人端粒结合蛋白CST复合体的研究进展北大核心CSCD

端粒是线性染色体末端的重要保护结构,由端粒DNA和多种端粒结合蛋白构成,参与了细胞的衰老、凋亡和永生化。端粒结合蛋白在端粒沉默、端粒末端的保护及端粒长度的调节中起了重要的作用。既往研究普遍认为:酵母和脊椎动物的核心端粒结合蛋白分别为CST复合体(Cdc13、Stn1及Ten1)和shelterin复合体(TRF1、TRF2、TIN2、Rap1、TPP1和POT1)。然而,最近的研究发现CST样复合体也同样存在于许多高等真核生物细胞中。本文概述了近年来国内外关于不同类型真核生物CST复合体结构和功能的研究进展,以揭示其在肿瘤防治中的潜在应用价值。     

端粒长度调节的分子机制

真核生物染色体末端有一串联排列的简单重复序列,称为端粒.端粒的长度与细胞老化、永生化、癌变有密切的关系.端粒酶的激活是端粒延长的重要原因.然而,细胞本身可能即存在一种监控端粒长度的机制,通过反馈调节作用,将端粒维持在一定长度范围内,如果这种监控作用减弱或消失,细胞的生物特性将发生改变.细胞的监控机制是通过端粒结合蛋白来实现的.     

端粒保护蛋白TRF2在肿瘤发生和治疗中作用机制的研究进展

端粒重复序列结合因子2（telomeric-repeat binding factor 2，TRF2）是一种重要的端粒保护蛋白，与端粒保护蛋白1（protection of telomeres 1，POT1）、端粒重复序列结合因子1（telomeric-repeat binding factor 1，TRF1）、相互作用核蛋白2（TRF1-interacting nuclear protein 2，TIN2）、阻滞活化蛋白1（repressor activator protein 1，Rap1）以及TPP1，5个核心蛋白通过一系列相互作用共同形成端粒保护蛋白复合体（shelterin）以维持端粒结构和功能的稳定性和完整性。越来越多的研究显示TRF2在多种肿瘤中异常表达并与肿瘤的发生，肿瘤细胞的耐药以及肿瘤血管的生成密切相关。因此，本文就TFR2的结构与生理功能，以及其在肿瘤发生，发展与治疗中的作用研究进展进行综述，以期为肿瘤的预防和治疗提供新的思路。      

端粒蛋白复合物shelterin的 结构及功能研究进展

端粒是稳固真核生物染色体末端、保护遗传信息完整的必要组分,端粒酶、端粒蛋白复合物（shelterin）等与端粒相互作用的蛋白因子,均参与端粒结构和功能的维持。其中,shelterin复合物可通过特异结合到端粒区,帮助端粒成帽,在调控端粒长度、维护端粒结构和功能完整性方面发挥重要作用。Shelterin复合物6种核心蛋白组分的结构及其端粒维护功能的研究,对端粒生物学、端粒相关的退行性疾病研究具有重要意义,各组分表达异常或结构改变与端粒功能失调、细胞衰老加速相关,将导致基因组不稳定性以及疾病发生。本文对shelterin复合物的结构和各组分的功能研究进展进行综述。     

端粒、端粒酶和肿瘤

端粒是真核细胞染色体末端的特有结构,由串联重复的NDA序列和特异蛋白构成,这些序列在进化上高度保守,能维持染色体的稳定和完整,端粒酶是RNA和蛋白质组成的复合体,其核心序列是拷贝端粒DNA的模板。端粒酶在肿瘤的发生,发展中具有重要作用。研究端粒酶的活性表达及其调节和作用机理,对肿瘤的早期诊断和治疗有着重要的意义。     

以端粒为靶的肿瘤治疗研究进展

端粒自身的鸟嘌呤四联体（guanine-quadruplex）和一些端粒特异性结合蛋白对端粒长度、端粒稳定甚至对端粒酶活性都有调节作用，在肿瘤形成和生长中发挥重要作用，以此为靶点抑制肿瘤，直接作用于端粒，不依赖端粒酶的存在，对端粒酶阴性肿瘤亦有作用。因此，端粒可牟成为新的肿瘤抑制靶点。     

以端粒为靶的肿瘤治疗研究进展

端粒自身的鸟嘌呤四联体(guanine-quadruplex)和一些端粒特异性结合蛋白对端粒长度、端粒稳定甚至对端粒酶活性都有调节作用，在肿瘤形成和生长中发挥重要作用。以此为靶点抑制肿瘤，直接作用于端粒，不依赖端粒酶的存在，对端粒酶阴性肿瘤亦有作用。因此，端粒可能成为新的肿瘤抑制靶点。     

端粒长度在细胞癌变和预测肿瘤风险中的作用

端粒与人类的衰老和肿瘤密切相关。端粒是真核生物染色体末端一段特殊的结构,能够防止染色体末端降解、融合和重排,在维持基因组的稳定性和完整性方面具有重要作用。在整个生命周期中,体细胞的端粒长度是逐渐缩短的,但肿瘤细胞由于端粒酶的重新激活具有稳定的端粒长度和永生化的特征。端粒长度缩短往往增加衰老相关疾病的风险而端粒长度增长往往增加肿瘤的风险。近年来,大量研究证实端粒长度的改变影响肿瘤发生发展,而且端粒长度可以预测肿瘤风险,显示出端粒长度在精准诊疗中的重要意义,因此,本文将对端粒长度在细胞癌变和预测肿瘤风险中的作用进行综述。      

端粒维持基因多态性与膀胱癌易感性的关系

端粒维持基因在保持端粒稳定、维护染色体完整方面起重要作用,端粒功能紊乱将会导致肿瘤发生。在维持端粒稳定的基因中,已经发现多种基因存在单核苷酸多态性。随着研究的深入,越来越多的证据表明,个体间端粒稳定性及长度的差异主要取决于端粒维持基因的多态性。端粒维持基因单核苷酸多态性可能是导致端粒长度及稳定性存在个体差异及增加肿瘤易感性的主要因素之一。本文就端粒维持基因多态性与膀胱癌易感性关系的研究进展作一综述。     

端粒酶与细胞周期调控

端粒的缩短和端粒酶的激活在细胞衰老和永生化过程中的作用已被证实并受到广泛关注，许多酶和蛋白参与细胞周期调控，其中以G1/S期转换关卡的调控最为重要。细胞染色体的稳定主要取决于端粒长度和细胞周期转移关卡之间的相互作用。而肿瘤细胞端粒长度的维持大部分情况下与端粒酶活性表达有关，端粒酶与细胞周期调控因子之间关系的研究将为我们探索肿瘤病因及新的肿瘤治疗途径开壁广阔的前景。     

Telosome: biological function and its relationship with tumor

Recent studies show that the structure integrality and function are more important than the length of telomere in maintaining tumor cells reproduction and proliferation. Mammalian telomeres bound by a specialized six protein(TRF1,TRF2,TIN2,RAP1,POT1 and TPP1)complex known as telosome or shelterin, has fundamental role in the regulation of telomere activity, maintaining the length and protecting telomere chromosome terminal. Telosome is involved in the complex regulation of cell mitosis process. The study of the role of telomere binding protein complex in tumorigenesis is very important for searching new methods of tumor diagnosis and treatment.     

Impaired telomere regulation mechanism by TRF1 (telomere-binding protein), but not TRF2 expression, in acute leukemia cells

Telomere regulation is suggested to be an important mechanism in cellular proliferation and cellular senescence not only in normal diploid cells but also in neoplastic cells, including human leukemia cells. We studied the possible correlation among telomere length, telomerase (a ribonuclear protein that synthesizes the telemeres de novo) activity, hTERT (a catalytic subunit of telomerase) expression, and TRF1 and TRF2 (telomere DNA binding proteins) expression in human acute leukemia cells. The hTERT expression level was strongly associated with telomerase activity (P=0.0001), indicating that the expression level of the catalytic subunit (hTERT) regulates telomerase activity in human acute leukemia cells. TRF1 expression, which is believed to control telomere length, was significantly elevated in patients with acute lymphoblastic leukemia (ALL) (P=0.0232) compared to those in acute myeloid leukemia (AML); TRF1 expression tended to be higher in patients without telomere shortening (P=0.077) and in those with hTERT expression (P=0.055). This indicates that TRF1 may act to monitor telomere length under the condition of up-regulated telomerase activity in some neoplastic cells. In contrast, TRF2 expression in acute leukemia did not show any correlation with telomere parameters in this study. Although the precise regulation mechanism of telomere length is still uncertain, these results may suggest that regulation of telomere length is partially associated with TRF1 expression, whereas dysfunction of TRF1 expression may be speculated in a subset of acute leukemia.     

端粒长度调控机制的研究进展

端粒长度调控是一个复杂的机制 ,端粒酶、端粒特异性结合蛋白 (TRF1、TRF2、Pot1等 )均通过直接或间接与端粒结合发挥其对端粒长度、端粒稳定性的调节作用 ,此外还存在不常见的ALT途径。综述了有关端粒长度调控机制的研究进展及其调节因素     

Heregulin,a new interactor of the telosome/shelterin complex in human telomeres

Telomere length, shape and function depend on a complex of six core telomere-associated proteins referred to as the telosome or shelterin complex. We here demonstrate that the isoform β2 of the heregulin family of growth factors (HRGβ2) is a novel interactor of the telosome/shelterin complex in human telomeres. Analysis of protein-protein interactions using a high-throughput yeast two-hybrid (Y2H) screen identified RAP1, the only telomere protein that is conserved from yeasts to mammals, as a novel interacting partner of HRGβ2. Deletion analysis of RAP1 revealed that the linker domain, a region previously suggested to recruit negative regulators of telomere length, interacts specifically with HRGβ2. Co-immunoprecipitation and imaging experiments demonstrated that, in addition to RAP1, HRGβ2 could associate with the RAP1-associated telomeric repeat binding factor 2 (TRF2). Deletion analysis of HRGβ2 confirmed that a putative nuclear localization signal (NLS) was necessary for nuclear HRGβ2 to exert a negative regulation of telomere length whereas the N-terminus (extracellular) amino acids of HRGβ2 were sufficient to interact with RAP1/TRF2 and promote telomere shortening. Taken together, our studies identify nuclear HRGβ2 as one of the previously unknown regulators predicted to be recruited by the RAP1 linker domain to negatively regulate telomere length in human cells. Our current findings reveal that a new, but likely not the last, unexpected visitor has arrived to the “telosome/shelterin town”.     

TRF2 promotes multidrug resistance in gastric cancer cells

The role of telomere in drug resistance has not been clearly understood. Recent studies have been focused on telomerase activity and telomere length, but the findings are still controversial. It's been found that DNA double-strand breaks induced by anticancer drugs or irradiation increase TRF2 expression as an early response to DNA damage, which inhibits activation of ATM-dependent DNA damage response network, indicating TRF2 might probably be a general DNA-repair factor rather than merely a telomere-binding factor. In this study, the possible involvement of telomerase, telomere and TRF2 in DNA damage response and drug resistance was investigated. Telomere length was found elongated in multidrug-resistant variants of gastric cancer cell line SGC7901 treated with adriamycin or etoposide, however, drug-treatment per se had no effect on telomere length. Telomerase activity and TRF2 expression were upregulated after treatment, but not TRF1. TRF2 upregulation was more dramatic in drug-resistant cells and occurred before the expression of ATM, gammaH2AX and p53. Moreover, TRF2 inhibited the expression of ATM-dependent DSB responsive genes. Inhibition of TRF2 expression by RNA interference in drug-resistant cells partially reversed its resistance phenotype and overexpression of TRF2 in SGC7901 promoted its resistance phenotype. Taken together, current results indicate that TRF2 plays an important role in DNA damage response, and is involved in drug resistance of gastric cancer. Further study of the biological functions of TRF2 might be helpful to dissect the molecular mechanism of multiple drug-resistance and generate novel target to overcome it.     

Gene Expression for Suppressors of Telomerase Activity (Telomeric-repeat Binding Factors) in Breast Cancer

Mechanisms regulating telomerase activity and telomere length remain incompletely understood in human breast cancer. We therefore studied gene expression for telomeric-repeat binding factors (TRFs) in relation to telomerase activity, telomere length, and clinicopathologic factors in human breast cancer. Telomerase activity was detected in 65.8% of 38 breast cancers, but none of 16 non-cancerous samples. Terminal restriction fragments were longer in noncancerous than in cancerous tissues, but not significantly. Among 8 patients with both cancer and paired noncancerous tissue available for terminal restriction fragments length assay, terminal restriction fragments were shorter in cancers than in paired noncancerous samples in all but one. Significantly more mRNA encoding TRF1 and 2 was detected in noncancerous than in cancer tissues. Additionally, expression of TRF1 and 2 mRNA was significantly higher in cancers without detectable telomerase activity than in cancers showing activity. Expression of these genes tended to show a negative correlation with terminal restriction fragments length, but this was not statistically significant. No correlation was seen between TRF1 or 2 mRNA expression, and clinicopathologic factors except for TRF1 with respect to tumor size and progesterone receptor status. In addition to reactivation of telomerase activity, escape from negative regulation of this activity is needed to maintain telomere length during cell proliferation in breast cancer. Genes encoding telomerase inhibitors might be of value in gene therapy against human breast cancer.     

Gene expression for suppressors of telomerase activity (telomeric-repeat binding factors) in breast cancer.

Mechanisms regulating telomerase activity and telomere length remain incompletely understood in human breast cancer. We therefore studied gene expression for telomeric-repeat binding factors (TRFs) in relation to telomerase activity, telomere length, and clinicopathologic factors in human breast cancer. Telomerase activity was detected in 65.8% of 38 breast cancers, but none of 16 noncancerous samples. Terminal restriction fragments were longer in noncancerous than in cancerous tissues, but not significantly. Among 8 patients with both cancer and paired noncancerous tissue available for terminal restriction fragments length assay, terminal restriction fragments were shorter in cancers than in paired noncancerous samples in all but one. Significantly more mRNA encoding TRF1 and 2 was detected in noncancerous than in cancer tissues. Additionally, expression of TRF1 and 2 mRNA was significantly higher in cancers without detectable telomerase activity than in cancers showing activity. Expression of these genes tended to show a negative correlation with terminal restriction fragments length, but this was not statistically significant. No correlation was seen between TRF1 or 2 mRNA expression, and clinicopathologic factors except for TRF1 with respect to tumor size and progesterone receptor status. In addition to reactivation of telomerase activity, escape from negative regulation of this activity is needed to maintain telomere length during cell proliferation in breast cancer. Genes encoding telomerase inhibitors might be of value in gene therapy against human breast cancer.     

Modulation of telomere shelterin by TRF1 [corrected] and TRF2 interacts with telomerase to maintain the telomere length in non-small cell lung cancer

Our previous report demonstrated good correlations between the expressions of h-TERT and its associated genes, such as c-Myc, TRF1 and TRF2. To observe the interaction between telomerase activity and expression of its associated genes in regulation of the telomere restriction fragment length (TRFL) in non-small cell lung cancer (NSCLC), 79 NSCLC specimens were examined. Telomerase activity, h-TERT, TRF1 and TRF2 genes expression were observed in 60.8, 66.7, 74.7, and 83.5% of the tumour tissues, respectively. The TRFL were shorter in both tumour tissues and telomerase positive tissues, as compared to their counterparts. The t/n-TRFLR (tumour-to-normal TRFL ratio) was also lower in telomerase positive tissues. When telomerase was negative, the t/n-TRFLR was lower in both TRF1 positive and TFR2 positive. However, when telomerase was positive, the t/n-TRFLR was only lower in the TFR2 positive group. When t/n-TRFLR level was equal to or less than 75%, the majority of the specimens became TRF1 and TRF2 positive. To explain these findings, our hypothesis is that when the TRF length becomes shorter during tumour progression, the tumour cells can sustain a better tolerance to shorter telomere with the help of both TRF1 and TRF2, but without immediate activation of the telomerase. However, when the TRF length reaches a critical level, changing the telomere shelterin by persistent expression of the TRF2, which in combination with telomerase activation reverses the telomere shortening.