RESUMO
Ionizing radiation has been reported to promote accelerated or premature senescence in both normal and tumour cell lines. The current studies were designed to characterize the accelerated senescence response to radiation in the breast tumour cell in terms of its dependence on functional p53 and its relationship to telomerase activity, telomere lengths, expression of human telomerase reverse transcriptase (hTERT, the catalytic subunit of telomerase) and human telomerase RNA (hTR, the RNA subunit of telomerase), as well as the induction of cytogenetic aberrations. Studies were performed in p53 wild-type MCF-7 cells, MCF-7/E6 cells with attenuated p53 function, MDA-MB231 cells with mutant p53 and MCF-7/hTERT cells with constitutive expression of hTERT. Telomerase activity was measured by the telomeric repeat amplification protocol (TRAP assay), telomere lengths by the terminal restriction fragment (TRF) assay, hTR and hTERT expression by reverse transcriptase-polymerase chain reaction (RT-PCR), senescence by beta-galactosidase staining, and apoptosis by TdT-mediated d-UTP-X nick-end labelling (TUNEL assay). Widespread and extensive expression of beta-galactosidase, a marker of cellular senescence, was evident in MCF-7 breast tumour cells following exposure to 10 Gy of ionizing radiation. Radiation did not suppress expression of either hTERT or hTR, alter telomerase activity or induce telomere shortening. Senescence arrest was also observed in irradiated MCF-7/hTERT cells, which have elongated telomeres due to the ectopic expression of the catalytic component of telomerase. In contrast to MCF-7 cells, irradiated MDA-MB231 breast tumour cells and MCF-7/E6 cells failed to senesce and instead demonstrated a delayed apoptotic cell death. Irradiation produced chromosome end associated abnormalities, including end-to-end fusions (an indicator of telomere dysfunction) in MCF-7 cells, MCF-7/hTERT cells, as well as in MCF-7/E6 cells. When cells were maintained in culture following irradiation, proliferative recovery was evident exclusively after senescence while the cell lines which responded to radiation by apoptosis continued to decline in cell number. Accelerated senescence in response to ionizing radiation is p53 dependent and associated with telomer dysfunction but is unrelated to changes in telomerase activity or telomere lengths, expression of hTERT and hTR. In the absence of functional p53, cells are unable to arrest for an extended period, resulting in apoptotic cell death while accelerated senescence in cells expressing p53 is succeeded by proliferative recovery.
Assuntos
Neoplasias da Mama/radioterapia , Senescência Celular/efeitos da radiação , Proteína Supressora de Tumor p53/fisiologia , Apoptose/efeitos da radiação , Neoplasias da Mama/genética , Neoplasias da Mama/patologia , Ciclo Celular/efeitos da radiação , Linhagem Celular Tumoral , Aberrações Cromossômicas , Proteínas de Ligação a DNA/metabolismo , Feminino , Humanos , RNA/análise , Telomerase/análise , Telomerase/metabolismo , TelômeroRESUMO
Telomerase activity has been detected in >85% of all malignant human cancers, including 90% of prostate carcinomas. Using a well-characterized experimental prostate cancer system, we have found that telomerase activity is notably increased (>10-fold) during tumorigenic conversion. Expression profiles of the telomerase components (hTR and hTERT) revealed no substantive changes, which suggests a nontranscriptional mechanism for increased activity. Because the hsp90 chaperone complex functionally associates with telomerase, we investigated that relationship and found that along with telomerase activity, a number of hsp90-related chaperones are markedly elevated during transformation, as well as in advanced prostate carcinomas. Using the nontumorigenic cell protein extract as the source of telomerase, addition of purified chaperone components enhanced reconstitution of telomerase activity, which suggests a novel mechanism of increased telomerase assembly via a hsp90 chaperoning process during prostate cancer progression.
Assuntos
Proteínas de Choque Térmico HSP90/metabolismo , Neoplasias da Próstata/metabolismo , Telomerase/metabolismo , Animais , Transformação Celular Neoplásica/metabolismo , Proteínas de Ligação a DNA , Progressão da Doença , Proteínas de Choque Térmico HSP90/biossíntese , Humanos , Oxirredutases Intramoleculares , Masculino , Camundongos , Camundongos Nus , Chaperonas Moleculares/biossíntese , Chaperonas Moleculares/metabolismo , Fosfoproteínas/biossíntese , Fosfoproteínas/metabolismo , Prostaglandina-E Sintases , Neoplasias da Próstata/enzimologia , Neoplasias da Próstata/patologia , RNA/metabolismo , Telomerase/biossíntese , Moldes GenéticosRESUMO
The ribonucleoprotein telomerase holoenzyme is minimally composed of a catalytic subunit, hTERT, and its associated template RNA component, hTR. We have previously found two additional components of the telomerase holoenzyme, the chaperones p23 and heat shock protein (hsp) 90, both of which are required for efficient telomerase assembly in vitro and in vivo. Both hsp90 and p23 bind specifically to hTERT and influence its proper assembly with the template RNA, hTR. We report here that the hsp70 chaperone also associates with hTERT in the absence of hTR and dissociates when telomerase is folded into its active state, similar to what occurs with other chaperone targets. Our data also indicate that hsp90 and p23 remain associated with functional telomerase complexes, which differs from other hsp90-folded enzymes that require only a transient hsp90.p23 binding. Our data suggest that components of the hsp90 chaperone complex, while required for telomerase assembly, remain associated with active enzyme, which may ultimately provide critical insight into the biochemical properties of telomerase assembly.
Assuntos
Proteínas de Choque Térmico HSP70/metabolismo , Proteínas de Choque Térmico HSP90/metabolismo , Proteínas de Choque Térmico/metabolismo , RNA , Telomerase/metabolismo , Sítios de Ligação , Domínio Catalítico , Proteínas de Ligação a DNA , Proteínas de Drosophila , Proteínas de Choque Térmico HSP70/química , Proteínas de Choque Térmico HSP90/química , Proteínas de Choque Térmico/química , Humanos , Cinética , Modelos Moleculares , Conformação Proteica , Proteínas Recombinantes/química , Proteínas Recombinantes/metabolismo , Telomerase/química , Moldes GenéticosRESUMO
The onset of human cancer typically requires numerous genetic mutations, generally specific for the tissue type from which the cancer originates. Thus, it has been difficult to screen all tumor types for a single mutation. In recent years, telomerase activity has been associated with at least 85% of human malignancies as well as with some lesions considered preneoplastic by traditional cytology (1,2). Telomerase appears to be ubiquitously associated with a wide array of human cancers from a variety of tissue sources. Therefore, detection of telomerase activity relative to human cancer development is likely to be an important and novel method, in combination with cytology, for cancer diagnosis.
RESUMO
Telomerase activity is present in approximately 85% of malignant cancers and >30% of premalignant lesions. Extraction of telomerase from solid tumors and tissues remains difficult and inconvenient due to the presence of PCR inhibitors. Here we show that PCR inhibitors are easily removed during an initial extraction, allowing detection of telomerase activity in subsequent extractions from the same sample. In addition, telomerase activity may be enriched and detected from very small samples in a large background by utilizing a biotin/strept-avidin coated, magnetic bead retrieval assay. Our results provide alternative methods for telomerase extraction from solid tumors and small samples that are more convenient and accurate.
Assuntos
Neoplasias/enzimologia , Telomerase/isolamento & purificação , Neoplasias da Mama/enzimologia , Humanos , Métodos , Telomerase/metabolismoRESUMO
Introduction of telomerase into normal cells provides telomere maintenance and an extended cellular life span, establishing the critical role of telomere attrition in cellular senescence. Additional data surrounding this observation suggest that expression of telomerase renders these "mortal" cells genomically stable with decreased frequencies of mutation, ultimately leading to continued proliferation without signs of changes typically associated with progression to a cancer-like phenotype. Interestingly, oncogenic insult after exogenous telomerase expression does not result in cellular transformation, yet addition of an oncogene first followed by telomerase does transform cells. Taken together, these results imply that order of addition is important for telomerase-mediated genomic protection and that telomerase expression is critical for the transformation process. The hypothesis proposed here is that telomerase, via its function in telomere stabilization, is capable of protecting cells from acquiring the required mutations and genomic instability necessary for malignant transformation, suggesting that telomerase is not an oncogene but may act as a novel class of tumor suppressor.
Assuntos
Transformação Celular Neoplásica , Genes Supressores de Tumor , Telomerase , Telômero , Animais , Estabilidade Enzimática , HumanosRESUMO
Human fibroblasts expressing the catalytic component of human telomerase (hTERT) have been followed for 250-400 population doublings. As expected, telomerase activity declined in long term culture of stable transfectants. Surprisingly, however, clones with average telomere lengths several kilobases shorter than those of senescent parental cells continued to proliferate. Although the longest telomeres shortened, the size of the shortest telomeres was maintained. Cells with subsenescent telomere lengths proliferated for an additional 20 doublings after inhibiting telomerase activity with a dominant-negative hTERT mutant. These results indicate that, under conditions of limiting telomerase activity, cis-acting signals may recruit telomerase to act on the shortest telomeres, argue against the hypothesis that the mortality stage 1 mechanism of cellular senescence is regulated by telomere positional effects (in which subtelomeric loci silenced by long telomeres are expressed when telomeres become short), and suggest that catalytically active telomerase is not required to provide a protein-capping role at the end of very short telomeres.
Assuntos
Telomerase/metabolismo , Telômero/metabolismo , Linhagem Celular Transformada , Senescência Celular , Fibroblastos/citologia , Fibroblastos/fisiologia , Humanos , Cinética , Masculino , Proteínas Recombinantes/metabolismo , Pele/citologia , Telomerase/genética , Telômero/ultraestrutura , TransfecçãoRESUMO
The ribonucleoprotein, telomerase, is responsible for the maintenance of telomere length in most immortal and cancer cells. Telomerase appears to be a marker of human malignancy with at least 85% of human cancers expressing its activity. In the present study, we examined a series of tumor-derived and in vitro immortalized cell lines for telomerase activity levels, telomere lengths, and expression levels of the RNA and catalytic components of telomerase. We found significant variability in both telomere lengths and telomerase activity in clones from tumor cells. In addition, the levels of telomerase components or telomerase activity were not predictive of telomere length. Data from clonally derived cells suggest that critically shortened telomeres in these tumor-derived cell lines may signal activation of telomerase activity through an increase in the expression of the catalytic subunit of telomerase. Although clones with low telomerase shorten their telomeres over time, their subclones all have high levels of telomerase activity with no telomere shortening. In addition, analysis of early clones for telomerase activity indicates substantial variability, which suggests that activity levels fluctuate in individual cells. Our data imply that cell populations exhibit a cyclic expression of telomerase activity, which may be partially regulated by telomere shortening.
Assuntos
Células Clonais , Neoplasias/enzimologia , Neoplasias/ultraestrutura , Telomerase/metabolismo , Telômero/ultraestrutura , Linhagem Celular , Linhagem Celular Transformada , Ácidos Cólicos , Células Clonais/enzimologia , Células Clonais/ultraestrutura , Embrião de Mamíferos , Fibroblastos , Humanos , RNA Mensageiro/análise , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Telomerase/genética , Células Tumorais CultivadasRESUMO
Cell senescence and programmed cell death (apoptosis) are two fundamental biological mechanisms that regulate proliferative capacity, survival potential, aging, and death of cells. Here we report several independent lines of experimental evidence that support the hypothesis that telomerase function and telomere length perform important roles in cell survival during apoptosis. First, with serum starvation and matrix-independent survival experiments, we found that young normal diploid cells were more resistant to apoptosis than their older counterparts. In addition, normal cells with stable telomere lengths caused by ectopic expression of telomerase maintained an increased resistance to serum starvation- and matrix-deprivation-induced programmed cell death compared with aged normal cells without telomerase. Second, we found that telomerase-positive immortalized SW39 cells had a higher survival ability and resistance to apoptosis than their telomerase-negative immortalized counterparts, SW13 and SW26. Third, we showed that telomerase-positive cells with experimentally elongated telomeres (GTR-IDH4 and GTR-DU145) acquired increased survival ability and higher resistance to apoptosis than the parental cell lines with shorter telomeres (IDH4 and DU145). Higher resistance to apoptosis of these cells was associated with a deficiency in two major apoptosis execution pathways: induction of nuclear calcium-dependent endonucleases and activation of the interleukin-1 beta-converting enzyme-family of proteases (caspases). Taken together, these results provide the first direct experimental evidence supporting the hypothesis that telomerase activity and maintenance of telomere stability are associated with increased cellular resistance to apoptosis.
Assuntos
Apoptose/genética , Telomerase/metabolismo , Telômero , Linhagem Celular Transformada , Senescência Celular/genética , HumanosRESUMO
We have mapped the 5' and 3' boundaries of the region of the human telomerase RNA (hTR) that is required to produce activity with the human protein catalytic subunit (hTERT) by using in vitro assembly systems derived from rabbit reticulocyte lysates and human cell extracts. The region spanning nucleotides +33 to +325 of the 451-base hTR is the minimal sequence required to produce levels of telomerase activity that are comparable with that made with full-length hTR. Our results suggest that the sequence approximately 270 bases downstream of the template is required for efficient assembly of active telomerase in vitro; this sequence encompasses a substantially larger portion of the 3' end of hTR than previously thought necessary. In addition, we identified two fragments of hTR (nucleotides +33 to +147 and +164 to +325) that cannot produce telomerase activity when combined separately with hTERT but can function together to assemble active telomerase. These results suggest that the minimal sequence of hTR can be divided into two sections, both of which are required for de novo assembly of active telomerase in vitro.
Assuntos
RNA não Traduzido , RNA/química , RNA/metabolismo , Telomerase/química , Telomerase/metabolismo , Animais , Sequência de Bases , Domínio Catalítico/genética , Primers do DNA/genética , Proteínas de Ligação a DNA , Humanos , Técnicas In Vitro , Dados de Sequência Molecular , RNA/genética , RNA Longo não Codificante , Coelhos , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Reticulócitos/metabolismo , Telomerase/genéticaRESUMO
Telomerase is a cellular reverse transcriptase that helps to provide genomic stability in highly proliferative normal, immortal, and tumor cells by maintaining the integrity of the chromosome ends, the telomeres. The activity of telomerase is associated with the majority of malignant human cancers. Telomerase or another mechanism for telomere maintenance is required for continuous tumor cell proliferation. Telomerase-positive cells that exit the cell cycle via quiescence downregulate telomerase through a transcriptional repression pathway. In the case of cell cycle exit via terminal differentiation, proteolysis of telomerase may also be involved. In response to mitogenic or growth factor signaling, telomerase-competent quiescent cells reenter the cell cycle and express telomerase activity independent of DNA synthesis. Under normal growth conditions, inhibition of telomerase activity in tumor-derived cells results in continued cell division coupled with telomere shortening, eventually followed by cellular senescence or death. Thus, repression of telomerase activity may be a novel adjuvant therapy for the treatment of human cancer and detection of telomerase activity may be important for cancer diagnostics.
Assuntos
Células Germinativas/enzimologia , Neoplasias Experimentais/enzimologia , Telomerase/genética , Animais , Divisão Celular/fisiologia , Regulação Enzimológica da Expressão Gênica , Regulação Neoplásica da Expressão Gênica , Células Germinativas/citologia , Humanos , Telomerase/metabolismo , Telômero/enzimologiaRESUMO
A nonrandom sample (N = 41) of inmates from a maximum security prison were classified as either psychopathic or nonpsychopathic (using the Psychopathy Checklist-Revised (PCL-R)) and violent or sexually violent. Sadism was measured using the Millon Clinical Multiaxial Inventory-II (MCMI-II) Scale 6B, the Personality Disorder Examination (PDE) items for sadistic personality disorder, and the sexual sadism criteria of DSM-IV. Psychopaths were found to be significantly more sadistic than nonpsychopaths (MCMI-II and PDE). Overall power was relatively high. Sadism did not differentiate the violent and sexually violent groups. A diagnosis of sexual sadism was too infrequent (n = 3) for meaningful statistical analysis. The trait measures of sadism and psychopathy measures (PCL-R, Factor 1 and Factor 2) significantly and positively correlated. Results provide further empirical validity for the theoretically proposed and clinically observed relationship between sadistic traits and psychopathic personality.
Assuntos
Transtorno da Personalidade Antissocial/psicologia , Prisioneiros/psicologia , Sadismo/psicologia , Delitos Sexuais/psicologia , Violência/psicologia , Adulto , Análise de Variância , Transtorno da Personalidade Antissocial/epidemiologia , California/epidemiologia , Análise Fatorial , Humanos , Masculino , Prisioneiros/estatística & dados numéricos , Sadismo/epidemiologia , Delitos Sexuais/estatística & dados numéricos , Violência/estatística & dados numéricosRESUMO
Most normal human diploid cells have no detectable telomerase; however, expression of the catalytic subunit of telomerase is sufficient to induce telomerase activity and, in many cases, will bypass normal senescence. We and others have previously demonstrated in vitro assembly of active telomerase by combining the purified RNA component with the reverse transcriptase catalytic component synthesized in rabbit reticulocyte extract. Here we show that assembly of active telomerase from in vitro-synthesized components requires the contribution of proteins present in reticulocyte extracts. We have identified the molecular chaperones p23 and Hsp90 as proteins that bind to the catalytic subunit of telomerase. Blockade of this interaction inhibits assembly of active telomerase in vitro. Also, a significant fraction of active telomerase from cell extracts is associated with p23 and Hsp90. Consistent with in vitro results, inhibition of Hsp90 function in cells blocks assembly of active telomerase. To our knowledge, p23 and Hsp90 are the first telomerase-associated proteins demonstrated to contribute to telomerase activity.
Assuntos
Proteínas de Ligação a DNA/metabolismo , Proteínas de Choque Térmico HSP90/metabolismo , Telomerase/metabolismo , Trifosfato de Adenosina/metabolismo , Animais , Benzoquinonas , Western Blotting , Ciclosporina/metabolismo , Relação Dose-Resposta a Droga , Humanos , Técnicas In Vitro , Lactamas Macrocíclicas , Chaperonas Moleculares/metabolismo , Quinonas/metabolismo , DNA Polimerase Dirigida por RNA/metabolismo , Coelhos , Reticulócitos/metabolismo , Fatores de TempoRESUMO
The ectopic expression of telomerase in normal human cells results in an extended lifespan, indicating that telomere shortening regulates the timing of cellular senescence. As telomerase expression is a hallmark of cancer, we investigated the long-term effects of forced expression of human telomerase catalytic component (hTERT) in normal human fibroblasts. In vitro growth requirements, cell-cycle checkpoints and karyotypic stability in telomerase-expressing cells are similar to those of untransfected controls. In addition, co-expression of telomerase, the viral oncoproteins HPV16 E6/E7 (which inactivate p53 and pRB) and oncogenic HRAS does not result in growth in soft agar. Thus, although ectopic expression of telomerase in human fibroblasts is sufficient for immortalization, it does not result in changes typically associated with malignant transformation.
Assuntos
Domínio Catalítico , Senescência Celular , Fibroblastos/citologia , Proteínas/metabolismo , RNA , Proteínas Repressoras , Telomerase/metabolismo , Domínio Catalítico/genética , Divisão Celular , Linhagem Celular , Inibidor de Quinase Dependente de Ciclina p21 , Ciclinas/metabolismo , Proteínas de Ligação a DNA , Galactosidases/biossíntese , Humanos , Proteínas Oncogênicas Virais/genética , Proteínas E7 de Papillomavirus , Fosforilação , Proteínas/genética , Proteínas Proto-Oncogênicas p21(ras)/genética , Proteína do Retinoblastoma/metabolismo , Telomerase/genética , Proteína Supressora de Tumor p53/metabolismoRESUMO
The detectability of telomerase activity in human cells almost always correlates with indefinite proliferation capability (immortalization). To make quantitative statements about telomerase activity levels, complete extraction of the telomerase activity is needed. A series of detergents was tested for this purpose, and a combination of NP-40 and sodium deoxycholate (NaDOC) was found to be the most efficient for extracting telomerase activity. Tumor-derived cell lines originally thought to contain differing amounts of telomerase on the basis of the original CHAPS bases extraction procedures have nearly equivalent amounts of activity when extracted with the NP-40/NaDOC lysis buffer. These results indicate that these lysis conditions can be used to extract telomerase activity more efficiently from tumor-derived cell lines.
Assuntos
Bioquímica/métodos , Neoplasias/enzimologia , Telomerase/análise , Telomerase/metabolismo , Soluções Tampão , Detergentes , Humanos , Células Tumorais CultivadasRESUMO
Normal human cells undergo a finite number of cell divisions and ultimately enter a nondividing state called replicative senescence. It has been proposed that telomere shortening is the molecular clock that triggers senescence. To test this hypothesis, two telomerase-negative normal human cell types, retinal pigment epithelial cells and foreskin fibroblasts, were transfected with vectors encoding the human telomerase catalytic subunit. In contrast to telomerase-negative control clones, which exhibited telomere shortening and senescence, telomerase-expressing clones had elongated telomeres, divided vigorously, and showed reduced straining for beta-galactosidase, a biomarker for senescence. Notably, the telomerase-expressing clones have a normal karyotype and have already exceeded their normal life-span by at least 20 doublings, thus establishing a causal relationship between telomere shortening and in vitro cellular senescence. The ability to maintain normal human cells in a phenotypically youthful state could have important applications in research and medicine.
Assuntos
Divisão Celular , Senescência Celular , Proteínas/metabolismo , RNA , Telomerase/metabolismo , Telômero/fisiologia , Biomarcadores , Catálise , Linhagem Celular , Transformação Celular Neoplásica , Clonagem Molecular , Proteínas de Ligação a DNA , Fibroblastos/citologia , Homeostase , Humanos , Cariotipagem , Fenótipo , Epitélio Pigmentado Ocular/citologia , Proteínas/genética , DNA Polimerase Dirigida por RNA/genética , DNA Polimerase Dirigida por RNA/metabolismo , Células-Tronco/citologia , Células-Tronco/enzimologia , Telomerase/genética , Telômero/metabolismo , Telômero/ultraestrutura , Transfecção , Células Tumorais Cultivadas , beta-Galactosidase/metabolismoRESUMO
The maintenance of chromosome termini, or telomeres, requires the action of the enzyme telomerase, as conventional DNA polymerases cannot fully replicate the ends of linear molecules. Telomerase is expressed and telomere length is maintained in human germ cells and the great majority of primary human tumours. However, telomerase is not detectable in most normal somatic cells; this corresponds to the gradual telomere loss observed with each cell division. It has been proposed that telomere erosion eventually signals entry into senescence or cell crisis and that activation of telomerase is usually required for immortal cell proliferation. In addition to the human telomerase RNA component (hTR; ref. 11), TR1/TLP1 (refs 12, 13), a protein that is homologous to the p80 protein associated with the Tetrahymena enzyme, has been identified in humans. More recently, the human telomerase reverse transcriptase (hTRT; refs 15, 16), which is homologous to the reverse transcriptase (RT)-like proteins associated with the Euplotes aediculatus (Ea_p123), Saccharomyces cerevisiae (Est2p) and Schizosaccharomyces pombe (5pTrt1) telomerases, has been reported to be a telomerase protein subunit. A catalytic function has been demonstrated for Est2p in the RT-like class but not for p80 or its homologues. We now report that in vitro transcription and translation of hTRT when co-synthesized or mixed with hTR reconstitutes telomerase activity that exhibits enzymatic properties like those of the native enzyme. Single amino-acid changes in conserved telomerase-specific and RT motifs reduce or abolish activity, providing direct evidence that hTRT is the catalytic protein component of telomerase. Normal human diploid cells transiently expressing hTRT possessed telomerase activity, demonstrating that hTRT is the limiting component necessary for restoration of telomerase activity in these cells. The ability to reconstitute telomerase permits further analysis of its biochemical and biological roles in cell aging and carcinogenesis.
Assuntos
DNA Polimerase Dirigida por RNA/genética , DNA Polimerase Dirigida por RNA/metabolismo , RNA/metabolismo , Telomerase/genética , Sequência de Aminoácidos , Animais , Catálise , Humanos , Dados de Sequência Molecular , Mutagênese Sítio-Dirigida , RNA/biossíntese , RNA/genética , DNA Polimerase Dirigida por RNA/biossíntese , Coelhos , Alinhamento de Sequência , Moldes GenéticosRESUMO
Conflicting reports have appeared concerning the cell cycle regulation of telomerase activity and its possible repression during quiescence and cell differentiation. We have reexamined these issues in an attempt to uncover the basis for the discrepancies. Variations in extracted telomerase activity during the cell cycle are not observed in cells sorted on the basis of DNA content. Variations are observed in cells synchronized using some biochemical cell cycle inhibitors, but only with those agents where cellular toxicity is evident. A progressive decline in telomerase activity is observed in cells whose growth rate is reduced from seven to eight population doublings per week to one to two doublings per week. Telomerase is largely absent in cells that truly exit the cell cycle and do not divide over the 7-day period. Although it is not necessary for all cell types to regulate telomerase in the same way, we conclude that in the immortal cultured cell lines examined, extracted telomerase activity does not change significantly during progression through the stages of the cell cycle. Telomerase activity generally correlates with growth rate and is repressed in cells that exit the cell cycle and become quiescent.
Assuntos
Ciclo Celular , Telomerase/metabolismo , Ciclo Celular/efeitos dos fármacos , Linhagem Celular Transformada , Citometria de Fluxo , Meia-Vida , HumanosRESUMO
The ends of vertebrate chromosome are composed of large tracts of a repeated sequence, TTAGGG, which are known as telomeres. Normal somatic cells progressively lose telomeric repeats with each successive cell division due to incomplete replication. Immortal and cancer cells compensate for telomeric loss by expressing the enzyme telomerase, an RNA-dependent DNA polymerase that maintains telomere length. Telomerase activity has been detected in almost 90% of all human cancers. Telomerase activity is generally absent in normal somatic tissues but is detected in adult testes, activated lymphocytes, and lower levels are expressed in proliferative cells of renewal tissues. Telomerase activity is downregulated in cells that exit the cell cycle via either terminal differentiation or (reversible) quiescence. Inhibition of telomerase activity in tumour cells may provide an effective way to treat cancer by potentially reducing the recurrence of tumours due to occult micro-metastases. An understanding of the pathways involved in telomerase regulation will be important for determining the most practical means of inhibiting its activity.
Assuntos
Senescência Celular/fisiologia , Proteínas de Neoplasias/metabolismo , Neoplasias/genética , Transdução de Sinais/fisiologia , Telomerase/metabolismo , Telômero/fisiologia , Divisão Celular/fisiologia , Humanos , Linfócitos/enzimologia , Proteínas de Neoplasias/antagonistas & inibidores , Neoplasias/enzimologia , Células-Tronco/enzimologia , Telomerase/antagonistas & inibidoresRESUMO
While there has been substantial experimental evidence in support of the telomere-telomerase hypothesis of aging and cancer, it has been suggested that the theory has been oversimplified to the exclusion of alternative mechanisms in the progression of cancer. This review strives to present an overview of some of the areas that have not been well explained and to indicate where multiple interpretations of the data are possible.