Telomerase does not counteract telomere shortening but protects mitochondrial function under oxidative stress

The catalytic subunit of human telomerase is a unique caspase-6 and caspase-7 substrate

Telomerase is a ribonucleoprotein complex that is essential for persistent cellular proliferation. The catalytic subunit of human telomerase, hTERT, functions as a reverse transcriptase and promotes vitality by maintaining telomeric DNA length. hTERT is tightly regulated with complex but poorly understood positive and negative regulation at several levels including transcription, protein-protein interactions, and post-translation modifications. Because evidence implicates hTERT as an apoptosis inhibitor and because telomerase activity tends to decrease during apoptosis, we hypothesized that hTERT is a caspase substrate leading to down regulation during apoptosis. Caspases are proteases that initiate and execute apoptosis by cleaving target proteins. Indeed, we found that caspases-6 and -7 cleave hTERT during apoptosis in cultured cells. Caspase-6 cleaves at residues D129 and D637, and caspase-7 cleaves at E286 and D628. Three of the caspase cleavage sites are unique motifs. All four caspase motifs appear conserved in TERTs from Old World monkeys and apes, and the caspase-6 sites appear conserved in all primates. The caspase site that cleaves at D129 appears conserved in amniotes. hTERT fragments generated by cleavage were remarkably persistent, lasting hours after caspase activation. These results reveal a new biologically relevant mechanism for telomerase down regulation through caspase-mediated cleavage of hTERT and expand the list of known caspase motifs.

Human telomerase expression regulation

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

Sirtuin 1 is upregulated in a subset of hepatocellular carcinomas where it is essential for telomere maintenance and tumor cell growth

Hepatocellular carcinoma (HCC) is a highly malignant tumor with a poor prognosis. Treatment of HCC is complicated by the fact that the disease is often diagnosed at an advanced stage when it is no longer amenable to curative surgery, and current systemic chemotherapeutics are mostly inefficacious. Sirtuin 1 (SIRT1) is a class III histone deacetylase that is implicated in gene regulations and stress resistance. In this study, we found that SIRT1 is essential for the tumorigenesis of HCC. We showed that although SIRT1 was expressed at very low levels in normal livers, it was overexpressed in HCC cell lines and in a subset of HCC. Tissue microarray analysis of HCC and adjacent nontumoral liver tissues revealed a positive correlation between the expression levels of SIRT1 and advancement in tumor grades. Downregulation of SIRT1 consistently suppressed the proliferation of HCC cells via the induction of cellular senescence or apoptosis. SIRT1 silencing also caused telomere dysfunction-induced foci and nuclear abnormality that were clearly associated with reduced expressions of telomerase reverse transcriptase (TERT), and PTOP, which is a member of the shelter in complex. Ectopic expression of either TERT or PTOP in SIRT1-depleted cells significantly restored cell proliferation. There was also a positive correlation between the level of induction of SIRT1 and TERT [corrected] in human HCC. Finally, SIRT1-silencing sensitized HCC cells to doxorubicin treatment. Together, our findings reveal a novel function for SIRT1 in telomere maintenance of HCC, and they rationalize the clinical exploration of SIRT1 inhibitors for HCC therapy.

Non-telomeric activities of telomerase

Recent results suggest that telomerase is involved in many more cellular processes than merely telomere elongation. These include telomere-independent anti-apoptotic, cytoprotective and pro-proliferative effects of telomerase or protection of mitochondrial DNA against oxidative stress. Telomerase also participates in DNA repair and its essential subunits, hTR and hTERT, are able to modulate independently the cell's response to DNA damage. Recent high throughput analyses of gene expression showed that hTERT expression modulates expression of about 300 genes, including genes involved in the regulation of cell cycle progression, proliferation and differentiation. Besides the well-known telomerase catalytic activity of RNA-dependent DNA polymerase, its RNA-dependent RNA polymerase activity was recently described in association with the RNA subunit of mitochondrial RNA processing endoribonuclease, thus suggesting involvement of telomerase in RNA interference processes. These recent discoveries open novel possibilities and entirely unexpected research perspectives, branching off from the mainstream telomere and telomerase research.

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.

Telomerase reverse transcriptase protects ATM-deficient hematopoietic stem cells from ROS-induced apoptosis through a telomere-independent mechanism

Telomerase reverse transcriptase (TERT) contributes to the prevention of aging by a largely unknown mechanism that is unrelated to telomere lengthening. The current study used ataxia-telangiectasia mutated (ATM) and TERT doubly deficient mice to evaluate the contributions of 2 aging-regulating molecules, TERT and ATM, to the aging process. ATM and TERT doubly deficient mice demonstrated increased progression of aging and had shorter lifespans than ATM-null mice, while TERT alone was insufficient to affect lifespan. ATM-TERT doubly null mice show in vivo senescence, especially in hematopoietic tissues, that was dependent on p16(INK4a) and p19(ARF), but not on p21. As their HSCs show decreased stem cell activities, accelerated aging seen in these mice has been attributed to impaired stem cell function. TERT-deficient HSCs are characterized by reactive oxygen species (ROS) fragility, which has been suggested to cause stem cell impairment during aging, and apoptotic HSCs are markedly increased in these mice. p38MAPK activation was indicated to be partially involved in ROS-induced apoptosis in TERT-null HSCs, and BCL-2 is suggested to provide a part of the protective mechanisms of HSCs by TERT. The current study demonstrates that TERT mitigates aging by protecting HSCs under stressful conditions through telomere length-independent mechanisms.

Biologic function and clinical potential of telomerase and associated proteins in cardiovascular tissue repair and regeneration

Telomeres comprise long tracts of double-stranded TTAGGG repeats that extend for 9-15 kb in humans. Telomere length is maintained by telomerase, a specialized ribonucleoprotein that prevents the natural ends of linear chromosomes from undergoing inappropriate repair, which could otherwise lead to deleterious chromosomal fusions. During the development of cardiovascular tissues, telomerase activity is strong but diminishes with age in adult hearts. Dysfunction of telomerase is associated with the impairment of tissue repair or regeneration in several pathologic conditions, including heart failure and infarction. Under both physiologic and pathophysiologic conditions, telomerase interacts with promyogenic nuclear transcription factors (e.g. myocardin, serum response factor) to augment the potency of cardiovascular cells during growth, survival, and differentiation. We review recent findings on the biologic function of telomerase and its potential for clinical application in cardiovascular development and repair. Understanding the roles of telomerase and its associated proteins in the functional regulation of cardiovascular cells and their progenitors may lead to new strategies for cardiovascular tissue repair and regeneration.

Deficiency of telomerase activity aggravates the blood-brain barrier disruption and neuroinflammatory responses in a model of experimental stroke

Epidemiology and genetic studies indicate that patients with telomere length shorter than average are at higher risk of dying from heart disease or stroke. Telomeres are located at the ends of eukaryotic chromosomes, which demonstrate progressive length reduction in most somatic cells during aging. The enzyme telomerase can compensate for telomere loss during cell replication. The present study sought to investigate the contribution of telomerase to stroke and blood-brain barrier (BBB) dysfunction. Telomerase reverse transcriptase knockout (TERT(-/-)) mice and littermate controls with normal TERT expression were subjected to a 24-hr permanent middle cerebral artery occlusion (pMCAO). The stroke outcomes were assessed in terms of neurological scores and infarct volumes. In addition, we evaluated oxidative stress, permeability across the BBB, and integrity of tight junctions in brain microvessels. Neurological testing revealed that TERT(-/-) mice showed enhanced deficits compared with controls. These changes were associated with a greater infarct volume. The expression of tight junction protein ZO-1 decreased markedly in ischemic hemispheres of TERT(-/-) mice. The brain microvessels of TERT(-/-) mice also were more susceptible to oxidative stress, revealing higher superoxide and lower glutathione levels compared with mice with normal TERT expression. Importantly, TERT deficiency potentiated the production of inflammatory mediators, such as tumor necrosis factor-alpha, interleukin-1 beta, and intercellular adhesion molecule-1, in the ischemic hemispheres of mice with pMCAO. Our study suggests that TERT deficiency can predispose to the development of stroke in an experimental model of this disease.

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.

Ageing, telomeres, senescence, and liver injury

Populations in developed countries continue to grow older and an understanding of the ageing process to allow healthy ageing carries important medical implications. Older individuals are more susceptible to most acquired liver disorders and more vulnerable to the consequences of liver disease. Accordingly, age is a critical determinant of outcome for hepatitis C virus infection and liver transplantation. In this review we describe changes in the ageing liver and discuss mechanisms of senescence at the cellular level. In particular, we focus on mechanisms by which inflammation, oxidative stress, and oncogenic stress accelerate cellular senescence. In the setting of chronic hepatic injury and inflammation, cellular senescence functions as an essential stress-response mechanism to limit the proliferation of damaged cells and reduce the risk of malignancy, but this benefit is achieved at the expense of senescence-related organ dysfunction. The dual role of cell senescence in chronic liver disease will make this an intriguing but challenging area for future clinical interventions.

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.

Telomerase protects adult rodent olfactory ensheathing glia from early senescence

Adult olfactory bulb ensheathing glia (OB-OEG) promote the repair of acute, subacute, and chronic spinal cord injuries and autologous transplantation is a feasible approach. There are interspecies differences between adult rodent and primate OB-OEG related to their longevity in culture. Whereas primate OB-OEG exhibit a relatively long life span, under the same culture conditions rodent OB-OEG divide just three to four times, are sensitive to oxidative stress and become senescent after the third week in vitro. Telomerase is a "physiological key regulator" of the life span of normal somatic cells and also has extratelomeric functions such as increased resistance to oxidative stress. To elucidate whether telomerase has a role in the senescence of rodent OB-OEG, we have introduced the catalytic subunit of telomerase mTERT into cultures of these cells by retroviral infection. Native and modified adult rat OB-OEG behaved as telomerase-competent cells as they divided while expressing mTERT but entered senescence once the gene switched off. After ectopic expression of mTERT, OB-OEG resumed division at a nonsenescent rate, expressed p75 and other OEG markers, and exhibited the morphology of nonsenescent OB-OEG. The nonsenescent period of mTERT-OEG lasted 9weeks and then ectopic mTERT switched off and cells entered senescence again. Our results suggest a role of telomerase in early senescence of adult rodent OB-OEG cultures and a protection from oxidative damage. This article is part of a Special Issue entitled: Understanding olfactory ensheathing glia and their prospect for nervous system repair.

Well-known signaling proteins exert new functions in the nucleus and mitochondria

One distinguishing feature of eukaryotic cells is their compartmentalization into organelles, which all have a unique structural and functional identity. Some proteins are exclusively localized in a single organelle, whereas others are found in more than one. A few proteins, whose function was thought to be completely understood, were only recently found to be present in the mitochondria. Although these proteins come from diverse functional classes, their common new denominator is the regulation of respiratory chain activity. Therefore, this review focuses on new functions of the Signal Transducer and Activator of Transcription 3, originally described as a transcription factor, the most prominent Src kinase family members, Src, Fyn, and Yes, which were so far known as plasma membrane-associated molecular effectors of a variety of extracellular stimuli, the tyrosine phosphatase Shp-2 previously characterized as a modulator of cytosolic signal transduction involved in cell growth, development, inflammation, and chemotaxis, and Telomerase Reverse Transcriptase, the key enzyme preventing telomere erosion in the nucleus. Their unexpected localization in other organelles and regulation of mitochondrial and/or nuclear functions by them adds a new layer of regulatory complexity. This extends the flexibility to cope with changing environmental demands using a limited number of genes and proteins.

Dual targeting of mitochondrial proteins: mechanism, regulation and function

One solution found in evolution to increase the number of cellular functions, without increasing the number of genes, is distribution of single gene products to more than one cellular compartment. It is well documented that in eukaryotic cells, molecules of one protein can be located in several subcellular locations, a phenomenon termed dual targeting, dual localization, or dual distribution. The differently localized proteins are coined in this review "echoforms" indicating repetitious forms of the same protein (echo in Greek denotes repetition) distinctly placed in the cell. This term replaces the term to "isoproteins" or "isoenzymes" which are reserved for proteins with the same activity but different amino acid sequences. Echoforms are identical or nearly identical, even though, as referred to in this review may, in some cases, surprisingly have a totally different function in the different compartments. With regard to mitochondria, our operational definition of dual targeted proteins refers to situations in which one of the echoforms is translocated through/into a mitochondrial membrane. In this review we ask how, when and why mitochondrial proteins are dual localized in the cell. We describe mechanisms of dual targeting of proteins between mitochondria and other compartments of the eukaryotic cell. In particular, we have paid attention to situations in which dual localization is regulated in time, location or function. In addition, we have attempted to provide a broader view concerning the phenomenon of dual localization of proteins by looking at mechanisms that are beyond our simple definition of dual targeting. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

TGF-beta induces telomerase-dependent pancreatic tumor cell cycle arrest

Recent studies suggest that transforming growth factor beta (TGF-beta) inhibits telomerase activity by repression of the telomerase reverse transcriptase (TERT) gene. In this report, we show that TGF-beta induces TERT repression-dependent apoptosis in pancreatic tumor, vascular smooth muscle, and cervical cancer cell cultures. TGF-beta activates Smad3 signaling, induces TERT gene repression and results in G1/S phase cell cycle arrest and apoptosis. TERT over-expression stimulates the G1/S phase transition and alienates TGF-beta-induced cell cycle arrest and apoptosis. Our data suggest that telomere maintenance is a limiting factor of the transition of the cell cycle. TGF-beta triggers cell cycle arrest and death by a mechanism involving telomerase deregulation of telomere maintenance.

Endogenous oxidative stress prevents telomerase-dependent immortalization of human endothelial cells

INTRODUCTION

With aging, oxidative stress accelerates vascular endothelial cell (EC) telomere shortening-induced senescence, and may promote atherosclerosis in humans. Our aim was to investigate whether an antioxidant treatment combined with telomerase (hTERT) over-expression would prevent senescence of EC isolated from patients with severe atherosclerosis.

METHODS

Cells were isolated from internal mammary arteries (n=11 donors), cultured until senescence with or without N-acetylcystein (NAC) and infected, or not, with a lentivirus over-expressing hTERT.

RESULTS

Compared to control EC, hTERT-NAC cells had increased telomerase activity, longer telomeres and underwent more cell divisions. According to the donor, hTERT-NAC either delayed (n=5) or prevented (n=4) EC senescence, the latter leading to cell immortalization. Lack of cell immortalization by hTERT-NAC was accompanied by an absence of beneficial effect of NAC alone in paired EC. Accordingly, lack of EC immortalization by hTERT-NAC was associated with high endogenous susceptibility to oxidation. In EC where hTERT-NAC did not immortalize EC, p53, p21 and p16 expression increased with senescence, while oxidative-dependent DNA damage associated with senescence was not prevented.

CONCLUSION

Our data suggest that irreversible oxidative stress-dependent damages associated with cardiovascular risk factors are responsible for senescence of EC from atherosclerotic patients.

A mutant telomerase defective in nuclear-cytoplasmic shuttling fails to immortalize cells and is associated with mitochondrial dysfunction

Telomerase is a reverse transcriptase specialized in telomere synthesis. The enzyme is primarily nuclear where it elongates telomeres, but many reports show that the catalytic component of telomerase (in humans called hTERT) also localizes outside of the nucleus, including in mitochondria. Shuttling of hTERT between nucleus and cytoplasm and vice versa has been reported, and different proteins shown to regulate such translocation. Exactly why telomerase moves between subcellular compartments is still unclear. In this study we report that mutations that disrupt the nuclear export signal (NES) of hTERT render it nuclear but unable to immortalize cells despite retention of catalytic activity in vitro. Overexpression of the mutant protein in primary fibroblasts is associated with telomere-based cellular senescence, multinucleated cells and the activation of the DNA damage response genes ATM, Chk2 and p53. Mitochondria function is also impaired in the cells. We find that cells expressing the mutant hTERT produce high levels of mitochondrial reactive oxygen species and have damage in telomeric and extratelomeric DNA. Dysfunctional mitochondria are also observed in an ALT (alternative lengthening of telomeres) cell line that is insensitive to growth arrest induced by the mutant hTERT showing that mitochondrial impairment is not a consequence of the growth arrest. Our data indicate that mutations involving the NES of hTERT are associated with defects in telomere maintenance, mitochondrial function and cellular growth, and suggest targeting this region of hTERT as a potential new strategy for cancer treatment.

Mitochondrial energetics and therapeutics

Mitochondrial dysfunction has been linked to a wide range of degenerative and metabolic diseases, cancer, and aging. All these clinical manifestations arise from the central role of bioenergetics in cell biology. Although genetic therapies are maturing as the rules of bioenergetic genetics are clarified, metabolic therapies have been ineffectual. This failure results from our limited appreciation of the role of bioenergetics as the interface between the environment and the cell. A systems approach, which, ironically, was first successfully applied over 80 years ago with the introduction of the ketogenic diet, is required. Analysis of the many ways that a shift from carbohydrate glycolytic metabolism to fatty acid and ketone oxidative metabolism may modulate metabolism, signal transduction pathways, and the epigenome gives us an appreciation of the ketogenic diet and the potential for bioenergetic therapeutics.

DNA damaging bystander signalling from stem cells, cancer cells and fibroblasts after Cr(VI) exposure and its dependence on telomerase

The bystander effect is a feature of low dose radiation exposure and is characterized by a signaling process from irradiated cells to non irradiated cells, which causes DNA and chromosome damage in these 'nearest neighbour' cells. Here we show that a low and short dose of Cr(VI) can induce stem cells, cancer cells and fibroblasts to chronically secrete bystander signals, which cause DNA damage in neighboring cells. The Cr(VI) induced bystander signaling depended on the telomerase status of either cell. Telomerase negative fibroblasts were able to receive DNA damaging signals from telomerase positive or negative fibroblasts or telomerase positive cancer cells. However telomerase positive fibroblasts were resistant to signals from Cr(VI) exposed telomerase positive fibroblasts or cancer cells. Human embryonic stem cells, with positive Oct4 staining as a marker of pluripotency, showed no significant increase of DNA damage from adjacent Cr and mitomycin C exposed fibroblasts whilst those cells that were negatively stained did. This selectivity of DNA damaging bystander signaling could be an important consideration in developing therapies against cancer and in the safety and effectiveness of tissue engineering and transplantation using stem cells.

Administration with telomeric DNA telomere-like oligonucleotides induces enhancement of telomerase activity and resistance against oxidative stress in telomere reverse transcriptase gene-transfected human fibroblasts

Out of normal human fibroblasts OUMS-36 and three clones (T1, T2 and T3) of telomere reverse transcriptase gene (hTERT)-transfectants, telomere length is in order: T3 >T2 >T1 >>OUMS-36 (young) >>OUMS-36 (old), and telomerase activity is in order: T2 >T3 >T1 >>OUMS-36 (young, old), suggesting that telomere length may be roughly governed by telomerase activity. Telomere-like oligonucleotides [5'- (TTAGGG) (1-3)-3'] (ON mono-/di-/trimer) and a mononucleotide (T:A:G=2:1:3, mol/mol) mixture (MN mix), as candidates for telomerase activators, maintained above 80% of cell viability at marginally higher doses of 2 μM for MN-mix or ON monomer, 1 μM for ON dimmer and 0.67 μM for ON trimer, respectively, in OUMS-36 and T2 cells, and administered for 4 weeks, resulting in no elongation of telomere length in both the cell lines. In contrast, telomerase activity was enhanced by administration with ON mono-/di-/trimer, but not MN mix, in a manner dependent on treatment periods, in T2 transfectants, whereas similar effects were not observed in OUMS-36 parents. The 4-week treatment with ON mono-/di-/trimers, but not MN mix, also suppressed cell-viability diminishment induced by the oxidative-stressor tert-butylhydroperoxide in T2 cells, but scarcely in OUMS-36 cells. Thus, the promoting effects of oligonucleotide [5'-(TTAGGG)(1-3)-3'] on both telomerase enhancement and oxidative-stress resistance can be exerted for telomerase-abundant T2 hTERT-transfectants, but not for telomerase-poor OUMS-36 parents.

Downregulation of mitochondrial telomerase reverse transcriptase induced by H2O2 is Src kinase dependent

Telomerase with its catalytic subunit telomerase reverse transcriptase (TERT) prevents telomere erosion in the nucleus. In addition, telomerase has also telomere-independent functions in protection from apoptosis. Unexpectedly, TERT was found in the mitochondria. However, its regulation in this organelle is completely unknown. Here, we demonstrate that mitochondrial TERT is downregulated by exposure to H(2)O(2) in primary human endothelial cells. This depletion is dependent on the Src phosphorylation site within TERT, tyrosine 707. In accordance with this finding, we also detected Src in the mitochondria and demonstrated that Src is activated upon H(2)O(2) treatment. This regulation of mitochondrial TERT is reminiscent of the situation in the nucleus from where TERT is exported under conditions of oxidative stress in a Src kinase dependent manner. In addition, Akt1 was also found in the mitochondria and H(2)O(2) treatment led to reduced active Akt1 in these organelles, suggesting that similar regulatory mechanisms operate in mitochondria and the nucleus.

Mitochondrial redox metabolism: aging, longevity and dietary effects

Mitochondrial redox metabolism has long been considered to play important roles in mammalian aging and the development of age-related pathologies in the major oxidative organs. Both genetic and dietary manipulations of mitochondrial redox metabolism have been associated with the extension of lifespan. Here we provide a broad overview of the circumstantial evidence showing associations between mitochondrial reactive oxygen species (ROS) metabolism, aging and longevity. We address most aspects of mitochondrial ROS metabolism, from superoxide production, to ROS detoxification and the repair/removal of ROS-mediated macromolecular damage. Finally, we discuss the effects of dietary manipulations (e.g. caloric restriction, methionine restriction), dietary deficiencies (e.g. folate) and dietary supplementation (e.g. resveratrol) on mitochondrial ROS metabolism and lifespan.

Telomere shortening sensitizes cancer cells to selected cytotoxic agents: in vitro and in vivo studies and putative mechanisms

Background

Telomere/telomerase system has been recently recognized as an attractive target for anticancer therapy. Telomerase inhibition results in tumor regression and increased sensitivity to various cytotoxic drugs. However, it has not been fully established yet whether the mediator of these effects is telomerase inhibition per se or telomere shortening resulting from inhibition of telomerase activity. In addition, the characteristics and mechanisms of sensitization to cytotoxic drugs caused by telomerase inhibition has not been elucidated in a systematic manner.

Methodology/Principal Findings

In this study we characterized the relative importance of telomerase inhibition versus telomere shortening in cancer cells. Sensitization of cancer cells to cytotoxic drugs was achieved by telomere shortening in a length dependent manner and not by telomerase inhibition per se. In our system this sensitization was related to the mechanism of action of the cytotoxic drug. In addition, telomere shortening affected also other cancer cell functions such as migration. Telomere shortening induced DNA damage whose repair was impaired after administration of cisplatinum while doxorubicin or vincristine did not affect the DNA repair. These findings were verified also in in vivo mouse model. The putative explanation underlying the phenotype induced by telomere shortening may be related to changes in expression of various microRNAs triggered by telomere shortening.

Conclusions/Significance

To our best knowledge this is the first study characterizing the relative impact of telomerase inhibition and telomere shortening on several aspects of cancer cell phenotype, especially related to sensitivity to cytotoxic drugs and its putative mechanisms. The microRNA changes in cancer cells upon telomere shortening are novel information. These findings may facilitate the development of telomere based approaches in treatment of cancer.

Cancer as a metabolic disease

Emerging evidence indicates that impaired cellular energy metabolism is the defining characteristic of nearly all cancers regardless of cellular or tissue origin. In contrast to normal cells, which derive most of their usable energy from oxidative phosphorylation, most cancer cells become heavily dependent on substrate level phosphorylation to meet energy demands. Evidence is reviewed supporting a general hypothesis that genomic instability and essentially all hallmarks of cancer, including aerobic glycolysis (Warburg effect), can be linked to impaired mitochondrial function and energy metabolism. A view of cancer as primarily a metabolic disease will impact approaches to cancer management and prevention.

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.

Prevention of senescence progression in reversibly immortalized human ensheathing glia permits their survival after deimmortalization

Reversible immortalization holds great potential for primary tissue expansion to develop cell-based therapies as well as for basic research. Human olfactory ensheathing glia (hOEG) are promising candidates for treating spinal cord injury and for studying extrinsic neuroregenerative mechanisms. We used lentivectors with Cre/loxP technology to achieve reversible gene transfer of BMI1, SV40 large T antigen (TAg), a short hairpin RNA against p53 (shp53), and the catalytic subunit of telomerase (TERT) in primary cultures of hOEG from human donor cadaver olfactory bulbs. Several combinations of these genes were able to immortalize hOEG, conserving their antigenic markers and neuroregenerative properties but only those transduced by BMI1/TERT did not accumulate karyotypic alterations or increase senescence marker levels. Strikingly, these were also the only cells which continued to proliferate after transgene removal by Cre recombinase delivery, whereas hOEG immortalized by shp53 or TAg in combination with TERT entered into growth arrest and died. These data support the idea that immortalization and halting senescent changes are separate processes; hOEG immortalized by BMI1/TERT can revert back to their former primary cell replicative state when deimmortalized, whereas those transduced by the other combinations depend on the presence of these transgenes to maintain their aberrant proliferative state.

Telomere shortening may be associated with human keloids

BACKGROUND

Keloids are benign skin tumors that are the effect of a dysregulated wound-healing process in genetically predisposed patients. They are inherited with an autosomal dominant mode with incomplete clinical penetrance and variable expression. Keloids are characterized by formation of excess scar tissue beyond the boundaries of the wound. The exact etiology is still unknown and there is currently no appropriate treatment for keloid disease.

METHODS

We analyzed sample tissues were obtained from 20 patients with keloid skin lesions and normal skin was obtained from 20 healthy donors. The telomeres were measured by Terminal Restriction Fragment (TRF) analysis and Real-Time PCR assay. Quantitative Real-Time RT-PCR analysis of hTERT gene expression was performed and intracellular ROS generation was measured.

RESULTS

In this study, we determined whether telomeric shortening and the expression of human telomerase reverse transcriptase (hTERT) occurs in keloid patients. Using Terminal Restriction Fragment (TRF) analysis and Real-Time PCR assay, we detected a significant telomere shortening of 30% in keloid specimens compared to normal skin. Using quantitative Real-Time RT-PCR, telomerase activity was found absent in the keloid tissues. Moreover, an increase in ROS generation was detected in fibroblasts cell cultures from keloid specimens as more time elapsed compared to fibroblasts from normal skin.

CONCLUSION

Telomere shortening has been reported in several metabolic and cardiovascular diseases. We found that telomere shortening can also be associated with human keloids. Chronic oxidative stress plays a major role in the pathophysiology of several chronic inflammatory diseases. Here we found increased ROS generation in fibroblasts from keloid fibroblasts cell cultures when compared to normal skin fibroblasts. Hence we conclude that oxidative stress might be an important modulator of telomere loss in keloid because of the absence of active telomerase that counteracts telomere shortening.

Energetics, epigenetics, mitochondrial genetics

The epigenome has been hypothesized to provide the interface between the environment and the nuclear DNA (nDNA) genes. Key factors in the environment are the availability of calories and demands on the organism's energetic capacity. Energy is funneled through glycolysis and mitochondrial oxidative phosphorylation (OXPHOS), the cellular bioenergetic systems. Since there are thousands of bioenergetic genes dispersed across the chromosomes and mitochondrial DNA (mtDNA), both cis and trans regulation of the nDNA genes is required. The bioenergetic systems convert environmental calories into ATP, acetyl-Coenzyme A (acetyl-CoA), s-adenosyl-methionine (SAM), and reduced NAD(+). When calories are abundant, ATP and acetyl-CoA phosphorylate and acetylate chromatin, opening the nDNA for transcription and replication. When calories are limiting, chromatin phosphorylation and acetylation are lost and gene expression is suppressed. DNA methylation via SAM can also be modulated by mitochondrial function. Phosphorylation and acetylation are also pivotal to regulating cellular signal transduction pathways. Therefore, bioenergetics provides the interface between the environment and the epigenome. Consistent with this conclusion, the clinical phenotypes of bioenergetic diseases are strikingly similar to those observed in epigenetic diseases (Angelman, Rett, Fragile X Syndromes, the laminopathies, cancer, etc.), and an increasing number of epigenetic diseases are being associated with mitochondrial dysfunction. This bioenergetic-epigenomic hypothesis has broad implications for the etiology, pathophysiology, and treatment of a wide range of common diseases.

Molecular distinctions between stasis and telomere attrition senescence barriers shown by long-term culture of normal human mammary epithelial cells

Normal human epithelial cells in culture have generally shown a limited proliferative potential of approximately 10 to 40 population doublings before encountering a stress-associated senescence barrier (stasis) associated with elevated levels of cyclin-dependent kinase inhibitors p16 and/or p21. We now show that simple changes in medium composition can expand the proliferative potential of human mammary epithelial cells (HMEC) initiated as primary cultures to 50 to 60 population doublings followed by p16-positive, senescence-associated beta-galactosidase-positive stasis. We compared the properties of growing and senescent pre-stasis HMEC with growing and senescent post-selection HMEC, that is, cells grown in a serum-free medium that overcame stasis via silencing of p16 expression and that display senescence associated with telomere dysfunction. Cultured pre-stasis populations contained cells expressing markers associated with luminal and myoepithelial HMEC lineages in vivo in contrast to the basal-like phenotype of the post-selection HMEC. Gene transcript and protein expression, DNA damage-associated markers, mean telomere restriction fragment length, and genomic stability differed significantly between HMEC populations at the stasis versus telomere dysfunction senescence barriers. Senescent isogenic fibroblasts showed greater similarity to HMEC at stasis than at telomere dysfunction, although their gene transcript profile was distinct from HMEC at both senescence barriers. These studies support our model of the senescence barriers encountered by cultured HMEC in which the first barrier, stasis, is retinoblastoma-mediated and independent of telomere length, whereas a second barrier (agonescence or crisis) results from telomere attrition leading to telomere dysfunction. Additionally, the ability to maintain long-term growth of genomically stable multilineage pre-stasis HMEC populations can greatly enhance experimentation with normal HMEC.

A comprehensive review on mesenchymal stem cell growth and senescence

In recent years mesenchymal stem cells (MSCs) have generated a great deal of excitement as an attractive alternative to embryonic stem cells (ESCs) in cell-based regenerative medicine. In contrast to cells of embryonic origin, however, the clinical application of MSCs is heavily restricted by their finite ability of self-renewal, in which they resemble the rest of the somatic cells. Yet the mechanisms controlling MSC proliferation and senescence remain unclear. This review summarizes recent advances in our understanding of the factors affecting MSC expansion in vitro and discusses the pattern of their senescence with particular emphasis on the role of telomere shortening, activation of effectory pathways, and oxidative stress. The issues associated with MSC growth and senescence will be shown in the context of other somatic cells, and all of the parallels and disparities will be delineated precisely.

Mitochondrial gateways to cancer

Mitochondria are required for cellular survival, yet can also orchestrate cell death. The peculiar biochemical properties of these organelles, which are intimately linked to their compartmentalized ultrastructure, provide an optimal microenvironment for multiple biosynthetic and bioenergetic pathways. Most intracellular ATP is generated by mitochondrial respiration, which also represents the most relevant source of intracellular reactive oxygen species. Mitochondria participate in a plethora of anabolic pathways, including cholesterol, cardiolipin, heme and nucleotide biosynthesis. Moreover, mitochondria integrate numerous pro-survival and pro-death signals, thereby exerting a decisive control over several biochemical cascades leading to cell death, in particular the intrinsic pathway of apoptosis. Therefore, it is not surprising that cancer cells often manifest the deregulation of one or several mitochondrial functions. The six classical hallmarks of cancer (i.e., limitless replication, self-provision of proliferative stimuli, insensitivity to antiproliferative signals, disabled apoptosis, sustained angiogenesis, invasiveness/metastatic potential), as well as other common features of tumors (i.e., avoidance of the immune response, enhanced anabolic metabolism, disabled autophagy) may directly or indirectly implicate deregulated mitochondria. In this review, we discuss several mechanisms by which mitochondria can contribute to malignant transformation and tumor progression.

Mitochondria and aging: innocent bystanders or guilty parties?

There are many theories of aging and a number of them encompass the role of mitochondria in this process. Mitochondrial DNA mutations and deletions have been shown to accumulate in many tissues in mammals during aging. However, there is little evidence that these mutations could affect the functioning of aging tissues.

DNA damage response and cellular senescence in tissues of aging mice

The impact of cellular senescence onto aging of organisms is not fully clear, not at least because of the scarcity of reliable data on the mere frequency of senescent cells in aging tissues. Activation of a DNA damage response including formation of DNA damage foci containing activated H2A.X (gamma-H2A.X) at either uncapped telomeres or persistent DNA strand breaks is the major trigger of cell senescence. Therefore, gamma-H2A.X immunohistochemistry (IHC) was established by us as a reliable quantitative indicator of senescence in fibroblasts in vitro and in hepatocytes in vivo and the age dependency of DNA damage foci accumulation in ten organs of C57Bl6 mice was analysed over an age range from 12 to 42 months. There were significant increases with age in the frequency of foci-containing cells in lung, spleen, dermis, liver and gut epithelium. In liver, foci-positive cells were preferentially found in the centrilobular area, which is exposed to higher levels of oxidative stress. Foci formation in the intestine was restricted to the crypts. It was not associated with either apoptosis or hyperproliferation. That telomeres shortened with age in both crypt and villus enterocytes, but telomeres in the crypt epithelium were longer than those in villi at all ages were confirmed by us. Still, there was no more than random co-localization between gamma-H2A.X foci and telomeres even in crypts from very old mice, indicating that senescence in the crypt enterocytes is telomere independent. The results suggest that stress-dependent cell senescence could play a causal role for aging of mice.

Telomerase, mitochondria and oxidative stress

Telomerase plays an important role in cellular proliferation capacity and survival under conditions of stress. A large part of this protective function is due to telomere capping and maintenance. Thus it contributes to cellular immortality in stem cells and cancer. Recently, evidence has accumulated that telomerase can contribute to cell survival and stress resistance in a largely telomere-independent manner. Telomerase has been shown to shuttle dynamically between different cellular locations. Under increased oxidative stress telomerase is excluded from the nucleus and can be found within the mitochondria. This phenotype correlates with decreased oxidative stress within telomerase expressing cells and improved mitochondrial function by currently largely unknown mechanisms. Our data suggest that mitochondrial protection could be an important non-canonical function for telomerase in cell survival and ageing. This review summarises briefly our knowledge about extra-telomeric functions of telomerase and discusses the potential significance of its mitochondrial localisation.

Mitochondrial telomerase reverse transcriptase binds to and protects mitochondrial DNA and function from damage

OBJECTIVE

The enzyme telomerase and its catalytic subunit the telomerase reverse transcriptase (TERT) are important for maintenance of telomere length in the nucleus. Recent studies provided evidence for a mitochondrial localization of TERT. Therefore, we investigated the exact localization of TERT within the mitochondria and its function.

METHODS AND RESULTS

Here, we demonstrate that TERT is localized in the matrix of the mitochondria. TERT binds to mitochondrial DNA at the coding regions for ND1 and ND2. Binding of TERT to mitochondrial DNA protects against ethidium bromide-induced damage. TERT increases overall respiratory chain activity, which is most pronounced at complex I and dependent on the reverse transcriptase activity of the enzyme. Moreover, mitochondrial reactive oxygen species are increased after genetic ablation of TERT by shRNA. Mitochondrially targeted TERT and not wild-type TERT revealed the most prominent protective effect on H(2)O(2)-induced apoptosis. Lung fibroblasts from 6-month-old TERT(-/-) mice (F2 generation) showed increased sensitivity toward UVB radiation and heart mitochondria exhibited significantly reduced respiratory chain activity already under basal conditions, demonstrating the protective function of TERT in vivo.

CONCLUSIONS

Mitochondrial TERT exerts a novel protective function by binding to mitochondrial DNA, increasing respiratory chain activity and protecting against oxidative stress-induced damage.

Mitochondrial reticulum network dynamics in relation to oxidative stress, redox regulation, and hypoxia

A single mitochondrial network in the cell undergoes constant fission and fusion primarily depending on the local GTP gradients and the mitochondrial energetics. Here we overview the main properties and regulation of pro-fusion and pro-fission mitodynamins, i.e. dynamins-related GTPases responsible for mitochondrial shape-forming, such as pro-fusion mitofusins MFN1, MFN2, and the inner membrane-residing long OPA1 isoforms, and pro-fission mitodynamins FIS1, MFF, and DRP1 multimers required for scission. Notably, the OPA1 cleavage into non-functional short isoforms at a diminished ATP level (collapsed membrane potential) and the DRP1 recruitment upon phosphorylation by various kinases are overviewed. Possible responses of mitodynamins to the oxidative stress, hypoxia, and concomitant mtDNA mutations are also discussed. We hypothesize that the increased GTP formation within the Krebs cycle followed by the GTP export via the ADP/ATP carrier shift the balance between fission and fusion towards fusion by activating the GTPase domain of OPA1 located in the peripheral intermembrane space (PIMS). Since the protein milieu of PIMS is kept at the prevailing oxidized redox potential by the TOM, MIA40 and ALR/Erv1 import-redox trapping system, redox regulations shift the protein environment of PIMS to a more reduced state due to the higher substrate load and increased respiration. A higher cytochrome c turnover rate may prevent electron transfer from ALR/Erv1 to cytochrome c. Nevertheless, the putative links between the mitodynamin responses, mitochondrial morphology and the changes in the mitochondrial bioenergetics, superoxide production, and hypoxia are yet to be elucidated, including the precise basis for signaling by the mitochondrion-derived vesicles.

Lung alveolar integrity is compromised by telomere shortening in telomerase-null mice

Shortened telomeres are a normal consequence of cell division. However, telomere shortening past a critical point results in cellular senescence and death. To determine the effect of telomere shortening on lung, four generations of B6.Cg-Terc(tm1Rdp) mice, null for the terc component of telomerase, the holoenzyme that maintains telomeres, were bred and analyzed. Generational inbreeding of terc-/- mice caused sequential shortening of telomeres. Lung histology from the generation with the shortest telomeres (terc-/- F4) showed alveolar wall thinning and increased alveolar size. Morphometric analysis confirmed a significant increase in mean linear intercept (MLI). terc-/- F4 lung showed normal elastin deposition but had significantly decreased collagen content. Both airway and alveolar epithelial type 1 cells (AEC1) appeared normal by immunohistochemistry, and the percentage of alveolar epithelial type 2 cells (AEC2) per total cell number was similar to wild type. However, because of a decrease in distal lung cellularity, the absolute number of AEC2 in terc-/- F4 lung was significantly reduced. In contrast to wild type, terc-/- F4 distal lung epithelium from normoxia-maintained mice exhibited DNA damage by terminal deoxynucleotidyltransferase (TdT)-mediated dUTP nick end labeling (TUNEL) and 8-oxoguanine immunohistochemistry. Western blotting of freshly isolated AEC2 lysates for stress signaling kinases confirmed that the stress-activated protein kinase (SAPK)/c-Jun NH(2)-terminal kinase (JNK) stress response pathway is stimulated in telomerase-null AEC2 even under normoxic conditions. Expression of downstream apoptotic/stress markers, including caspase-3, caspase-6, Bax, and HSP-25, was also observed in telomerase-null, but not wild-type, AEC2. TUNEL analysis of freshly isolated normoxic AEC2 showed that DNA strand breaks, essentially absent in wild-type cells, increased with each successive terc-/- generation and correlated strongly with telomere length (R(2) = 0.9631). Thus lung alveolar integrity, particularly in the distal epithelial compartment, depends on proper telomere maintenance.

Nuclear protein tyrosine phosphatase Shp-2 is one important negative regulator of nuclear export of telomerase reverse transcriptase

Aging is one major risk factor for numerous diseases. The enzyme telomerase reverse transcriptase (TERT) plays an important role for aging and apoptosis. Previously, we demonstrated that inhibition of oxidative stress-induced Src kinase family-dependent nuclear export of TERT results in delayed replicative senescence and reduced apoptosis sensitivity. Therefore, the aim of this study was to investigate mechanisms inhibiting nuclear export of TERT. First, we demonstrated that H2O2-induced nuclear export of TERT was abolished in Src, Fyn, and Yes-deficient embryonic fibroblasts. Next, we wanted to identify one potential negative regulator of this export process. One candidate is the protein tyrosine phosphatase Shp-2 (Shp-2), which can counteract activities of the Src kinase family. Indeed, Shp-2 was evenly distributed between the nucleus and cytosol. Nuclear Shp-2 associates with TERT in endothelial cells and dissociates from TERT prior to its nuclear export. Overexpression of Shp-2 wt inhibited H2O2-induced export of TERT. Overexpression of the catalytically inactive, dominant negative Shp-2 mutant (Shp-2(C459S)) reduced endogenous as well as overexpressed nuclear TERT protein and telomerase activity, whereas it had no influence on TERT(Y707F). Binding of TERT(Y707F) to Shp-2 is reduced compared with TERTwt. Ablation of Shp-2 expression led only to an increased tyrosine phosphorylation of TERTwt, but not of TERT(Y707F). Moreover, reduced Shp-2 expression decreased nuclear telomerase activity, whereas nuclear telomerase activity was increased in Shp-2-overexpressing endothelial cells. In conclusion, Shp-2 retains TERT in the nucleus by regulating tyrosine 707 phosphorylation.

The many faces of telomerase: emerging extratelomeric effects

Telomeres, the ends of chromosomes, shorten with each cell division. To expand their replicative potential, various cell types use the ribonucleoprotein telomerase, which lengthens telomeres by its reverse transcriptase activity. Because of its ability to immortalize cancer cells, telomerase also plays a significant role in tumor growth. However, in recent years, a wide variety of non-canonical effects of telomerase that are independent of telomere lengthening have been discovered, and even the notion that telomerase is restricted to very few cell types has been questioned. These effects also seem to be important in carcinogenesis and might explain the tumor-promoting effects of telomerase independently of telomere elongation. Here, the current understanding of the extratelomeric roles of telomerase and their physiological and pathological significance is reviewed.

Permanent embryo arrest: molecular and cellular concepts

Developmental arrest is one of the mechanisms responsible for the elevated levels of embryo demise during the first week of in vitro development. Approximately 10-15% of IVF embryos permanently arrest in mitosis at the 2- to 4-cell cleavage stage showing no indication of apoptosis. Reactive oxygen species (ROS) are implicated in this process and must be controlled in order to optimize embryo production. A stress sensor that can provide a key understanding of permanent cell cycle arrest and link ROS with cellular signaling pathway(s) is p66Shc, an adaptor protein for apoptotic-response to oxidative stress. Deletion of the p66Shc gene in mice results in extended lifespan, which is linked to their enhanced resistance to oxidative stress and reduced levels of apoptosis. p66Shc has been shown to generate mitochondrial H(2)O(2) to trigger apoptosis, but may also serve as an integration point for many signaling pathways that affect mitochondrial function. We have detected elevated levels of p66Shc and ROS within arrested embryos and believe that p66Shc plays a central role in regulating permanent embryo arrest. In this paper, we review the cellular and molecular aspects of permanent embryo arrest and speculate on the mechanism(s) and etiology of this method of embryo demise.