The role of telomerase expression and telomere length maintenance in human and mouse

Telomeres and immunodeficiencies

The function of the immune system is highly dependent on cellular differentiation and clonal expansion of antigen-specific lymphocytes. Telomeres are conserved DNA-protein structures of linear chromosome termini. Telomere length has been investigated to be different in various lymphocyte subpopulations depending on their function and to change with aging. Association of accelerated telomere loss compared to matched controls has already been confirmed in many syndromes with immune dysregulation. Immunodeficiencies connected with dysfunction of telomere termini are dyskeratosis congenita, ICF syndrome (Immunodeficiency, centromeric instability and facial anomalies syndrome) genetic disorders involving DNA repair and disorders involving the VDJ recombination.

hTERT Transduction Extends the Lifespan of Primary Pediatric Low-Grade Glioma Cells While Preserving the Biological Response to NGF

The neurotrophin nerve growth factor (NGF) modulates the growth of human gliomas and is able to induce cell differentiation through the engagement of tropomyosin receptor kinase A (TrkA) receptor, although the role played in controlling glioma survival has proved controversial. Unfortunately, the slow growth rate of low-grade gliomas (LGG) has made it difficult to investigate NGF effects on these tumors in preclinical models. In fact, patient-derived low-grade human astrocytoma cells duplicate only a limited number of times in culture before undergoing senescence. Nevertheless, replicative senescence can be counteracted by overexpression of hTERT, the catalytic subunit of telomerase, which potentially increases the proliferative potential of human cells without inducing cancer-associated changes. We have extended, by hTERT transduction, the proliferative in vitro potential of a human LGG cell line derived from a pediatric pilocytic astrocytoma (PA) surgical sample. Remarkably, the hTERT-transduced LGG cells showed a behavior similar to that of the parental line in terms of biological responses to NGF treatment, including molecular events associated with induction of NGF-related differentiation. Therefore, transduction of LGG cells with hTERT can provide a valid approach to increase the in vitro life-span of patient-derived astrocytoma primary cultures, characterized by a finite proliferative potential.

Telomere Biology of Human Hematopoietic Stem Cells

BACKGROUND

Telomeres are protein DNA structures present at the ends of chromosomes and are essential for genetic stability and cell replication. Telomerase is the enzyme complex that maintains telomere integrity. Hematopoietic stem cells express telomerase and contain long telomeres, which become shorter as cells differentiate and mature. The extent of telomere shortening and the level of telomerase activity often correlate with the presence and severity of some hematopoietic diseases.

METHODS

The fundamentals of telomeres and telomerase are reviewed, and the telomere biology of human hematopoietic cells is discussed.

RESULTS

Telomere length and telomerase activity are important in the self-renewal of hematopoietic stem cells. Changes within these compartments affect both normal hematopoietic cells and the generation of hematopoietic disease. Telomere length provides information pertaining to the proliferative history and potential of a hematopoietic cell.

CONCLUSIONS

The role of telomerase and telomeres within the hematopoietic compartment needs further clarification. Advances in our knowledge in this field may improve clinical outcomes for the treatment of hematologic disease.

Transplantation of Immortalized CD34+ and CD34- Adipose-Derived Stem Cells Improve Cardiac Function and Mitigate Systemic Pro-Inflammatory Responses

Adipose-derived stem cells (ADSCs) have the potential to differentiate into various cell lineages and they are easily obtainable from patients, which makes them a promising candidate for cell therapy. However, a drawback is their limited life span during in vitro culture. Therefore, hTERT-immortalized CD34+ and CD34- mouse ADSC lines (mADSCs^hTERT) tagged with GFP were established. We evaluated the proliferation capacity, multi-differentiation potential, and secretory profiles of CD34+ and CD34- mADSCs^hTERTin vitro, as well as their effects on cardiac function and systemic inflammation following transplantation into a rat model of acute myocardial infarction (AMI) to assess whether these cells could be used as a novel cell source for regeneration therapy in the cardiovascular field. CD34+ and CD34- mADSCs^hTERT demonstrated phenotypic characteristics and multi-differentiation potentials similar to those of primary mADSCs. CD34+ mADSCs^hTERT exhibited a higher proliferation ability compared to CD34- mADSCs^hTERT, whereas CD34- mADSCs^hTERT showed a higher osteogenic differentiation potential compared to CD34+ mADSCs^hTERT. Primary mADSCs, CD34+, and CD34- mADSCs^hTERT primarily secreted EGF, TGF-β1, IGF-1, IGF-2, MCP-1, and HGFR. CD34+ mADSCs^hTERT had higher secretion of VEGF and SDF-1 compared to CD34- mADSCs^hTERT. IL-6 secretion was severely reduced in both CD34+ and CD34- mADSCs^hTERT compared to primary mADSCs. Transplantation of CD34+ and CD34- mADSCs^hTERT significantly improved the left ventricular ejection fraction and reduced infarct size compared to AMI-induced rats after 28 days. At 28 days after transplantation, engraftment of CD34+ and CD34- mADSCs^hTERT was confirmed by positive Y chromosome staining, and differentiation of CD34+ and CD34- mADSCs^hTERT into endothelial cells was found in the infarcted myocardium. Significant decreases were observed in circulating IL-6 levels in CD34+ and CD34- mADSCs^hTERT groups compared to the AMI-induced control group. Transplantation of CD34- mADSCs^hTERT significantly reduced circulating MCP-1 levels compared to the AMI control and CD34+ mADSCs^hTERT groups. GFP-tagged CD34+ and CD34- mADSCs^hTERT are valuable resources for cell differentiation studies in vitro as well as for regeneration therapy in vivo.

Plasma irisin levels predict telomere length in healthy adults

The ageing process is strongly influenced by nutrient balance, such that modest calorie restriction (CR) extends lifespan in mammals. Irisin, a newly described hormone released from skeletal muscles after exercise, may induce CR-like effects by increasing adipose tissue energy expenditure. Using telomere length as a marker of ageing, this study investigates associations between body composition, plasma irisin levels and peripheral blood mononuclear cell telomere length in healthy, non-obese individuals. Segmental body composition (by bioimpedance), telomere length and plasma irisin levels were assessed in 81 healthy individuals (age 43 ± 15.8 years, BMI 24.3 ± 2.9 kg/m(2)). Data showed significant correlations between log-transformed relative telomere length and the following: age (p < 0.001), height (p = 0.045), total body fat percentage (p = 0.031), abdominal fat percentage (p = 0.038), visceral fat level (p < 0.001), plasma leptin (p = 0.029) and plasma irisin (p = 0.011), respectively. Multiple regression analysis using backward elimination revealed that relative telomere length can be predicted by age (b = -0.00735, p = 0.001) and plasma irisin levels (b = 0.04527, p = 0.021). These data support the view that irisin may have a role in the modulation of both energy balance and the ageing process.

IFN-α inhibits telomerase in human CD8⁺ T cells by both hTERT downregulation and induction of p38 MAPK signaling

The cytokine IFN-α is secreted during viral infections and has been shown to inhibit telomerase activity and accelerate T cell differentiation in vivo. However, the mechanism for this inhibition is not clear. In this study, we show that IFN-α inhibits both the transcription and translation of human telomerase reverse transcriptase (hTERT), the catalytic component of telomerase, in activated CD8(+) T cells. This was associated with increased activity of the repressor of hTERT transcription E2 transcription factor and decreased activation of NF-κB that promotes hTERT transcription. However IFN-α did not affect the translocation of hTERT from the cytoplasm to the nucleus. IFN-α also inhibits AKT kinase activation but increases p38 MAPK activity, and both of these events have been shown previously to inhibit telomerase activity. Addition of BIRB796, an inhibitor of p38 activity, to IFN-α-treated cells reversed, in part, the inhibition of telomerase by this cytokine. Therefore, IFN-α can inhibit the enzyme telomerase in CD8(+) T cells by transcriptional and posttranslational mechanisms. Furthermore, the addition of IFN-α to CD8(+)CD27(+)CD28(+) T cells accelerates the loss of both these costimulatory molecules. This suggests that persistent viral infections may contribute to the accumulation of highly differentiated/senescent CD8(+)CD27(-)CD28(-) T cells during aging by promoting IFN-α secretion during repeated episodes of viral reactivation.

Mosaicism for trisomy 21 and ring (21) in a male born to normal parents: a case report

We present a case of a ring (21) in a mentally challenged patient with mosaicism for trisomy 21 showing karyotype 47, XY,+21/47,XY,+21(r)/46,XY, born to normal parents. The parents and female sibling were phenotypically normal. This is a unique case report from Central India, on occurrence of trisomy 21 and r (21) in the same individual born to normal parents. Also being documented for the first time is the immuno-FISH analysis revealing differential expression of hTERT and a linked over expression of TRF2 in proband, probably corresponding to a high percentage of acrocentric associations.

Aging, cellular senescence, and cancer

For most species, aging promotes a host of degenerative pathologies that are characterized by debilitating losses of tissue or cellular function. However, especially among vertebrates, aging also promotes hyperplastic pathologies, the most deadly of which is cancer. In contrast to the loss of function that characterizes degenerating cells and tissues, malignant (cancerous) cells must acquire new (albeit aberrant) functions that allow them to develop into a lethal tumor. This review discusses the idea that, despite seemingly opposite characteristics, the degenerative and hyperplastic pathologies of aging are at least partly linked by a common biological phenomenon: a cellular stress response known as cellular senescence. The senescence response is widely recognized as a potent tumor suppressive mechanism. However, recent evidence strengthens the idea that it also drives both degenerative and hyperplastic pathologies, most likely by promoting chronic inflammation. Thus, the senescence response may be the result of antagonistically pleiotropic gene action.

Dynamics of telomere's length and telomerase activity in Philadelphia chromosome negative myeloproliferative neoplasms

Telomere exhaustion and increased telomerase activity are associated with the acquisition of aggressive molecular events in a variety of haematological malignancies. In Philadelphia chromosome negative myeloproliferative neoplasms (Ph(neg)MPN's), telomere dynamics during clonal evolution of these diseases have not yet been fully elucidated. Herein we demonstrated that telomere shortening is a global phenomenon in Ph(neg)MPN's, irrespective of disease phenotype, treatment administration and JAK2V617F mutational status but the presence of additional cytogenetic abnormalities further affects them. Consistent with the above finding, TA was upregulated in CD34+ haemopoietic progenitors from almost all Ph(neg)MPN subgroups compared to healthy donors. Moreover, TL below the cut-off value of 27% could predict disease progression in Ph(neg)MPN patients (PFS at 5 years 39% vs 81%). Thus, TL emerges as a new prognostic marker in Ph(neg)MPN, reflecting probably the genetic instability of highly proliferating MPN clones.

NFAT5 regulates transcription of the mouse telomerase reverse transcriptase gene

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

Inhibition of telomerase activity alters tight junction protein expression and induces transendothelial migration of HIV-1-infected cells

Telomerase, via its catalytic component telomerase reverse transcriptase (TERT), extends telomeres of eukaryotic chromosomes. The importance of this reaction is related to the fact that telomere shortening is a rate-limiting mechanism for human life span that induces cell senescence and contributes to the development of age-related pathologies. The aim of the present study was to evaluate whether the modulation of telomerase activity can influence human immunodeficiency virus type 1 (HIV-1)-mediated dysfunction of human brain endothelial cells (hCMEC/D3 cells) and transendothelial migration of HIV-1-infected cells. Telomerase activity was modulated in hCMEC/D3 cells via small interfering RNA-targeting human TERT (hTERT) or by using a specific pharmacological inhibitor of telomerase, TAG-6. The inhibition of hTERT resulted in the upregulation of HIV-1-induced overexpression of intercellular adhesion molecule-1 via the nuclear factor-kappaB-regulated mechanism and induced the transendothelial migration of HIV-1-infected monocytic U937 cells. In addition, the blocking of hTERT activity potentiated a HIV-induced downregulation of the expression of tight junction proteins. These results were confirmed in TERT-deficient mice injected with HIV-1-specific protein Tat into the cerebral vasculature. Further studies revealed that the upregulation of matrix metalloproteinase-9 is the underlying mechanisms of disruption of tight junction proteins in hCMEC/D3 cells with inhibited TERT and exposed to HIV-1. These results indicate that the senescence of brain endothelial cells may predispose to the HIV-induced upregulation of inflammatory mediators and the disruption of the barrier function at the level of the brain endothelium.

Bone marrow-derived cells: the influence of aging and cellular senescence

During the course of an entire lifespan, tissue repair and regeneration is made possible by the presence of adult stem cells. Stem cell expansion, maintenance, and differentiation must be tightly controlled to assure longevity. Hematopoietic stem cells (HSC) are greatly solicited given the daily high blood cell turnover. Moreover, several bone marrow-derived cells including HSC, mesenchymal stromal cells (MSC), and endothelial progenitor cells (EPC) also significantly contribute to peripheral tissue repair and regeneration, including tumor formation. Therefore, factors influencing bone marrow-derived cell proliferation and functions are likely to have a broad impact. Aging has been identified as one of these factors. One hypothesis is that aging directly affects stem cells as a consequence of exhaustive proliferation. Alternatively, it is also possible that aging indirectly affects stem cells by acting on their microenvironment. Cellular senescence is believed to have evolved as a tumor suppressor mechanism capable of arresting growth to reduce risk of malignancy. In opposition to apoptosis, senescent cells accumulate in tissues. Recent evidence suggests their accumulation contributes to the phenotype of aging. Senescence can be activated by both telomere-dependent and telomere-independent pathways. Genetic alteration, genome-wide DNA damage, and oxidative stress are inducers of senescence and have recently been identified as occurring in bone marrow-derived cells. Below is a review of the link between cellular senescence, aging, and bone marrow-derived cells, and the possible consequences aging may have on bone marrow trans plantation procedures and emerging marrow-derived cell-based therapies.

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

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

Age-associated deficiency in activation-induced up-regulation of telomerase activity in CD4+ T cells

For lymphocytes, the ability to undergo clonal expansion is crucial for effective immune function. Telomerase activity compensates for telomere erosion during cell division and contributes to the capability of lymphocytes to maintain cellular proliferation. In addition, telomerase activity may have a fundamental role in cell growth and survival. To determine whether age-related immune dysfunction is associated with an abnormality in telomerase activity, we investigated telomerase activity in T cell populations from young adult and aged mice. Our data show that the ability of T cells from aged mice to up-regulate telomerase activity after activation was significantly diminished. This age-related deficiency in telomerase induction is restricted to CD4(+) T cells, as CD8(+) T cells retain the capability to up-regulate telomerase activity. These findings reinforce the notion that age-related immune dysfunction results mainly from impairment of helper T cells, and may have important implications for designing novel means to improve immune responses in aged individuals by enhancing CD8(+) T cell functions, which are crucial in both viral and tumour immunity.

Measurable immune dysfunction and telomere attrition in long-term allogeneic transplant recipients

This study was conducted to determine if the accelerated telomere attrition that occurs as a consequence of allogeneic stem cell transplantation leads to measurable functional defects. Telomere lengths in mononuclear leukocytes obtained from 15 long-term allogeneic stem cell transplant recipients and their respective donors were determined by Southern hybridization and densitometric analysis. Functional assays evaluated the ability of these cells to proliferate in response to a mitogenic stimulus and to differentiate under appropriate cytokine stimulation. Lymphocyte proliferation in response to phytohemagglutinin was determined by measurement of (3)[H]thymidine uptake. The ability of circulating myeloid cells to differentiate was determined after incubation of peripheral blood mononuclear cells with IL-3 and GM-CSF. A total of 13 patients demonstrated telomeric loss, ranging from 0.1 to 3.7 kbp. Strikingly, lymphocytes from 14 of the 15 patients demonstrated a significant decrease in proliferation when compared to their respective donors (68%+/-22, P=0.001). All patients demonstrated at least a 50% decrease in the number of myeloid colony-forming units when compared to their respective donors (P<0.0001). A decreased ability of hematopoietic cells to proliferate and differentiate is phenotypically consistent with an aged immune system. This may correlate with diminished clinically relevant immune responses to infection or vaccination, as seen in the elderly.

Expression of telomerase RNA template, but not telomerase reverse transcriptase, is limiting for telomere length maintenance in vivo

Telomerase consists of two essential components, the telomerase RNA template (TR) and telomerase reverse transcriptase (TERT). The haplo-insufficiency of TR was recently shown to cause one form of human dyskeratosis congenita, an inherited disease marked by abnormal telomere shortening. Consistent with this finding, we recently reported that mice heterozygous for inactivation of mouse TR exhibit a similar haplo-insufficiency and are deficient in the ability to elongate telomeres in vivo. To further assess the genetic regulation of telomerase activity, we have compared the abilities of TR-deficient and TERT-deficient mice to maintain or elongate telomeres in interspecies crosses. Homozygous TERT knockout mice had no telomerase activity and failed to maintain telomere length. In contrast, TERT(+/-) heterozygotes had no detectable defect in telomere elongation compared to wild-type controls, whereas TR(+/-) heterozygotes were deficient in telomere elongation. Levels of TERT mRNA in heterozygous mice were one-third to one-half the levels expressed in wild-type mice, similar to the reductions in telomerase RNA observed in TR heterozygotes. These findings indicate that both TR and TERT are essential for telomere maintenance and elongation but that gene copy number and transcriptional regulation of TR, but not TERT, are limiting for telomerase activity under the in vivo conditions analyzed.

Telomere cap components influence the rate of senescence in telomerase-deficient yeast cells

Cells lacking telomerase undergo senescence, a progressive reduction in cell division that involves a cell cycle delay and culminates in "crisis," a period when most cells become inviable. In telomerase-deficient Saccharomyces cerevisiae cells lacking components of the nonsense-mediated mRNA decay (NMD) pathway (Upf1,Upf2, or Upf3 proteins), senescence is delayed, with crisis occurring approximately 10 to 25 population doublings later than in Upf+ cells. Delayed senescence is seen in upfDelta cells lacking the telomerase holoenzyme components Est2p and TLC1 RNA, as well as in cells lacking the telomerase regulators Est1p and Est3p. The delay of senescence in upfDelta cells is not due to an increased rate of survivor formation. Rather, it is caused by alterations in the telomere cap, composed of Cdc13p, Stn1p, and Ten1p. In upfDelta mutants, STN1 and TEN1 levels are increased. Increasing the levels of Stn1p and Ten1p in Upf+ cells is sufficient to delay senescence. In addition, cdc13-2 mutants exhibit delayed senescence rates similar to those of upfDelta cells. Thus, changes in the telomere cap structure are sufficient to affect the rate of senescence in the absence of telomerase. Furthermore, the NMD pathway affects the rate of senescence in telomerase-deficient cells by altering the stoichiometry of telomere cap components.

Homeostatic control of T-cell generation in neonates

T cells are produced through 2 mechanisms, thymopoiesis and proliferative expansion of postthymic T cells. Thymic output generates diversity of the pool, and proliferation achieves optimal clonal size of each individual T cell. To determine the contribution of these 2 mechanisms to the formation of the initial T-cell repertoire, we examined neonates of 30 to 40 weeks' gestation. Peripheral T cells were in a state of high proliferative turnover. In premature infants, 10% of T cells were dividing; the proliferation rates then declined but were still elevated in mature newborns. Throughout the third trimester, concentrations of T-cell-receptor excision circles (TRECs) were 10 per 100 T cells. Stability of TREC frequencies throughout the period of repertoire generation suggested strict regulation of clonal size to approximately 10 to 20 cells. Neonatal naive CD4+ and CD8+ T cells were explicitly responsive to IL-7; growth-promoting properties of IL-15 were selective for newborn CD8+ T cells. Neonatal T cells expressed telomerase and, in spite of the high turnover, built up a telomeric reserve. Thus, proliferative expansion, facilitated by increased cytokine responsiveness, and thymopoiesis complement each other as mechanisms of T-cell production in neonates. Maintaining optimal clonal size instead of filling the space in a lymphopenic host appears to regulate homeostatic T-cell proliferation during fetal development.

Telomere shortening in leukocyte subpopulations from baboons

To address questions about telomere length regulation in nonhuman primates, we studied the telomere length in subpopulations of leukocytes from the peripheral blood of baboons aged 0.2-26.5 years. Telomere length in granulocytes, B cells, and subpopulations of T cells all decreased with age. Overall, telomere length kinetics were lineage- and cell subset-specific. T cells showed the most pronounced, overall decline in telomere length. Levels of telomerase in stimulated T cells from old animals were lower than in corresponding cells from young animals. Memory T cells with very short telomeres accumulated in old animals. In contrast, the average telomere length values in B cells remained relatively constant from middle age onward. Individual B cells showed highly variable telomere length, and B cells with very long telomeres were observed after the ages of 1-2 years. In general, cell type-specific telomere kinetics in baboons were remarkably similar to those observed in humans.

Induction of telomerase activity and maintenance of telomere length in virus-specific effector and memory CD8+ T cells

Acute viral infections induce extensive proliferation and differentiation of virus-specific CD8+ T cells. One mechanism reported to regulate the proliferative capacity of activated lymphocytes is mediated by the effect of telomerase in maintaining the length of telomeres in proliferating cells. We examined the regulation of telomerase activity and telomere length in naive CD8+ T cells and in virus-specific CD8+ T cells isolated from mice infected with lymphocytic choriomeningitis virus. These studies reveal that, compared with naive CD8+ T cells, which express little or no telomerase activity, Ag-specific effector and long-lived memory CD8+ T cells express high levels of telomerase activity. Despite the extensive clonal expansion that occurs during acute lymphocytic choriomeningitis virus infection, telomere length is maintained in both effector and memory CD8+ T cells. These results suggest that induction of telomerase activity in Ag-specific effector and memory CD8+ T cells is important for the extensive clonal expansion of both primary and secondary effector cells and for the maintenance and longevity of the memory CD8+ T cell population.

Cancer and aging: a model for the cancer promoting effects of the aging stroma

The incidence of cancer rises exponentially with age in humans and many other mammalian species. Malignant tumors are caused by an accumulation of oncogenic mutations. In addition, malignant tumorigenesis requires a permissive tissue environment in which mutant cells can survive, proliferate, and express their neoplastic phenotype. We propose that the age-related increase in cancer results from a synergy between the accumulation of mutations and age-related, pro-oncogenic changes in the tissue milieu. Most age-related cancers derive from the epithelial cells of renewable tissues. An important element of epithelial tissues is the stroma, the sub-epithelial layer composed of extracellular matrix and several cell types. The stroma is maintained, remodeled and repaired by resident fibroblasts, supports and instructs the epithelium, and is essential for epithelial function. One change that occurs in tissues during aging is the accumulation of epithelial cells and fibroblasts that have undergone cellular senescence. Cellular senescence irreversibly arrests proliferation in response to damage or stimuli that put cells at risk for neoplastic transformation. Senescent cells secrete factors that can disrupt tissue architecture and/or stimulate nearby cells to proliferate. We therefore speculate that their presence may create a pro-oncogenic tissue environment that synergizes with oncogenic mutations to drive the rise in cancer incidence with age. Recent evidence lends support to this idea, and suggests that senescent stromal fibroblasts may be particularly adept at creating a tissue environment that can promote the development of age-related epithelial cancers.

Reversible manipulation of telomerase expression and telomere length. Implications for the ionizing radiation response and replicative senescence of human cells

Most human cells do not express telomerase and irreversibly arrest proliferation after a finite number of divisions (replicative senescence). Several lines of evidence suggest that replicative senescence is caused by short dysfunctional telomeres, which arise when DNA is replicated in the absence of adequate telomerase activity. We describe a method to reversibly bypass replicative senescence and generate mass cultures that have different average telomere lengths. A retrovirus carrying hTERT flanked by excision sites for Cre recombinase rendered normal human fibroblasts telomerase-positive and replicatively immortal. Superinfection with retroviruses carrying wild-type or mutant forms of TIN2, a negative regulator of telomere length, created telomerase-positive, immortal populations with varying average telomere lengths. Subsequent infection with a Cre-expressing retrovirus abolished telomerase activity, creating mortal cells with varying telomere lengths. Using these cell populations, we show that, after hTERT excision, cells senesce with shorter telomeres than parental cells. Moreover, long telomeres, but not telomerase, protected cells from the loss of division potential caused by ionizing radiation. Finally, although telomerase-negative cells with short telomeres senesced after fewer doublings than those with long telomeres, telomere length per se did not correlate with senescence. Our results support a role for telomere structure, rather than length, in replicative senescence.

Proliferation and telomere length in acutely mobilized blood mononuclear cells in HIV infected patients

The aim of the study was to investigate the mobilization of T cells in response to a stressful challenge (adrenalin stimulation), and to access T cells resided in the peripheral lymphoid organs in HIV infected patients. Seventeen patients and eight HIV seronegative controls received an adrenalin infusion for 1 h. Blood was sampled before, during and 1 h after adrenalin infusion. Proliferation and mean telomere restriction fragment length (telomeres) of blood mononuclear cells (BMNC) and purified CD8+ and CD4+ cells were investigated at all time points. In patients, the proliferation to pokeweed mitogens (PWM) was lower and decreased more during adrenalin infusion. After adrenalin infusion the proliferation to PWM was restored only in the controls. In all subjects telomeres in CD4+ cells declined during adrenalin infusion. Additionally, the patients had shortened telomeres in their CD8+ cells, and particularly HAART treated patients had shortened telomeres in all cell-subtypes. The finding that patients mobilized cells with an impaired proliferation to PWM during and after adrenalin infusion has possible clinical relevance for HIV infected patients during pathological stressful conditions, such as sepsis, surgery and burns. However, this study did not find a correlation between impaired proliferation and telomeres. It is concluded that physiological stress further aggravates the HIV-induced immune deficiency.

Telomerase in cancer and aging

The telomere-telomerase hypothesis is the science of cellular aging (senescence) and cancer. The ends of chromosomes, telomeres, count the number of divisions a cell can undergo before entering permanent growth arrest. As divisions are being counted, events occur on the cellular and molecular level, which may either delay or hasten this arrest. As humans age, a particular concern is the accumulation of events that lead to the progression of cancer. Telomerase is a mechanism that most normal cells do not possess, but almost all cancer cells acquire, to overcome their mortality and extend their lifespan. This review aims to provide a comprehensive understanding of the role of telomerase in cancer development, progression, diagnosis, and in the future, treatment. The ultimate goal of telomerase research is to use our understanding to develop anti-telomerase therapies, an almost universal tumor target.

Telomeres and telomerase: basic science implications for aging

Life expectancy in the United States and other developed nations has increased remarkably over the past century, and continues to increase. However, lifespan has remained relatively unchanged over this period. As life expectancy approaches maximum human lifespan, further increase in life expectancy would only be possible if lifespan could also be increased. Although little is known about the aging process, increasing lifespan and delaying aging are the research challenges of the new century, and have caused intense debate and research activities among biogerontologists. Many theories have been proposed to explain the aging process. However, damage to deoxyribonucleic acid (DNA) is the centerpiece of most of these. Recently telomere shortening has been described to be associated with DNA damage. Located at the ends of eukaryotic chromosomes and synthesized by telomerase, telomeres maintain the length of chromosomes. The loss of telomeres can lead to DNA damage. The association between cellular senescence and telomere shortening in vitro is well established. In the laboratory, telomerase-negative differentiated somatic cells maintain a youthful state, instead of aging, when transfected with vectors encoding telomerase. Many human cancer cells demonstrate high telomerase activity. Evidence is also accumulating that telomere shortening is associated with cellular senescence in vivo. What causes changes in expression of telomerase in different cell types and premature aging syndromes? Does the key to "youthfulness" lie in our ability to control the expression of telomerase? We have reviewed the contemporary literature to find answers to these questions and explore the association between aging, telomeres, and telomerase.