Regulated expression of telomerase activity in human T lymphocyte development and activation

Stem cell biology for the transfusionist

The limited life-span of most blood cells requires continuous production of cells which in adults may exceed 1012 cells/day. This impressive production of cells (approximately 4.1015 cells over a life time) is achieved by the proliferation and differentiation of committed progenitor cells which themselves are derived from a population of pluripotent stem cells with self-renewal potential. In adults, the large majority of stem cells are found in the bone marrow among cells with a CD34 + CD38- phenotype. Interestingly, small but significant numbers of such cells can be found in the circulation. The frequency of circulating CD34 + CD38- cells can be dramatically increased by treatment with certain compounds including cytokines. Such "mobilized" peripheral blood stem cells have become an important alternative to bone marrow in stem cell transplantation procedures primarily because engraftment is more rapid. The latter is almost certainly related to the increased numbers of primitive CD34 + CD38- cells capable of engrafting the bone marrow in blood versus bone marrow stem cell grafts [1]. Paradoxically, the large majority of "candidate" stem cells in adult bone marrow are quiescent cells. One possibility is that stem cells, like other somatic cells, have only a limited replicative potential (< 100 divisions). This hypothesis is supported by two key observations and the consideration that, in theory, 52 divisions can yield 4.1015 cells. First, it was shown that "candidate" stem cells purified from fetal and adult tissue display marked functional differences in turn-over time and the ability to produce cells with stem cell properties [2]. Secondly, these functional differences were found to correlate with a measurable loss of telomere repeats [3], despite the presence of low but readily detectable levels of telomerase in all purified cell fractions [4,5]. In order to address questions about the role of telomeres in normal and malignant hematopoiesis, we developed quantitative fluorescence in situ hybridization [6]. With this technique the length of telomere repeats at individual chromosome ends can be reliably estimated using optical density measurements from digital images of metaphase chromosomes after fluorescence in situ hybridization with directly labeled (CCCTAA)3--Peptide Nucleic Acid Probe [6,7]. Furthermore, we recently showed that this method can be adapted to measure the total telomere repeat content of cells by flow cytometry [8]. Here some issues in studies of hematopoietic stem cells are discussed in relation to rapidly accumulating information about telomere biology.

T Cell Renewal Rates, Telomerase, and Telomere Length Shortening

Measurements on the average telomere lengths of normal human naive and memory T cells suggested that 1) naive and memory human T cells have similar division rates, and 2) that the difference between naive and memory cells reflects the degree of clonal expansion during normal immune reactions. Here we develop mathematic models describing how the population average of telomere length depends on the cell division rates of naive and memory T cells during clonal expansion and normal renewal. The results show that 1) telomeres shorten with twice the cell division rate, 2) that the conventional approach of estimating telomere length shortening per mean population doubling gives rise to estimates that are 39% larger than the “true” loss per cell division, 3) that naive and memory T cells are expected to shorten their telomeres at rates set by the division rate of the naive T cells only, i.e., irrespective of the division rate of memory T cells, 4) that the measured difference in the average telomere length between naive and memory T cells may largely reflect the difference in renewal rates between these subpopulations rather than the clonal expansion, and 5) that full telomerase compensation during clonal expansion is consistent with all data on the shortening of telomere length in, and between, naive and memory T cells. Thus we reconcile the apparent contradictions between the demonstrated difference in division rates between human naive and memory T cells and their similar rates of telomere shortening, and the demonstrated telomere shortening in the presence of telomerase activity.

Cutting Edge: Antigen-Dependent Regulation of Telomerase Activity in Murine T Cells

Telomeres, structures on the ends of linear chromosomes, function to maintain chromosomal integrity. Telomere shortening occurs with cell division and provides a mechanism for limiting the replicative potential of normal human somatic cells. Telomerase, a ribonucleoprotein enzyme, synthesizes telomeric repeats on chromosomal termini, potentially extending the capacity for cell division. The present study demonstrates that resting T cells express little/no activity, and optimal Ag-specific induction of telomerase activity in vitro requires both TCR and CD28-B7 costimulatory signals. Regulation of telomerase in T cells during in vivo Ag-dependent activation was also assessed by adoptive transfer of TCR transgenic T cells and subsequent Ag challenge. Under these conditions, telomerase was induced in transgenic T cells coincident with a phase of extensive clonal expansion. These findings suggest that telomerase may represent an adoptive response that functions to preserve replicative potential in Ag-reactive lymphocytes.

HHV-8 is not associated with follicular dendritic cell tumours

Follicular dendritic cell tumours are rare malignancies derived from the follicular dendritic cells of lymphoid follicles. These tumours have been associated with Epstein-Barr virus infections and with the hyaline vascular subtype of Castleman's disease. Because many examples of Castleman's disease have been associated with Kaposi's sarcoma associated herpes virus (HHV-8), this study uses polymerase chain reaction technology to examine five cases of follicular dendritic cell tumours for HHV-8. One of these cases had previously been documented to arise from pre-existing Castleman's disease. HHV-8 DNA was not detected in any of the follicular dendritic cell tumours examined, or in the original case of Castleman's disease. These findings suggest that HHV-8 plays no role in the aetiology of follicular dendritic cell tumours and the cause of this tumour remains obscure.

The correlation of telomerase and IL-10 with leukemia transformation in a mouse model of chronic lymphocytic leukemia (CLL)

Telomerase activity is upregulated in activated and malignant lymphocytes. We studied the correlation of telomerase and IL-10 to leukemia transformation in the NZB mouse model of human chronic lymphocytic leukemia (CLL). Telomerase levels increased from early to late leukemic stages, likewise IL-10 gene expression levels increased with the leukemic progression. The inverse relationship of telomerase and IL-10 levels to the survival of NZB mice was also established. Our data suggested that telomerase and IL-10 were involved in transformation in the murine model of CLL and the detection of telomerase activities might be of value in the prediction of CLL progression.

Telomeres, the nucleolus and aging

Reactivation of telomerase in cultured human cells extends their replicative life span beyond the Hayflick limit. How telomere shortening triggers cell senescence and whether it contributes to aging in vivo are under investigation. Studies in yeast have revealed another site critical to cellular aging: the nucleolus. The accumulation of ribosomal DNA circles is a cause of aging in this organism. The possible relevance of this mechanism to human aging is also being considered.

Telomerase activity might persist in the human thymus throughout life

Telomerase activity has been demonstrated at low levels in peripheral blood lymphocytes but at high levels in germinal centre B cells and thymocyte subpopulations. This study shows that telomerase is activated in the normal human thymus at different times of life. Telomerase activity was detected in thymic protein extracts from two newborn babies and from a 12 year old boy, as well as in extracts from two of six adult patients. The two positive cases were patients aged 54 and 66 years. These results suggest strongly that the thymus can remain functional despite involution in elderly patients.

Telomerase Activity in Candidate Stem Cells From Fetal Liver and Adult Bone Marrow

Telomerase is a ribonucleoprotein polymerase that synthesizes telomeric repeats onto the 3′ ends of eukaryotic chromosomes. Activation of telomerase may prevent telomeric shortening and correlates with cell immortality in the germline and certain tumor cells. Candidate hematopoietic stem cells (HSC) from adult bone marrow express low levels of telomerase, which is upregulated with proliferation and/or differentiation. To address this issue, we stimulated purified candidate HSC from human adult bone marrow with stem cell factor (SCF), interleukin-3 (IL-3), and Flt3-ligand (FL). After 5 days in culture, activity was detected in total cell extracts from IL-3–, SCF + FL–, SCF + IL-3–, FL + IL-3–, and SCF + IL-3 + FL–stimulated cultures, but not from cells cultured in SCF or FL alone. Within the CD34+fraction of the cultured cells, significant activity was found in the CD34+CD71+ fraction. In addition, PKH26 staining confirmed that detectable telomerase activity was present in dividing PKH26lo cells, whereas nondividing PKH26hi cells were telomerase negative. Because in these experiments no distinction could be made between cycling “candidate” stem cells that had retained or had lost self-renewal properties, fetal liver cells with a CD34+CD38− phenotype, highly enriched for cycling stem cells, were also examined and found to express readily detectable levels of telomerase activity. Given the replication-dependent loss of telomeric DNA in hematopoietic cells, these observations suggest that the observed telomerase activity in candidate stem cells is either expressed in a minor subset of stem cells or, more likely, is not sufficient to prevent telomere shortening.

Telomerase Activity in Candidate Stem Cells From Fetal Liver and Adult Bone Marrow

Telomerase is a ribonucleoprotein polymerase that synthesizes telomeric repeats onto the 3′ ends of eukaryotic chromosomes. Activation of telomerase may prevent telomeric shortening and correlates with cell immortality in the germline and certain tumor cells. Candidate hematopoietic stem cells (HSC) from adult bone marrow express low levels of telomerase, which is upregulated with proliferation and/or differentiation. To address this issue, we stimulated purified candidate HSC from human adult bone marrow with stem cell factor (SCF), interleukin-3 (IL-3), and Flt3-ligand (FL). After 5 days in culture, activity was detected in total cell extracts from IL-3–, SCF + FL–, SCF + IL-3–, FL + IL-3–, and SCF + IL-3 + FL–stimulated cultures, but not from cells cultured in SCF or FL alone. Within the CD34+fraction of the cultured cells, significant activity was found in the CD34+CD71+ fraction. In addition, PKH26 staining confirmed that detectable telomerase activity was present in dividing PKH26lo cells, whereas nondividing PKH26hi cells were telomerase negative. Because in these experiments no distinction could be made between cycling “candidate” stem cells that had retained or had lost self-renewal properties, fetal liver cells with a CD34+CD38− phenotype, highly enriched for cycling stem cells, were also examined and found to express readily detectable levels of telomerase activity. Given the replication-dependent loss of telomeric DNA in hematopoietic cells, these observations suggest that the observed telomerase activity in candidate stem cells is either expressed in a minor subset of stem cells or, more likely, is not sufficient to prevent telomere shortening.

Telomeres, telomerase, and the cell cycle

Telomeres protect the ends of chromosomes from degradation and fusion. In most eukaryotes telomeres are replicated by a specialised polymerase, telomerase. Telomerase synthesises one strand of the telomere; while conventional DNA polymerases synthesise the complementary strand. Additional processing of telomeres occurs in ciliates and yeast during each cell cycle. Telomerase activity and RNA levels change as cells enter and exit the cell cycle. Gradual telomere shortening in the absence of telomerase does not immediately affect cell cycling; however, "critically" short telomeres are hypothesised to play a role in senescence and the triggering of DNA damage checkpoints.

Essential role of mouse telomerase in highly proliferative organs

We have investigated the role of the enzyme telomerase in highly proliferative organs in successive generations of mice lacking telomerase RNA. Late-generation animals exhibited defective spermatogenesis, with increased programmed cell death (apoptosis) and decreased proliferation in the testis. The proliferative capacity of haematopoietic cells in the bone marrow and spleen was also compromised. These progressively adverse effects coincided with substantial erosion of telomeres (the termini of eukaryotic chromosomes) and fusion and loss of chromosomes. These findings indicate an essential role for telomerase, and hence telomeres, in the maintenance of genomic integrity and in the long-term viability of high-renewal organ systems.

Telomeres in the haemopoietic system

The limited life span of most blood cells requires the continuous production of cells, which in adults exceeds 10(12) cells/day. This impressive production of cells (approximately 4 x 10(16) cells over a lifetime) is achieved by the proliferation and differentiation of committed progenitor cells, which themselves are derived from a population of pluripotent stem cells with self-renewal potential. Paradoxically, the large majority of stem cells in adult bone marrow are quiescent cells. One possibility is that stem cells, like other somatic cells, have only a limited replicative potential (< 100 divisions). This hypothesis is supported by two key observations and the consideration that, in theory, 55 divisions can yield 4 x 10(16) cells. First, it was shown that 'candidate' stem cells purified from fetal and adult tissue showed dramatic functional differences in turn-over time and the ability to produce cells with stem cell properties, Second, these functional differences were found to correlate with a measurable loss of telomere repeats despite the presence of low but readily detectable levels of telomerase in all purified cell fractions. In order to address questions about the role of telomeres in normal and malignant haemopoiesis, we developed a quantitative fluorescence in situ hybridization technique. Here we review the characteristics of this novel tool to assess the number of telomere repeats at the end of individual chromosomes and provide an overview of recent observations.

Telomeres and HIV-1 infection: in search of exhaustion

Telomere length analysis could be helpful in determining if exhaustion and replicative senescence are involved in HIV-1 pathogenesis. Evidence that CD8+ T cells have shorter telomeres may point towards an increased turnover of CD8+ T cells and exhaustion of the CD8+ T-cell responses in HIV-1 infection. In CD4+ T cells, the relationship between telomere length and turnover remains controversial; however, telomere length analysis argues against exhaustion of CD4+ T cells.

Telomere length dynamics in telomerase-positive immortal human cell populations

It has been proposed that the progressive shortening of telomeres in somatic cells eventually results in senescence. Previous experiments have demonstrated that many immortal cell lines have acquired telomerase activity leading to stabilization of telomere length. Telomere dynamics and telomerase activity were examined in the telomerase-positive immortal cell lines HeLa and 293 and subclones derived from them. A mass culture of HeLa cells had a stable mean telomere length over 60 population doublings (PD) in vitro. Subclones of this culture, however, had a range of mean telomere lengths indicating that telomeric heterogeneity exists within a population with a stable mean telomere length. Some of the subclones lacked detectable telomerase activity soon after isolation but regained it by PD 18, suggesting that at least some of the variation in telomere length can be attributed to variations in telomerase activity levels. 293 subclones also varied in telomere length and telomerase activity. Some telomerase-positive 293 subclones contained long telomeres that gradually shortened, demonstrating that factors other than telomerase also act to modulate telomere length. Fluctuations in telomere length in telomerase-positive immortalized cells may contribute to chromosomal instability and clonal evolution.

Human lymphocyte telomerase is genetically regulated

Research on both cancer and aging has focused attention on the regulation of telomerase, the enzyme that synthesizes the ends of chromosomal DNA. To analyze the relative importance of genetic vs. environmental factors in determining telomerase inducibility, we have compared telomerase activity in phytohemagglutinin-stimulated peripheral blood lymphocytes from monozygotic and dizygotic twin pairs. The heritability calculated was 0.814, indicating that lymphocyte inducible telomerase activity is determined principally by genetic rather than by environmental factors.

Telomerase activity in acute myelogenous leukaemia: clinical and biological implications

We examined telomerase activity in myeloid leukaemic cell lines, normal haemopoietic cells, and leukaemic blasts from acute myelogenous leukaemia (AML) patients. Normal bone marrow mononuclear (BMNC) cells expressed low telomerase activity. Higher telomerase activity was detected in 10 myeloid leukaemic cell lines compared to normal BMNC cells. Treatment with 1,25(OH)2D3, and vitamin D3 analogues, EB1089 and KH1060, reduced telomerase activity in vitamin D3-sensitive HL-60 cells, whereas vitamin D3 insensitive K562 cells did not change its activity. This down-regulation of telomerase activity by EB1089 was associated with induction of p21 protein. The rank order of telomerase activity was leukaemic CD34- cells > leukaemic CD34+ cells > normal CD34- cells > normal CD34+ cells. Telomerase activity was positive in all of the AML patients tested; however, heterogeneity of telomerase activity was found amongst this group. Therefore we compared telomerase activity with clinical response. Unexpectedly, we found that a higher rate of complete remission was noted in AML patients with higher telomerase activity. No association between telomerase activity and biological parameters including percentage of S-phase, cytotoxicity to cytosine arabinoside and percentage of CD34+ cells in AML blasts was found. These results suggest that telomerase activity in AML patients is detected with high frequency, but is heterogenous. Expression level of telomerase activity may have a clinical implication in AML patients regarding clinical response.

Inhibition of telomerase activity by PKC inhibitors in human nasopharyngeal cancer cells in culture

Telomerase is a specialized ribonucleoprotein polymerase that adds hexanucleotides (TTAGGG) onto human chromosomal ends. The expression of telomerase activity has been associated with cell immortalization and the malignant phenotype in most cancers. How the telomerase activity is regulated in cancer cells is presently not known. In this work, the effects of cell cycle blockers, DNA damaging agents, TopII inhibitors and proteins kinase inhibitors on the telomerase activity were examined in cultured nasopharyngeal carcinoma cells NPC-076. Agents which interfere with tubulin assembly (Taxol and vinblastine) and agents which arrest cells at S phase (methotrexate and 5-fluorouracil) did not inhibit telomerase activity of treated cells. Agents which damage DNA (cisplatin, methyl methanesulfonate, and UV radiation) and TopII inhibitors (etoposide and daunorubicin) also did not inhibit telomerase activity of treated cells. Among the protein kinase inhibitors examined, no significant inhibition of telomerase activity was observed with cells treated with quercetin, H-89, or herbimycin A. On the other hand, two protein kinase C (PKC) inhibitors (bisindolylmaleimide I and H-7) were found to produce a big inhibition of telomerase activity in treated cells. Staurosporine produced a moderate inhibition, and sphingosine had a small inhibitory effect. The inhibition of telomerase activity by PKC inhibitors appears to be specific since the treated cells were mostly viable (i.e., greater than 75%) and still retained significant levels of protein synthesis capability. These results implicate that protein kinase C is involved in the regulation of telomerase activity in vivo.

Tales of tails: regulation of telomere length and telomerase activity during lymphocyte development, differentiation, activation, and aging

Telomerase activity and the regulation of telomere length are factors which have been implicated in the control of cellular replication. These variables have been examined during human lymphocyte development, differentiation, activation, and aging. It was found that telomere length of peripheral blood CD4+ T cells decreases with age as well as with differentiation from naive to memory cells in vivo, and decreases with cell division in vitro. These results provide evidence that telomere length correlates with lymphocyte replicative history and residual replicative potential. In contrast, telomere length appears to increase during tonsil B-cell differentiation and germinal center (GC) formation in vivo. It was also found that telomerase activity is highly regulated during T-cell development and B-cell differentiation in vivo, with high levels of telomerase activity expressed in thymocytes and GC B cells, and low levels of telomerase activity in resting mature peripheral blood lymphocytes. Finally, resting lymphocytes retain the ability to upregulate telomerase activity upon activation, and this capacity does not appear to decline with age. Although the precise role of telomerase in lymphocyte function remains to be elucidated, telomerase may contribute to protection from telomere shortening in T and B lymphocytes, and may thus play a critical role in lymphocyte development, differentiation and activation. The future study of telomerase and its regulation of telomere length may enhance our understanding of how the replicative lifespan is regulated in lymphocytes.

Effects of the immunosuppressive drugs CsA and FK506 on intracellular signalling and gene regulation

The isolation of Cyclosporin A (CsA) from cultures of the fungus Tolypocladium inflatum and its subsequent elucidation of immunosuppressive properties by Borel et al. (1) was of great clinical consequence. In the early 80s CsA was introduced in the field of organ transplantation resulting in extraordinary improvements of graft survival. CsA has become a first choice drug for patients with allograft organs. The discovery of FK506 by Kino et al. (2) as a novel immuno-suppressant and its introduction into clinics in 1989 (3) extended the available regimen for immunosuppressive therapy. Yet despite their advantages both CsA and FK506 display unwanted side effects and a possible preference of one drug over another remains controversial (4, 5). Although identification of the involvement of the transcription factor NF-AT was an important step forward (6), it has become clear that immunosuppressant action is more complex. CsA and FK506 selectively interact with certain cellular signal transduction pathways. This review briefly describes these effects on signal transduction. We further concentrate on the major known effect of these immunosuppressants, namely the inhibition of the PP2B phosphatase calcineurin. In addition we provide a compilation of effects of CsA and FK506 on gene expression at the level of transcription.

Nucleic acid-based methods for the detection of cancer

Continued elucidation of the genetic changes that drive cancer progression is yielding new and potentially powerful nucleic acid-based markers of neoplastic disease. Pilot studies indicate that these markers can be used to detect cancer cells in a variety of clinical settings with unprecedented precision. Nucleic acid-based markers may prove to be valuable tools for early detection of cancer in asymptomatic individuals, for confirmation or exclusion of a cancer diagnosis that is based on suspicious but nondiagnostic clinical material, for assessment of tumor burden in cancer patients, and for assessment of response to preventive approaches applied to healthy individuals who are at high risk of developing cancer. Examples of these markers, their potential applications, and the current practical limitations on their clinical use are reviewed here.

Loss of CD28 expression on T lymphocytes: a marker of replicative senescence

The CD28 molecule, a disulfide-linked homodimer expressed on peripheral T cells and thymocytes, mediates an essential costimulatory signal following engagement of the T cell receptor (TCR). Increased proportions of CD28- T cells have been observed during aging and in situations of chronic immune stimulation, but the origin and functional characteristics of these cells have been unclear. T cells which reach replicative senescence in culture after multiple rounds of cell division have shortened telomeres, respond poorly to stress stimuli, and no longer express CD28, suggesting that CD28- T cells observed in vivo may be the progeny of memory cells which have been repeatedly stimulated. This review describes the features of T cell replicative senescence, presents several possible mechanisms for the generation of senescent T cells in vivo, and proposes that replicative senescence may explain immune exhaustion.

Preclinical and clinical strategies for development of telomerase and telomere inhibitors

BACKGROUND

Telomerase is an important enzyme whose activity has been convincingly demonstrated in humans recently. It is required for maintenance of ends of chromosomes (telomeres) during cell division. Since its presence has been selectively demonstrated in dividing cells including tumor cells, it has generated considerable excitement as a potential anti-cancer strategy.

DESIGN

In this article, we review the current relevant biology of the enzyme, the challenges encountered in the preclinical phase of target development and the current efforts that focus on telomeres and telomerase as therapeutic targets. We also speculate on the potential toxicities and mechanisms of resistance that may be encountered during use of such therapies.

Telomere lengthening and telomerase activation during human B cell differentiation

The function of the immune system is highly dependent on cellular differentiation and clonal expansion of antigen-specific lymphocytes. However, little is known about mechanisms that may have evolved to protect replicative potential in actively dividing lymphocytes during immune differentiation and response. Here we report an analysis of telomere length and telomerase expression, factors implicated in the regulation of cellular replicative lifespan, in human B cell subsets. In contrast to previous observations, in which telomere shortening and concomitant loss of replicative potential occur in the process of somatic cell differentiation and cell division, it was found that germinal center (GC) B cells, a compartment characterized by extensive clonal expansion and selection, had significantly longer telomeric restriction fragments than those of precursor naive B cells. Furthermore, it was found that telomerase, a telomere-synthesizing enzyme, is expressed at high levels in GC B cells (at least 128-fold higher than those of naive and memory B cells), correlating with the long telomeres in this subset of B cells. Finally, both naive and memory B cells were capable of up-regulating telomerase activity in vitro in response to activation signals through the B cell antigen receptor in the presence of CD40 engagement and/or interleukin 4. These observations suggest that a novel process of telomere lengthening, possibly mediated by telomerase, functions in actively dividing GC B lymphocytes and may play a critical role in humoral immune response by maintaining the replicative potential of GC and descendant memory B cells.

Replicative senescence of T cells: does the Hayflick Limit lead to immune exhaustion?

Extensive in vitro research on fibroblasts has defined numerous genetic and phenotypic changes associated with replicative senescence. Identification of T-cell replicative senescence as a feature of human immunodeficiency virus (HIV) disease and ageing suggests this phenomenon merits more careful consideration by immunologists, especially with regard to chronic infection, memory and adoptive immunotherapy.

Protein phosphatase 2A inhibits nuclear telomerase activity in human breast cancer cells

Most cancer cells have increased levels of telomerase activity implicated in cell immortalization. Activation of telomerase, a ribonucleoprotein complex, catalyzes the elongation of the ends of mammalian chromosomal DNA (telomeres), the length of which regulates cell proliferation. Currently, how telomerase is regulated in cancer is not yet established. The present study shows that telomerase activity is regulated by protein phosphorylation in human breast cancer cells. Incubation of cell nuclear telomerase extracts with protein phosphatase 2A (PP2A) abolished the telomerase activity; in contrast cytoplasmic telomerase activity was unaffected, and protein phosphatases 1 and 2B were ineffective. Inhibition of telomerase activity by PP2A was both concentration- and time-dependent and was prevented by the protein phosphatase inhibitor okadaic acid. In addition, nuclear telomerase inhibited by PP2A was reactivated by endogenous protein kinase(s) in the presence of ATP, but not in the presence of ATPgammaS. Furthermore, telomerase activity in cultured human breast cancer PMC42 cells was stimulated by okadaic acid, consistent with a role for PP2A in the regulation of telomerase activity in intact cells. These findings suggest that protein phosphorylation reversibly regulates the function of telomerase and that PP2A is a telomerase inhibitory factor in the nucleus of human breast cancer cells.

Telomerase activity in human gynaecological malignancies

AIM

To evaluate whether increased telomerase activity can be clinically useful for detecting malignant cells in a variety of gynaecological specimens.

METHODS

Telomerase activity was examined in frozen tissue samples of histologically confirmed lesions of the endometrium, ovary, and cervix. It was also assessed in exfoliated cells in cervical smears from patients with premalignant and malignant lesions and in ascitic fluid obtained from cases with malignant or non-malignant ovarian tumours.

RESULTS

Solid tissues from carcinomas were telomerase positive in all specimens of endometrial (6/6) and cervical (6/6) origin, and in almost all from the ovary (12/13). Normal tissues from the cervix (0/5) and the ovary (0/5) were telomerase negative, but samples from normal endometrium were found to show telomerase activity, possibly due to the cyclical regenerative nature of this tissue. Conversely, dissociated cells in cervical smears from preneoplastic and frankly neoplastic lesions rarely showed detectable telomerase activity. Thus smears from patients with malignant tumours were only positive in one of two patients, whereas those from CIN-2 (0/5) and CIN-3 (1/17) lesions and from normal (0/10) samples were almost all negative. Telomerase activity was also scarcely detectable in cells obtained from ascitic fluid from patients with ovarian tumours.

CONCLUSIONS

As in many other organs, telomerase activity is increased in solid tissue specimens from malignant tumours of the female reproductive tract, but it is not yet a reliable indicator of the presence of exfoliated cancerous or precancerous cells in clinical specimens from such lesions. Interpretation should be guarded until more extensive studies have been conducted. The data on solid tissues presented here confirm that activation of this enzyme is a major hallmark of the neoplastic process.

Telomerase activity is elevated in early S phase in hamster cells

Telomerase is a ribonucleoprotein that elongates telomeric repeats de novo. We examined the possibility that telomerase activity is cell cycle regulated by examining telomerase activity in cell cycle synchronized Chinese hamster ovary (CHO) B11 cells. Overall telomerase activity was similar in growing and quiescent cells. Further, cells synchronized in G1, S, or G2/M showed similar levels of telomerase activity. However, a detailed analysis of cells within S phase showed that there was a higher level of telomerase activity in early S phase when compared with other points in the cell cycle. These results suggest a relationship between telomerase activity and cell cycle regulation.

Telomere length, telomerase activity, and replicative potential in HIV infection: analysis of CD4+ and CD8+ T cells from HIV-discordant monozygotic twins

To address the possible role of replicative senescence in human immunodeficiency virus (HIV) infection, telomere length, telomerase activity, and in vitro replicative capacity were assessed in peripheral blood T cells from HIV+ and HIV- donors. Genetic and age-specific effects on these parameters were controlled by studying HIV-discordant pairs of monozygotic twins. Telomere terminal restriction fragment (TRF) lengths from CD4+ T cells of HIV+ donors were significantly greater than those from HIV- twins. In contrast, telomere lengths in CD8+ T cells from HIV+ donors were shorter than in HIV- donors. The in vitro replicative capacity of CD4+ cells from HIV+ donors was equivalent to that of HIV- donors in response to stimulation through T cell receptor CD3 and CD28. Little or no telomerase activity was detected in freshly isolated CD4+ or CD8+ lymphocytes from HIV+ or HIV- donors, but was induced by in vitro stimulation of both HIV+ and HIV- donor cells. These results suggest that HIV infection is associated with alterations in the population dynamics of both CD4+ and CD8+ T cells, but fail to provide evidence for clonal exhaustion or replicative senescence as a mechanism underlying the decline in CD4+ T cells of HIV-infected donors.

Clinical implications of telomerase in cancer

Cellular immortality is believed to be a critical step in tumorigenesis. As an important component of the telomere maintenance mechanism, the activation of the enzyme telomerase is tightly associated with cellular immortality and cancer. Telomerase expression is detected in a majority of tumours, but is absent in most somatic tissues and correlates to clinical outcome in a number of cancer types. Telomerase expression is associated with the stage of differentiation but not necessarily with the rate of cell proliferation. Data also indicate that inhibition or absence of telomerase may result in cell crisis in cancer cells and tumour regression in cancer patients. These results suggest that cancer therapy based on telomerase inhibition could be a more effective and safer treatment for cancer, as well as provide a more accurate means for diagnosing and predicting clinical outcome in cancer. Complete understanding of the role of telomerase in tumorigenesis through well-designed clinical studies will have a significant clinical impact on the treatment and diagnosis of cancer.

Telomeres and telomerase in normal and malignant haematopoietic cells

The normal haematopoietic system harbours telomerase-competent cells with a capacity to upregulate the activity to notable levels in a telomere length-independent manner. Strong telomerase activity is found in progenitor stem cells and activated lymphocytes in vitro as well as in vivo, indicating that cells with high growth requirements can readily upregulate telomerase. Despite detection of telomerase activity, a gradual telomere erosion occurs in stem cells and lymphocytes, with significantly shortened telomeres at higher ages, a phenomenon that might be of importance for developing immunosenescence and exhausted haematopoiesis. In malignant haematopoietic disorders telomerase activity is a general finding with large differences in activity levels. The strongest telomerase expression has been shown in acute leukaemias and non-Hodgkin's lymphomas, especially high grade cases. There are indications that the level of activity might parallel tumour progression and be of prognostic relevance, but studies of larger patient materials are needed. An association between the cell cycle and telomerase activity exists, especially for normal haematopoietic cells, and induction of a differentiation programme in immortalised cell lines downregulates telomerase activity. The expression of telomerase activity seems to be regulated at different levels, since for immature bone marrow cells the level of activity seemed to parallel better the phenotype than the proliferation state. The frequent expression of telomerase in leukaemias and lymphomas makes these disorders interesting targets for future anti-telomerase therapy.

The biology of replicative senescence

Most cells cannot divide indefinitely due to a process termed cellular or replicative senescence. Replicative senescence appears to be a fundamental feature of somatic cells, with the exception of most tumour cells and possibly certain stem cells. How do cells sense the number of divisions they have completed? Although it has not yet been critically tested, the telomere shortening hypothesis is currently perhaps the best explanation for a cell division 'counting' mechanism. Why do cells irreversibly cease proliferation after completing a finite number of divisions? It is now known that replicative senescence alters the expression of a few crucial growth-regulatory genes. It is not known how these changes in growth-regulatory gene expression are related to telomere shortening in higher eukaryotes. However, lower eukaryotes have provided several plausible mechanisms. Finally, what are the physiological consequences of replicative senescence? Several lines of evidence suggest that, at least in human cells, replicative senescence is a powerful tumour suppressive mechanism. There is also indirect evidence that replicative senescence contributes to ageing. Taken together, current findings suggest that, at least in mammals, replicative senescence may have evolved to curtail tumorigenesis, but may also have the unselected effect of contributing to age-related pathologies, including cancer.

Changes in telomerase activity and telomere length during human T lymphocyte senescence

It has been proposed that telomeres shorten with every cell cycle because the normal mechanism of DNA replication cannot replicate the end sequences of the lagging DNA strand. Telomerase, a ribonucleoprotein enzyme that synthesizes telomeric DNA repeats at the DNA 3' ends of eukaryotic chromosomes, can compensate for such shortening, by extending the template of the lagging strand. Telomerase activity has been identified in human germline cells and in neoplastic immortal somatic cells, but not in most normal somatic cells, which senesce after a certain number of cell divisions. We and others have found that telomerase activity is present in normal human lymphocytes and is upregulated when the cells are activated. But, unlike the immortal cell lines, presence of telomerase activity is not sufficient to make T cells immortal and telomeres from these cells shorten continuously during in vitro culture. After senescence, telomerase activity, as detected by the TRAP technique, was downregulated. A cytotoxic T lymphocyte (CTL) cell line that was established in the laboratory has very short terminal restriction fragments (TRFs). Telomerase activity in this cell line is induced during activation and this activity is tightly correlated with cell proliferation. The level of telomerase activity in activated peripheral blood T cells, the CTL cell line, and two leukemia cell lines does not correlate with the average TRF length, suggesting that other factors besides telomerase activity are involved in the regulation of telomere length.

Proliferation-associated regulation of telomerase activity in human endometrium and its potential implication in early cancer diagnosis

Telomerase activity was detected in normal endometrium in association with proliferation and regulated during the menstrual cycle in a hormone-dependent manner. The activity was maximal at the late-proliferative phase to mid-secreting phase, and was absent or extremely low at early-proliferative phase and late-secreting phase. Activity was also detected in all endometrial simple hyperplasias tested (16 of 16) and in most cancers (28 of 30), but none was detected in endometrium of either pregnant or postmenopausal women in the absence of hyperplasia. Our data provide evidence that the telomerase activity in postmenopausal endometrium reflects a hyperproliferative condition. Therefore, we conclude that telomerase can provide a novel marker for early endometrial cancer diagnosis. Hormone-dependent regulation of telomerase suggests the possibility of therapeutic and preventive strategies for endometrial cancers through the management of ovarian steroid hormones or other agents that regulate telomerase activity.

Telomerase activity and in situ telomerase RNA expression in malignant and non-malignant lymph nodes

AIMS/BACKGROUND

Telomerase, an enzyme associated with cellular immortality, is expressed by most malignant tumours, but is inactive in normal somatic cells except for male germ cells and proliferating stem cells. Thus, the measurement of telomerase activity in tissue samples may provide useful diagnostic and prognostic information. The aim of this study was to determine whether telomerase expression is useful for the detection of occult malignant cells in lymph nodes.

METHODS

Telomerase activity was compared with histological findings in 123 surgically removed lymph nodes submitted for routine or frozen section diagnosis. Telomerase activity was measured using a modified, semi-quantitative PCR-based telomeric repeat amplification protocol (TRAP). The assay was adapted for single 5 microns OCT embedded cryostat sections. In either fresh tissues or cryostat sections, normalised activity was linear when compared with protein concentration. Furthermore, using an in situ hybridisation method, the human telomerase RNA (hTR) component was measured in a subset of negative and positive nodes.

RESULTS

Most (96%) of the 97 histologically negative nodes expressed low levels of activity (mean value of positive samples = 3.0 units/microgram protein) which may be derived from activated lymphocytes that express telomerase activity. All 26 malignant nodes (17 metastases, nine lymphomas) expressed telomerase (mean value = 17.8 units/microgram protein). The rank order levels between the two groups differed significantly (p = 0.0002). In situ results showed clearly that the hTR was expressed relatively highly in metastatic cancer cells and at lower levels in germinal centres of secondary follicles.

CONCLUSIONS

Although expression of telomerase by activated lymphocytes may limit its usefulness, measurement of enzyme activity combined with detection of hTR using in situ hybridisation may assist in the histopathological diagnosis of lymph nodes.

Telomerase activity concentrates in the mitotically active segments of human hair follicles

Telomerase is a ribonucleoprotein enzyme capable of adding hexanucleotide repeats onto the ends of linear chromosomal DNA. Whereas normal somatic cells with a limited replicative capacity fail to express telomerase activity, most immortal eukaryotic cells do. Cells of renewal tissues (e.g., skin, intestine, blood) require an extensive proliferative capacity. Some cells in such renewal tissues also express telomerase activity, most likely to prevent rapid erosion of their telomeres during cell proliferation. In this study, we measured the levels of telomerase activity in dissected compartments of the human hair follicle: hair shaft, gland-containing fragment, upper intermediate fragment (where it is thought undifferentiated stem cells reside), lower intermediate fragment, and in the bulb-containing fragment (an area with high mitotic activity containing a more differentiated pool of keratinocytes). In anagen follicles, high levels of telomerase activity were found almost exclusively in the bulb-containing fragment of the follicles, with low levels of telomerase in the bulge area (intermediate fragments) and gland-containing fragment. In comparison, catagen follicles had low levels of telomerase activity in the bulb-containing fragments as well as in other compartments. Such observations indicate that, in anagen hair follicles, the fragments containing cells actively dividing (e.g., transient amplifying cells) express telomerase activity, whereas fragments containing cells with low mitotic activity, for example, quiescent stem cells, express low levels of telomerase activity.

Telomeres, Telomerase and Leukaemia

There is increasing evidence supporting the hypothesis that telomere shortening both in vitro and in vivo is the clock that counts cell divisions and determines the onset of cellular senescence. Cells overcome the senescence mechanisms by stabilising telomere length, probably due to the activity of telomerase that specifically elongates telomeres. The striking observation that almost all malignant cancers have telomerase activity indicate that there is intensive selective pressure of telomerase activation with the progression of malignancy. Indirect support for this view is that benign or pre-cancerous lesions are telomerase silent. The fact that telomerase activity is observed in over 85% of human primary malignancies raises the possibilities that it may be a new marker of cancer with diagnostic and therapeutic potentials. Can such ideas be applied to leukaemias and preleukaemias? Since normal haematopoietic stem cells and their progeny express telomerase activity, it is important to consider whether or not telomere shortening and telomerase activity play any role in leukaemic progression. Telomere reduction has been observed in various leukaemias including in ALL, AML, transformed leukaemias from MDS and late stage of CML and CLL and might be indicative of the length of the disease. Elevated telomerase activity has also been found in ALL, AML, CML and CLL. In AML, patients with chromosomal abnormalities 11q, -5, -7 had higher telomerase activity and unfavourable prognosis; while those with favourable cytogenetics such as t(8:21), inversion 16 showed low levels of telomerase. This suggests that telomerase activity may be a marker for poor prognosis of AML. Unanswered at present is the potential role of telomeres and telomerase in the progression of benign to malignant tumours. Further studies on the expression and regulation of the individual components of telomerase may enable us to clarify the diagnostic and therapeutic potential of telomerase in leukaemias.

T cell telomere length in HIV-1 infection: no evidence for increased CD4+ T cell turnover

Progression to acquired immunodeficiency syndrome (AIDS) has been related to exhaustion of the regenerative capacity of the immune system resulting from high T cell turnover. Analysis of telomeric terminal restriction fragment (TRF) length, a marker for cellular replicative history, showed that CD8(+) T cell TRF length decreased but CD4(+) T cell TRF length was stable during the course of human immunodeficiency virus type-1 (HIV-1) infection, which was not explained by differential telomerase activity. This observation provides evidence that turnover in the course of HIV-1 infection can be increased considerably in CD8(+) T cells, but not in CD4(+) T cells. These results are compatible with CD4(+) T cell decline in HIV-1 infection caused by interference with cell renewal.

Telomerase regulation during entry into the cell cycle in normal human T cells

Telomerase activity is involved in telomere length maintenance. Leukocytes, unlike many human somatic tissues, have detectable telomerase activity. These cells provide a normal human cell type in which to study telomerase. We studied the regulation of telomerase activity and the telomerase RNA component as leukocytes were stimulated to enter the cell cycle. In primary human leukocytes stimulated with phytohemagglutinin, telomerase activity increased > 10-fold as naturally quiescent cells entered the cell cycle. Antibodies to the T cell receptor (TCR)/CD3 complex and the costimulatory CD28 receptor induced telomerase activity in a T cell-enriched population of cells. Rapamycin, an immunosuppressant that blocks TCR/CD3 signal transduction pathways and cdk2 activation, blocked telomerase induction. Hydroxyurea, an inhibitor of S phase, did not block cdk2 kinase activity or telomerase activation. In summary, telomerase is regulated in G1 phase as normal human T cells enter the cell cycle.

Telomerase activity in hematopoietic cells is associated with self-renewal potential

It has been proposed that the biological clock underlying the limited division potential of eukaryotic cells is telomere length. We assayed telomerase activity in single cells of the hematopoietic and immune systems. We examined hematopoietic stem cells at four stages of differentiation, lineage-committed progenitors, and mature myeloid and lymphoid cells. The frequency of telomerase-expressing cells within each population was proportional to the frequency of cells thought to have self-renewal potential. Among bone marrow hematopoietic stem cells, 70% exhibited detectable telomerase activity. The telomerase-expressing somatic cells observed in this study are not thought to be immortal, and expression was not correlated with cell cycle distribution or differentiation state. This study demonstrates that the developmental characteristic most consistently associated with telomerase expression is self-renewal potential.