoter mutations and the aggressive/invasive features displayed by the tumours remains unidentified. We and others have associated the presence of TERT promoter mutations with metastisation/survival in several types of cancer. In follicular cell-derived thyroid cancer, such mutations are associated with worse prognostic features (age of patients, tumour size and tumour stage) as well as with distant metastases, worse response to treatment and poorer survival. In this review, we analyse the data reported in several studies that imply TERT transcription reactivation/activity with cell proliferation, tumour invasion and metastisation. A particular attention is given to the putative connections between TERT transcriptional reactivation and signalling pathways frequently altered in cancer, such as c-MYC, NF-κB and B-Catenin.

HCV Induces Telomerase Reverse Transcriptase, Increases Its Catalytic Activity, and Promotes Caspase Degradation in Infected Human Hepatocytes

Introduction

Telomerase repairs the telomeric ends of chromosomes and is active in nearly all malignant cells. Hepatitis C virus (HCV) is known to be oncogenic and potential interactions with the telomerase system require further study. We determined the effects of HCV infection on human telomerase reverse transcriptase (TERT) expression and enzyme activity in primary human hepatocytes and continuous cell lines.

Results

Primary human hepatocytes and Huh-7.5 hepatoma cells showed early de novo TERT protein expression 2–4 days after infection and these events coincided with increased TERT promoter activation, TERT mRNA, and telomerase activity. Immunoprecipitation studies demonstrated that NS3-4A protease-helicase, in contrast to core or NS5A, specifically bound to the C-terminal region of TERT through interactions between helicase domain 2 and protease sequences. Increased telomerase activity was noted when NS3-4A was transfected into cells, when added to reconstituted mixtures of TERT and telomerase RNA, and when incubated with high molecular weight telomerase ‘holoenzyme’ complexes. The NS3-4A catalytic effect on telomerase was inhibited with primuline or danoprevir, agents that are known to inhibit NS3 helicase and protease activities respectively. In HCV infected cells, NS3-4A could be specifically recovered with telomerase holoenzyme complexes in contrast to NS5A or core protein. HCV infection also activated the effector caspase 7 which is known to target TERT. Activation coincided with the appearance of lower molecular weight carboxy-terminal fragment(s) of TERT, chiefly sized at 45 kD, which could be inhibited with pancaspase or caspase 7 inhibitors.

Conclusions

HCV infection induces TERT expression and stimulates telomerase activity in addition to triggering Caspase activity that leads to increased TERT degradation. These activities suggest multiple points whereby the virus can influence neoplasia. The NS3-4A protease-helicase can directly bind to TERT, increase telomerase activity, and thus potentially influence telomere repair and host cell neoplastic behavior.

Friend or foe? Telomerase as a pharmacological target in cancer and cardiovascular disease

Aging, cancer, and chronic disease have remained at the forefront of basic biological research for decades. Within this context, significant attention has been paid to the role of telomerase, the enzyme responsible for lengthening telomeres, the nucleotide sequences located at the end of chromosomes found in the nucleus. Alterations in telomere length and telomerase activity are a common denominator to the underlying pathology of these diseases. While nuclear-specific, telomere-lengthening effects of telomerase impact cellular/organismal aging and cancer development, non-canonical, extra-nuclear, and non-telomere-lengthening contributions of telomerase have only recently been described and their exact physiological implications are ill defined. Although the mechanism remains unclear, recent reports reveal that the catalytic subunit of telomerase, telomerase reverse transcriptase (TERT), regulates levels of mitochondrial-derived reactive oxygen species (mtROS), independent of its established role in the nucleus. Telomerase inhibition has been the target of chemotherapy (directed or indirectly) for over a decade now, yet no telomerase inhibitor is FDA approved and few are currently in late-stage clinical trials, possibly due to underappreciation of the distinct extra-nuclear functions of telomerase. Moreover, evaluation of telomerase-specific therapies is largely limited to the context of chemotherapy, despite reports of the beneficial effects of telomerase activation in the cardiovascular system in relation to such processes as endothelial dysfunction and myocardial infarction. Thus, there is a need for better understanding of telomerase-focused cell and organism physiology, as well as development of telomerase-specific therapies in relation to cancer and extension of these therapies to cardiovascular pathologies. This review will detail findings related to telomerase and evaluate its potential to serve as a therapeutic target.

Neuroprotective effects of the catalytic subunit of telomerase: A potential therapeutic target in the central nervous system

Senescence plays an important role in neurodegenerative diseases and involves key molecular changes induced by several mechanisms such as oxidative stress, telomere shortening and DNA damage. Potential therapeutic strategies directed to counteract these molecular changes are of great interest for the prevention of the neurodegenerative process. Telomerase is a ribonucleoprotein composed of a catalytic subunit (TERT) and a RNA subunit (TERC). It is known that the telomerase is involved in the maintenance of telomere length and is a highly expressed protein in embryonic stages and decreases in adult cells. In the last decade, a growing number of studies have shown that TERT has neuroprotective effects in cellular and animal models after a brain injury. Significantly, differences in TERT expression between controls and patients with major depressive disorder have been observed. More recently, TERT has been associated with the decrease in reactive oxygen species and DNA protection in mitochondria of neurons. In this review, we highlight the role of TERT in some neurodegenerative disorders and discuss some studies focusing on this protein as a potential target for neuroprotective therapies.

Conditional Knockout of Telomerase Reverse Transcriptase in Mesenchymal Cells Impairs Mouse Pulmonary Fibrosis

Telomerase is typically expressed in cellular populations capable of extended replication, such as germ cells, tumor cells, and stem cells, but is also induced in tissue injury, repair and fibrosis. Its catalytic component, telomerase reverse transcriptase (TERT) is induced in lung fibroblasts from patients with fibrotic interstitial lung disease and in rodents with bleomycin-induced pulmonary fibrosis. To evaluate the fibroblast specific role of TERT in pulmonary fibrosis, transgenic mice bearing a floxed TERT allele were generated, and then crossed with an inducible collagen α2(I)-Cre mouse line to generate fibroblast specific TERT conditional knockout mice. TERT-specific deficiency in mesenchymal cells caused attenuation of pulmonary fibrosis as manifested by reduced lung hydroxyproline content, type I collagen and α-smooth muscle actin mRNA levels. The TERT-deficient mouse lung fibroblasts displayed decreased cell proliferative capacity and higher susceptibility to induced apoptosis compared with control cells. Additionally TERT deficiency was associated with heightened α-smooth muscle actin expression indicative of myofibroblast differentiation. However the impairment of cell proliferation and increased susceptibility to apoptosis would cause a reduction in the myofibroblast progenitor population necessary to mount a successful myofibroblast-dependent fibrotic response. These findings identified a key role for TERT in fibroblast proliferation and survival essential for pulmonary fibrosis.

Telomerase regulates MYC-driven oncogenesis independent of its reverse transcriptase activity

Constitutively active MYC and reactivated telomerase often coexist in cancers. While reactivation of telomerase is thought to be essential for replicative immortality, MYC, in conjunction with cofactors, confers several growth advantages to cancer cells. It is known that the reactivation of TERT, the catalytic subunit of telomerase, is limiting for reconstituting telomerase activity in tumors. However, while reactivation of TERT has been functionally linked to the acquisition of several "hallmarks of cancer" in tumors, the molecular mechanisms by which this occurs and whether these mechanisms are distinct from the role of telomerase on telomeres is not clear. Here, we demonstrated that first-generation TERT-null mice, unlike Terc-null mice, show delayed onset of MYC-induced lymphomagenesis. We further determined that TERT is a regulator of MYC stability in cancer. TERT stabilized MYC levels on chromatin, contributing to either activation or repression of its target genes. TERT regulated MYC ubiquitination and proteasomal degradation, and this effect of TERT was independent of its reverse transcriptase activity and role in telomere elongation. Based on these data, we conclude that reactivation of TERT, a direct transcriptional MYC target in tumors, provides a feed-forward mechanism to potentiate MYC-dependent oncogenesis.

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).

Leishmania major telomerase TERT protein has a nuclear/mitochondrial eclipsed distribution that is affected by oxidative stress

In its canonical role the reverse transcriptase telomerase recovers the telomeric repeats that are lost during DNA replication. Other locations and activities have been recently described for the telomerase protein subunit TERT in mammalian cells. In the present work, using biochemistry, molecular biology, and electron microscopy techniques, we found that in the human parasite Leishmania major, TERT (and telomerase activity) shared locations between the nuclear, mitochondrial, and cytoplasmic compartments. Also, some telomerase activity and TERT protein could be found in ∼ 100-nm nanovesicles. In the mitochondrial compartment, TERT appears to be mainly associated with the kinetoplast DNA. When Leishmania cells were exposed to H2O2, TERT changed its relative abundance and activity between the nuclear and mitochondrial compartments, with the majority of activity residing in the mitochondrion. Finally, overexpression of TERT in Leishmania transfected cells not only increased the parasitic cell growth rate but also increased their resistance to oxidative stress.

Telomerase at the intersection of cancer and aging

Although cancer and aging have been studied as independent diseases, mounting evidence suggests that cancer is an aging-associated disease and that cancer and aging share many molecular pathways. In particular, recent studies validated telomerase activation as a potential therapeutic target for age-related diseases; in addition, abnormal telomerase expression and telomerase mutations have been associated with many different types of human tumor. Here, we revisit the connection between telomerase and cancer and aging in light of recent findings supporting a role for telomerase not only in telomere elongation, but also in metabolic fitness and Wnt activation. Understanding the physiological impact of telomerase regulation is fundamental given the therapeutic strategies that are being developed that involve telomerase modulation.

Telomerase directly regulates NF-κB-dependent transcription

Although elongation of telomeres is thought to be the prime function of reactivated telomerase in cancers, this activity alone does not account for all of the properties that telomerase reactivation attributes to human cancer cells. Here, we uncover a link between telomerase and NF-κB, a master regulator of inflammation. We observe that while blocking NF-κB signalling can inhibit effects of telomerase overexpression on processes relevant to transformation, increasing NF-κB activity can functionally substitute for reduced telomerase activity. Telomerase directly regulates NF-κB-dependent gene expression by binding to the NF-κB p65 subunit and recruitment to a subset of NF-κB promoters such as those of IL-6 and TNF-α, cytokines that are critical for inflammation and cancer progression. As NF-κB can transcriptionally upregulate telomerase levels, our findings suggest that a feed-forward regulation between them could be the key mechanistic basis for the coexistence of chronic inflammation and sustained telomerase activity in human cancers.

In vitro and ex vivo inhibition of human telomerase by anti-HIV nucleoside reverse transcriptase inhibitors (NRTIs) but not by non-NRTIs

Telomerase is a specialized reverse transcriptase responsible for the de novo synthesis of telomeric DNA repeats. In addition to its established reverse transcriptase and terminal transferase activities, recent reports have revealed unexpected cellular activities of telomerase, including RNA-dependent RNA polymerization. This telomerase characteristic, distinct from other reverse transcriptases, indicates that clinically relevant reverse transcriptase inhibitors might have unexpected telomerase inhibition profiles. This is particularly important for the newer generation of RT inhibitors designed for anti-HIV therapy, which have reported higher safety margins than older agents. Using an in vitro primer extension assay, we tested the effects of clinically relevant HIV reverse transcriptase inhibitors on cellular telomerase activity. We observed that all commonly used nucleoside reverse transcriptase inhibitors (NRTIs), including zidovudine, stavudine, tenofovir, didanosine and abacavir, inhibit telomerase effectively in vitro. Truncated telomere synthesis was consistent with the expected mode of inhibition by all tested NRTIs. Through dose-response experiments, we established relative inhibitory potencies of NRTIs on in vitro telomerase activity as compared to the inhibitory potencies of the corresponding dideoxynucleotide triphosphates. In contrast to NRTIs, the non-nucleoside reverse transcriptase inhibitors (NNRTIs) nevirapine and efavirenz did not inhibit the primer extension activity of telomerase, even at millimolar concentrations. Long-term, continuous treatment of human HT29 cells with select NRTIs resulted in an accelerated loss of telomere repeats. All tested NRTIs exhibited the same rank order of inhibitory potencies on telomerase and HIV RT, which, according to published data, were orders-of-magnitude more sensitive than other DNA polymerases, including the susceptible mitochondria-specific DNA polymerase gamma. We concluded that telomerase activity could be inhibited by common NRTIs, including currently recommended RTI agents tenofovir and abacavir, which warrants large-scale clinical and epidemiological investigation of the off-target effects of long-term highly active antiretroviral therapy (HAART) with these agents.

The long and short of it: the role of telomeres in fetal origins of adult disease

Placental insufficiency, maternal malnutrition, and other causes of intrauterine growth restriction (IUGR) can significantly affect short-term growth and long-term health. Following IUGR, there is an increased risk for cardiovascular disease and Type 2 Diabetes. The etiology of these diseases is beginning to be elucidated, and premature aging or cellular senescence through increased oxidative stress and DNA damage to telomeric ends may be initiators of these disease processes. This paper will explore the areas where telomere and telomerase biology can have significant effects on various tissues in the body in IUGR outcomes.

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.

Amplification of the telomerase RNA component gene in the process of human esophageal carcinogenesis

Amplification of the human telomerase RNA component (hTERC) gene occurs early in cervical cancer development. Telomerase, the product of the hTERC gene, plays an important role in tumor cell apoptosis and genomic stability. Given the numerous similarities between esophageal and cervical cancers, we hypothesized that hTERC gene amplification may also be related with esophageal cancer development. We therefore examined 189 tissue sections from 63 cases of esophageal cancer and preneoplastic lesions. hTERC gene amplification in the lesions was detected by interphase fluorescence in situ hybridization. Of the 189 tissue sections, 149 were successfully evaluated (40 samples were excluded because of inappropriately preparation) and were classified as normal (n=45), atypical hyperplasia I (n=27), atypical hyperplasia II/III (n=22), and squamous cell carcinomas (SCCs; the most common type of esophageal cancer) (n=55). hTERC gene expression was not detected in normal esophageal tissue, whereas its expression was detected in atypical hyperplasias I (25.9%), atypical hyperplasia II/III (54.5%), and SCCs (90.9%) (p<0.05). The average copy numbers of hTERC in atypical hyperplasias I and II/III, as well as SCCs were 2.19, 2.35, and 2.64, respectively. In particular, the numbers of abnormal nuclei in atypical hyperplasias II/III were significantly higher than those of in atypical hyperplasia I (p<0.05). The hTERC gene amplification was not related with patient gender, histological stage, lymph nodes metastasis, and SCC differentiation grade (p>0.05). All these findings suggest that hTERC gene amplification is associated with SCC development.

Aging and amyloid beta-induced oxidative DNA damage and mitochondrial dysfunction in Alzheimer's disease: implications for early intervention and therapeutics

Alzheimer's disease (AD) is an age-related progressive neurodegenerative disease affecting thousands of people in the world and effective treatment is still not available. Over two decades of intense research using AD postmortem brains, transgenic mouse and cell models of amyloid precursor protein and tau revealed that amyloid beta (Aβ) and hyperphosphorylated tau are synergistically involved in triggering disease progression. Accumulating evidence also revealed that aging and amyloid beta-induced oxidative DNA damage and mitochondrial dysfunction initiate and contributes to the development and progression of the disease. The purpose of this article is to summarize the latest progress in aging and AD, with a special emphasis on the mitochondria, oxidative DNA damage including methods of its measurement. It also discusses the therapeutic approaches against oxidative DNA damage and treatment strategies in AD.

Epithelial cell senescence impairs repair process and exacerbates inflammation after airway injury

Background

Genotoxic stress, such as by exposure to bromodeoxyuridine (BrdU) and cigarette smoke, induces premature cell senescence. Recent evidence indicates that cellular senescence of various types of cells is accelerated in COPD patients. However, whether the senescence of airway epithelial cells contributes to the development of airway diseases is unknown. The present study was designed to test the hypothesis that premature senescence of airway epithelial cells (Clara cells) impairs repair processes and exacerbates inflammation after airway injury.

Methods

C57/BL6J mice were injected with the Clara-cell-specific toxicant naphthalene (NA) on days 0, 7, and 14, and each NA injection was followed by a daily dose of BrdU on each of the following 3 days, during which regenerating cells were allowed to incorporate BrdU into their DNA and to senesce. The p38 MAPK inhibitor SB202190 was injected 30 minutes before each BrdU dose. Mice were sacrificed at different times until day 28 and lungs of mice were obtained to investigate whether Clara cell senescence impairs airway epithelial regeneration and exacerbates airway inflammation. NCI-H441 cells were induced to senesce by exposure to BrdU or the telomerase inhibitor MST-312. Human lung tissue samples were obtained from COPD patients, asymptomatic smokers, and nonsmokers to investigate whether Clara cell senescence is accelerated in the airways of COPD patients, and if so, whether it is accompanied by p38 MAPK activation.

Results

BrdU did not alter the intensity of the airway epithelial injury or inflammation after a single NA exposure. However, after repeated NA exposure, BrdU induced epithelial cell (Clara cell) senescence, as demonstrated by a DNA damage response, p21 overexpression, increased senescence-associated β-galactosidase activity, and growth arrest, which resulted in impaired epithelial regeneration. The epithelial senescence was accompanied by p38 MAPK-dependent airway inflammation. Senescent NCI-H441 cells impaired epithelial wound repair and secreted increased amounts of pro-inflammatory cytokines in a p38 MAPK-dependent manner. Clara cell senescence in COPD patients was accelerated and accompanied by p38 MAPK activation.

Conclusions

Senescence of airway epithelial cells impairs repair processes and exacerbates p38 MAPK-dependent inflammation after airway injury, and it may contribute to the pathogenesis of COPD.