RESUMO
Oncogene-induced senescence (OIS) is a critical tumor-suppressing mechanism that restrains cancer progression at premalignant stages, in part by causing telomere dysfunction. Currently it is unknown whether this proliferative arrest presents a stable and therefore irreversible barrier to cancer progression. Here we demonstrate that cells frequently escape OIS induced by oncogenic H-Ras and B-Raf, after a prolonged period in the senescence arrested state. Cells that had escaped senescence displayed high oncogene expression levels, retained functional DNA damage responses, and acquired chromatin changes that promoted c-Myc-dependent expression of the human telomerase reverse transcriptase gene (hTERT). Telomerase was able to resolve existing telomeric DNA damage response foci and suppressed formation of new ones that were generated as a consequence of DNA replication stress and oncogenic signals. Inhibition of MAP kinase signaling, suppressing c-Myc expression, or inhibiting telomerase activity, caused telomere dysfunction and proliferative defects in cells that had escaped senescence, whereas ectopic expression of hTERT facilitated OIS escape. In human early neoplastic skin and breast tissue, hTERT expression was detected in cells that displayed features of senescence, suggesting that reactivation of telomerase expression in senescent cells is an early event during cancer progression in humans. Together, our data demonstrate that cells arrested in OIS retain the potential to escape senescence by mechanisms that involve derepression of hTERT expression.
Assuntos
Neoplasias da Mama/genética , Regulação Neoplásica da Expressão Gênica , Proteína Oncogênica p21(ras)/genética , Proteínas Proto-Oncogênicas B-raf/genética , Neoplasias Cutâneas/genética , Telomerase/genética , Neoplasias da Mama/metabolismo , Neoplasias da Mama/patologia , Linhagem Celular , Linhagem Celular Tumoral , Sobrevivência Celular , Senescência Celular , Células Epiteliais/metabolismo , Células Epiteliais/patologia , Feminino , Fibroblastos/citologia , Fibroblastos/metabolismo , Humanos , Masculino , Proteína Oncogênica p21(ras)/metabolismo , Proteínas Proto-Oncogênicas B-raf/metabolismo , Proteínas Proto-Oncogênicas c-myc/genética , Proteínas Proto-Oncogênicas c-myc/metabolismo , Transdução de Sinais , Neoplasias Cutâneas/metabolismo , Neoplasias Cutâneas/patologia , Telomerase/metabolismo , Telômero/química , Telômero/metabolismo , Homeostase do TelômeroRESUMO
TA-65 is a dietary supplement based on an improved formulation of a small molecule telomerase activator that was discovered in a systematic screening of natural product extracts from traditional Chinese medicines. This study summarizes the findings on telomere length (TL) changes from a randomized, double blind, placebo controlled study of TA-65 over a 1 year period. The study was conducted on 117 relatively healthy cytomegalovirus-positive subjects aged 53-87 years old. Subjects taking the low dose of TA-65 (250 U) significantly increased TL over the 12 months period (530 ± 180 bp; p = 0.005), whereas subjects in the placebo group significantly lost TL (290 ± 100 bp; p = 0.01). The high dose of TA-65 (1000 U) showed a trend of improvements in TL compared with that of the placebo group; however, the improvements did not reach statistical significance. TL changes in the low-dose group were similar for both median and 20th percentile TLs. The findings suggest that TA-65 can lengthen telomeres in a statistically and possibly clinically significant manner.
Assuntos
Produtos Biológicos/farmacologia , Medicina Tradicional Chinesa , Telomerase/fisiologia , Telômero/efeitos dos fármacos , Idoso , Idoso de 80 Anos ou mais , Estudos Transversais , Relação Dose-Resposta a Droga , Método Duplo-Cego , Ativação Enzimática/efeitos dos fármacos , Humanos , Pessoa de Meia-IdadeRESUMO
Telomeres, the ends of our linear chromosomes, can function as 'replicometers', capable of counting cell division cycles as they progressively erode with every round of DNA replication. Once they are critically short, telomeres become dysfunctional and consequently activate a proliferative arrest called replicative senescence. For many years, telomeres were thought to be autonomous structures, largely isolated from cell intrinsic and extrinsic signals, whose function is to prevent limitless cellular proliferation, a characteristic of most cancer cells. It is becoming increasingly evident, however, that telomeres not only count cell divisions, but also function as sensors of genotoxic stresses to stop cell cycle progression prematurely and long before cells would have entered replicative senescence. This stable growth arrest, triggered by dysfunctional telomeres that are not necessarily critically short, likely evolved as a tumor-suppressing mechanism as it prevents proliferation of cells that are at risk for acquiring potentially hazardous and transforming mutations both in vitro and in vivo. Here, we review studies supporting the concept that telomeres are important cellular structures whose function not only is to count cell divisions, but also to act as molecular switches that can rapidly stop cell cycle progression permanently in response to a variety of stresses, including oncogenic signals.
Assuntos
Senescência Celular/fisiologia , Dano ao DNA/fisiologia , Telômero/metabolismo , Animais , Humanos , Telomerase/genética , Telomerase/metabolismo , Telômero/genéticaRESUMO
In normal human somatic cells, telomere dysfunction causes cellular senescence, a stable proliferative arrest with tumour suppressing properties. Whether telomere dysfunction-induced senescence (TDIS) suppresses cancer growth in humans, however, is unknown. Here, we demonstrate that multiple and distinct human cancer precursor lesions, but not corresponding malignant cancers, are comprised of cells that display hallmarks of TDIS. Furthermore, we demonstrate that oncogenic signalling, frequently associated with initiating cancer growth in humans, dramatically affected telomere structure and function by causing telomeric replication stress, rapid and stochastic telomere attrition, and consequently telomere dysfunction in cells that lack hTERT activity. DNA replication stress induced by drugs also resulted in telomere dysfunction and cellular senescence in normal human cells, demonstrating that telomeric repeats indeed are hypersensitive to DNA replication stress. Our data reveal that TDIS, accelerated by oncogene-induced DNA replication stress, is a biological response of cells in human cancer precursor lesions and provide strong evidence that TDIS is a critical tumour suppressing mechanism in humans.
Assuntos
Senescência Celular/fisiologia , Oncogenes/fisiologia , Telômero/fisiologia , Linhagem Celular , Transformação Celular Neoplásica/efeitos dos fármacos , Transformação Celular Neoplásica/genética , Senescência Celular/efeitos dos fármacos , Senescência Celular/genética , Replicação do DNA/efeitos dos fármacos , Replicação do DNA/genética , Replicação do DNA/fisiologia , Humanos , Oncogenes/efeitos dos fármacos , Oncogenes/genética , Inibidores da Síntese de Proteínas/farmacologia , Puromicina/farmacologia , Transdução de Sinais/efeitos dos fármacos , Transdução de Sinais/genética , Transdução de Sinais/fisiologia , Telômero/efeitos dos fármacos , Telômero/genéticaRESUMO
The tumor suppressor protein BRCA2 is a key component of the homologous recombination pathway of DNA repair, acting as the loader of RAD51 recombinase at sites of double-strand breaks. Here we show that BRCA2 associates with telomeres during the S and G2 phases of the cell cycle and facilitates the loading of RAD51 onto telomeres. Conditional deletion of Brca2 and inhibition of Rad51 in mouse embryonic fibroblasts (MEFs), but not inactivation of Brca1, led to shortening of telomeres and accumulation of fragmented telomeric signals--a hallmark of telomere fragility that is associated with replication defects. These findings suggest that BRCA2-mediated homologous recombination reactions contribute to the maintenance of telomere length by facilitating telomere replication and imply that BRCA2 has an essential role in maintaining telomere integrity during unchallenged cell proliferation. Mouse mammary tumors that lacked Brca2 accumulated telomere dysfunction-induced foci. Human breast tumors in which BRCA2 was mutated had shorter telomeres than those in which BRCA1 was mutated, suggesting that the genomic instability in BRCA2-deficient tumors was due in part to telomere dysfunction.
Assuntos
Proteína BRCA2/fisiologia , Rad51 Recombinase/metabolismo , Telômero/metabolismo , Animais , Proteína BRCA2/genética , Proteína BRCA2/metabolismo , Células Cultivadas , Imunoprecipitação da Cromatina , Reparo do DNA , Fase G2 , Deleção de Genes , Instabilidade Genômica , Camundongos , Rad51 Recombinase/antagonistas & inibidores , Rad51 Recombinase/genética , Fase S , Telômero/químicaRESUMO
A close association between brain metal dishomeostasis and the onset and/or progression of Alzheimer's disease (AD) has been clearly established in a number of studies, although the underlying biochemical mechanisms remain obscure. This observation renders chelation therapy an attractive pharmacological option for the treatment of this disease. However, a number of requirements must be fulfilled in order to adapt chelation therapy to AD so that the term "metal targeted strategies" seems now more appropriate. Indeed, brain metal redistribution rather than brain metal scavenging and removal is the major goal of this type of intervention. The most recent developments in metal targeted strategies for AD will be discussed using, as useful examples, clioquinol, curcumin, and epigallocatechin, and the future perspectives will also be outlined.
Assuntos
Doença de Alzheimer/tratamento farmacológico , Doença de Alzheimer/metabolismo , Quelantes/uso terapêutico , Terapia por Quelação/métodos , Metais/metabolismo , Animais , HumanosRESUMO
Cleavage of the amyloid precursor protein (APP) by beta- and gamma-secretases results in generation of the amyloid-beta protein (Abeta), which is characteristically deposited in the brain of Alzheimer's disease patients. Inhibitors of 3-hydroxy-3-methyl-glutaryl (HMG)-CoA reductase (the statins) reduce levels of cholesterol and isoprenoids such as geranylgeranyl pyrophosphate (GGPP). Previous studies have demonstrated that cholesterol increases and statins reduce Abeta levels mostly by regulating beta-secretase activity. In this study, we focused on the role of geranylgeranyl isoprenoids GGPP and geranylgeraniol (GGOH) in regulating Abeta production. Our data show that the inhibition of GGPP synthesis by statins plays an important role in statin-mediated reduction of Abeta secretion. Consistent with this finding, the geranylgeranyl isoprenoids preferentially increase the yield of Abeta of 42 residues (Abeta42) in a dose-dependent manner. Our studies further demonstrated that geranylgeranyl isoprenoids increase the yield of APP-CTFgamma (a.k.a. AICD) as well as Abeta by stimulating gamma-secretase-mediated cleavage of APP-CTFalpha and APP-CTFbeta in vitro. Furthermore, GGOH increases the levels of the active gamma-secretase complex in the detergent-insoluble membrane fraction along with its substrates, APP-CTFalpha and APP-CTFbeta. Our results indicate that geranylgeranyl isoprenoids may be an important physiological facilitator of gamma-secretase activity that can foster production of the pathologically important Abeta42.
Assuntos
Secretases da Proteína Precursora do Amiloide/metabolismo , Peptídeos beta-Amiloides/biossíntese , Precursor de Proteína beta-Amiloide/biossíntese , Fragmentos de Peptídeos/biossíntese , Fosfatos de Poli-Isoprenil/farmacologia , Sequência de Aminoácidos , Animais , Células CHO , Linhagem Celular , Cricetinae , Cricetulus , Ativação Enzimática , Ensaio de Imunoadsorção Enzimática , Humanos , Lovastatina/farmacologia , Dados de Sequência MolecularRESUMO
Alzheimer's disease (AD) is a complex neurodegenerative disorder pathologically identified by the presence of extracellular senile plaques (SP) with a proteinaceous core composed of aggregates of the amyloid peptide (Abeta) and intracellular aggregates of the microtubule-associated protein tau (tau) as neurofibrillary tangles (NFTs). These hallmarks consist of abnormally folded proteinaceous components that are believed to be neurotoxic in AD. The mechanisms of toxicity remain unclear although oxidative stress and inflammation are implicated as mediators of the toxicity and these lesions, in turn, are known to damage cellular components including proteins, lipids in the membrane and DNA. However effects on genotoxicity and its role in AD are less clear. The present review discusses various influences, in particular of amyloid, on the genetic material and their possible role in the neurodegeneration in AD. Further, the amalgamation of genomics and proteomics in understanding AD and therapeutic development is suggested.
Assuntos
Doença de Alzheimer/genética , Doença de Alzheimer/metabolismo , Peptídeos beta-Amiloides/metabolismo , Encéfalo/metabolismo , Dano ao DNA/fisiologia , Doença de Alzheimer/fisiopatologia , Peptídeos beta-Amiloides/toxicidade , Animais , Apoptose/fisiologia , Encéfalo/patologia , Encéfalo/fisiopatologia , Genes cdc/fisiologia , Humanos , Mutação/genética , Estresse Oxidativo/fisiologia , Transdução de Sinais/fisiologia , Proteína Supressora de Tumor p53/genética , Proteína Supressora de Tumor p53/metabolismoRESUMO
The amyloid hypothesis has dominated the thinking in our attempts to understand, diagnose and develop drugs for Alzheimer's disease (AD). This article presents a new hypothesis that takes into account the numerous familial AD (FAD) mutations in the amyloid precursor protein (APP) and its processing pathways, but suggests a new perspective beyond toxicity of forms of the amyloid beta-peptide (Abeta). Clearly, amyloid deposits are an invariable feature of AD. Moreover, although APP is normally processed to secreted and membrane-bound fragments, sAPPbeta and CTFbeta, by BACE, and the latter is subsequently processed by gamma-secretase to Abeta and CTFgamma, this pathway mostly yields Abeta of 40 residues, and increases in the levels of the amyloidogenic 42-residue Abeta (Abeta42) are seen in the majority of the mutations linked to the disease. The resulting theory is that the disease is caused by amyloid toxicity, which impairs memory and triggers deposition of the microtubule associated protein, Tau, as neurofibrillary tangles. Nevertheless, a few exceptional FAD mutations and the presence of large amounts of amyloid deposits in a group of cognitively normal elderly patients suggest that the disease process is more complex. Indeed, it has been hard to demonstrate the toxicity of Abeta42 and the actual target has been shifted to small oligomers of the peptide, named Abeta derived diffusible ligands (ADDLs). Our hypothesis is that the disease is more complex and caused by a failure of APP metabolism or clearance, which simultaneously affects several other membrane proteins. Thus, a traffic jam is created by failure of important pathways such as gamma-secretase processing of residual intramembrane domains released from the metabolism of multiple membrane proteins, which ultimately leads to a multiple system failure. In this theory, toxicity of Abeta42 will only contribute partially, if at all, to neurodegeneration in AD. More significantly, this theory would predict that focussing on specific reagents such as gamma-secretase inhibitors that hamper metabolism of APP, may initially show some beneficial effects on cognitive performance by elimination of acutely toxic ADDLs, but over the longer term may exacerbate the disease process by reducing membrane protein turnover.
Assuntos
Doença de Alzheimer/metabolismo , Proteínas de Membrana/metabolismo , Doença de Alzheimer/etiologia , Amiloide/metabolismo , Amiloide/toxicidade , Secretases da Proteína Precursora do Amiloide , Animais , Ácido Aspártico Endopeptidases , Endopeptidases/genética , Endopeptidases/metabolismo , Humanos , Proteínas de Membrana/genética , Degeneração Neural/fisiopatologia , Presenilina-1RESUMO
Alzheimer's disease (AD) is a complex neurodegenerative disorder. Our studies for the first time showed evidence for altered DNA conformation in the hippocampus of Alzheimer's disease affected brain. The Circular dichroism spectra of severely affected AD DNA showed a typical left-handed Z-DNA conformation, whereas normal, young, and aged brain DNA have the usual B-DNA conformation. Moderately affected AD DNA has modified B-DNA conformation (probable B-Z intermediate form). The ELISA, ethidium bromide binding pattern to DNA and melting temperature (Tm) profiles also revealed the conformational transition from B to Z DNA in AD brain DNA. The altered conformation of DNA will have tremendous implications in gene expressions.