Stabilization of telomere length and karyotypic stability are directly correlated with the level of hTERT gene expression in primary fibroblasts

Telomere shortening and lack of telomerase activity have been implicated in cellular senescence in human fibroblasts. Expression of the human telomerase (hTERT) gene in sheep fibroblasts reconstitutes telomerase activity and extends their lifespan. However, telomere length is not maintained in all cell lines, even though in vitro telomerase activity is restored in all of them. Cell lines expressing higher levels of hTERT mRNA do not exhibit telomere erosion or genomic instability. By contrast, fibroblasts expressing lower levels of hTERT do exhibit telomere shortening, although the telomeres eventually stabilize at a shorter length. The shorter telomere lengths and the extent of karyotypic abnormalities are both functions of hTERT expression level. We conclude that telomerase activity is required to bypass senescence but is not sufficient to prevent telomere erosion and genomic instability at lower levels of expression.     

Dyrk2-associated EDD-DDB1-VprBP E3 Ligase Inhibits Telomerase by TERT Degradation

Telomerase maintains the telomere, a specialized chromosomal end structure that is essential for genomic stability and cell immortalization. Telomerase is not active in most somatic cells, but its reactivation is one of the hallmarks of cancer. In this study, we found that dual-specificity tyrosine-(Y)-phosphorylation-regulated kinase 2 (Dyrk2) negatively regulates telomerase activity. Dyrk2 phosphorylates TERT protein, a catalytic subunit of telomerase. Phosphorylated TERT is then associated with the EDD-DDB1-VprBP E3 ligase complex for subsequent ubiquitin-mediated TERT protein degradation. During the cell cycle, Dyrk2 interacts with TERT at the G2/M phase and induces degradation. In contrast, depletion of endogenous Dyrk2 disrupts the cell cycle-dependent regulation of TERT and elicits the constitutive activation of telomerase. Similarly, a Dyrk2 nonsense mutation identified in breast cancer compromises ubiquitination-mediated TERT protein degradation. Our findings suggest the novel molecular mechanism of kinase-associated telomerase regulation.     

端粒、端粒酶与肿瘤的相关研究进展

端粒是真核细胞染色体末端的DNA序列,在维持染色体的稳定中起着重要的作用。快速生长的细胞通过端粒酶来合成端 粒重复序列以弥补其损耗。在人类恶性肿瘤细胞中,85%以上能检测到端粒酶的活性,使其成为一个几乎普遍的癌标志物,而在大 多数正常体细胞中,端粒酶是阴性的。端粒酶与肿瘤之间的最新研究已经在肿瘤生物学领域开辟了新的途径,可能会彻底改变抗 癌疗法。在这篇文章中,我们将会总结端粒和端粒酶在癌细胞中的作用。随着科技的发展,端粒和端粒酶拥有巨大的潜力,必将能 够为肿瘤的治疗带来更多的方法。     

Telomere and telomerase dynamics in human cells

Accumulating evidence now implicates telomeres and telomerase as critical regulators genomic stability and replicative lifespan in mammalian cells. Disruption of telomere maintenance and/or telomerase expression contributes to the etiology of some degenerative diseases and may participate in the process of aging. Although telomere dysfunction and aberrant telomerase expression clearly play important roles in cancer development, the contribution of telomere biology to cancer is complex and involves both positive and negative influences on tumor development. Indeed, recent work from several laboratories suggests additional roles for telomeres and telomerase in both normal and malignant physiology. Understanding the complexity of telomere biology will provide further insights into chromosome biology in both normal and malignant cells.     

Assessment of telomere length and factors that contribute to its stability.

Short strands of tandem hexameric repeats known as telomeres cap the ends of linear chromosomes. These repeats protect chromosomes from degradation and prevent chromosomal end-joining, a phenomenon that could occur due to the end-replication problem. Telomeres are maintained by the activity of the enzyme telomerase. The total number of telomeric repeats at the terminal end of a chromosome determines the telomere length, which in addition to its importance in chromosomal stabilization is a useful indicator of telomerase activity in normal and malignant tissues. Telomere length stability is one of the important factors that contribute to the proliferative capacity of many cancer cell types; therefore, the detection and estimation of telomere length is extremely important. Until relatively recently, telomere lengths were analyzed primarily using the standard Southern blot technique. However, the complexities of this technique have led to the search for more simple and rapid detection methods. Improvements such as the use of fluorescent probes and the ability to sort cells have greatly enhanced the ease and sensitivity of telomere length measurements. Recent advances, and the limitations of these techniques are evaluated. Drugs that assist in telomere shortening may contribute to tumor regression. Therefore, factors that contribute to telomere stability may influence the efficiency of the drugs that have potential in cancer therapy. These factors in relation to telomere length are also examined in this analysis.     

Telomerase activity is sufficient to allow transformed cells to escape from crisis

The introduction of simian virus 40 large T antigen (SVLT) into human primary cells enables them to proliferate beyond their normal replicative life span. In most cases, this temporary escape from senescence eventually ends in a second proliferative block known as "crisis," during which the cells cease growing or die. Rare immortalization events in which cells escape crisis are frequently correlated with the presence of telomerase activity. We tested the hypothesis that telomerase activation is the critical step in the immortalization process by studying the effects of telomerase activity in two mortal SVLT-Rasval12-transformed human pancreatic cell lines, TRM-6 and betalox5. The telomerase catalytic subunit, hTRT, was introduced into late-passage cells via retroviral gene transfer. Telomerase activity was successfully induced in infected cells, as demonstrated by a telomerase repeat amplification protocol assay. In each of nine independent infections, telomerase-positive cells formed rapidly dividing cell lines while control cells entered crisis. Telomere lengths initially increased, but telomeres were then maintained at their new lengths for at least 20 population doublings. These results demonstrate that telomerase activity is sufficient to enable transformed cells to escape crisis and that telomere elongation in these cells occurs in a tightly regulated manner.     

Telomere dysfunction and telomerase activation in cancer--a pathological paradox?

Telomerase is expressed in more than 90% of human cancers. Telomere maintenance by this enzyme is believed to safeguard genomic integrity in neoplastic cells. Nevertheless, many telomerase-expressing tumours exhibit chromosomal instability triggered by short, dysfunctional telomeres, implying that active telomerase is not sufficient for preserving a functional telosomic nucleoprotein complex in cancer cells. We here examine three possible solutions to this ostensible paradox. First, prior to telomerase activation, telomere erosion may have evolved to a level where telomeric repeat sequences are too short to provide a functional substrate for telomerase enzyme activity. Second, mechanisms other than the continuous telomere erosion counteracted by telomerase may contribute to rapid shortening of telomere repeats. Third, dysfunction of telomere-regulating proteins may result in direct telomere uncapping. Moreover, telomerase may contribute to tumour development also through mechanisms unrelated to telomere length maintenance. Taken together, the available data on the role of telomerase in cancer strongly support that inhibition of this enzyme is a feasible strategy for cancer therapy.     

DNA瞬态损伤的快速修复阻断癌变最早事件的发展

DNA瞬态损伤(DNA自由基)引起DNA碱基对断裂和双螺旋结构下沉,导致基因失稳,可能是构成癌变的最早事件。基因失稳是癌的共同特征标志之一,而DNA修复可通过维持基因完整性来降低癌的发生。修复DNA的能力低下也是癌的共同特征标志之一。近年新发现了一种独特的修复机制,称为快速修复,专一地修复DNA瞬态损伤,只需几个微秒就能完成,比已知的酶性修复快十亿倍,修复速率常数达到109L.mol-1.s-1。还发现凡是具有快速修复能力的植物多酚类几乎都能不同程度地削弱癌的特征标志,包括使癌细胞形态正常化、抑制癌细胞增殖、促进癌细胞分化或凋亡、降低端粒酶活性、减少植瘤率、阻止细胞迁移和扩散、抑制血管生成、延长存活时间、降低患癌风险等。     

An introduction to telomeres and telomerase

It is now more than a dozen years since the enzyme telomerase was discovered, and since that time, key studies have characterized the structural components of the enzyme and the associated telomeric proteins. Since the original discovery of telomerase, a clear association with cancer has been demonstrated. In normal somatic cells the telomeres at the ends of chromosomes shorten with every cell division, whereas in cancer cells telomere length is often maintained by reactivation of the enzyme telomerase. These discoveries have led to the proposal that telomerase expression can be used as a helpful marker for diagnostic and prognostic purposes in humans. Another area of research that has developed as a result of improving knowledge and understanding of the role of telomerase in malignancy is that of cancer therapeutics. This article is an introduction to the field of telomere and telomerase research, with an introduction to recent attempts to develop novel cancer treatments based on telomerase structure and function.     

Evolving views of telomerase and cancer

The maintenance of telomeres, nucleoprotein structures that constitute the ends of eukaryotic chromosomes, regulates many crucial cellular functions and might, in multicellular organisms, participate in the control of complex phenotypes such as aging and cancer. Stabilization of telomere length is strongly associated with cellular immortalization, and constitutive telomerase activation occurs in most human cancers. Such observations form the basis for the prevailing model that postulates that alterations in telomere biology both suppress and facilitate malignant transformation by regulating genomic stability and cell life span. However, recent findings suggest that telomere maintenance might not be an obligate requirement for initial tumor formation in some settings and that telomerase activation contributes to tumorigenesis independently of its role in maintaining telomere length. These recent developments indicate that our understanding of telomere biology remains incomplete and implicate additional complexity in the relationships among telomeres, telomerase and cancer.     

Telomere dynamics in genome stability

The past several years have seen an increasing interest in telomere recombinational interactions that provide many functions in telomere capping, in telomere size homeostasis and in overcoming the catastrophic effects of telomerase deficiency. Several key recombination mechanisms have emerged from recent investigations. In the yeasts, these mechanisms include exchange between subtelomeric regions and telomere sequences, rapid telomere expansion and telomere deletion. These processes proceed by pathways that use both the cellular recombination machinery and novel mechanisms such as rolling circle replication. The insights gained from recent studies extend our understanding of similar processes in higher eukaryotes and suggest that the recombinational dynamics of telomeres have additional roles that contribute to genomic stability and instability.