Attenuation of telomerase activity does not increase sensitivity of human melanoma cells to anticancer agents

In tumour cells, replicative immortality is attained through stabilisation of telomeres by telomerase. Recent evidence suggests that telomerase plays an anti-apoptotic role. Since apoptosis is the primary mode of cell death induced by several drugs, telomerase could be involved in determining the chemosensitivity profile of tumour cells. We investigated whether inhibition of telomerase activity through a hammerhead ribozyme targeting the RNA template of telomerase influences the susceptibility of human melanoma cells to a variety of anticancer agents (platinum compounds, taxanes, topoisomerase I inhibitors). The ribozyme sequence was inserted into an expression vector and the JR8 human melanoma cell line was transfected with it. The cell clones obtained showed a reduced telomerase activity. Growth inhibition curves generated after exposure of ribozyme-transfectant clones to individual drugs were superimposable to those obtained from parental cells. Moreover, telomerase inhibition did not promote apoptosis as a cellular response to drug treatment. Overall, our results indicate that downregulation of telomerase activity does not increase the sensitivity of melanoma cells to anticancer drugs.     

端粒酶反义脱氧核苷酸与多糖对HL-60细胞的影响

目的　研究人类端粒酶反义脱氧核苷酸及其与梁金菇多糖协同作用对HL 6 0细胞凋亡及端粒酶活性的影响。方法　端粒酶反义脱氧核苷酸单独或与梁金菇多糖协同处理HL 6 0细胞后 ,分别用涂片法、流式细胞仪法和电镜检测两种药物处理后对细胞凋亡的影响 ,以TRAP Elisa法检测对端粒酶活性的影响。结果　端粒酶反义核酸单独或与梁金菇多糖协同处理HL 6 0均可提高细胞的凋亡率并抑制端粒酶活性 ,两者协同处理效果更显著。结论　端粒酶反义核酸可降低细胞的端粒酶活性并诱导细胞调亡 ,梁金菇多糖则可加强这种作用。     

三氧化二砷对HL-60细胞hTERT基因表达和端粒酶活性的影响

目的：探讨三氧化二砷（As2O3)处理HL－60细胞前后人类端粒酶逆转灵酶（hTERT)基因表达和端粒酶活性的改变。方法：采用姬姆萨染色及碘化丙锭染色置流式细胞仪检测来观察细胞的凋亡，以RT－PCR分析hTERT 基因的mRNA表达水平；应用间接免疫荧光标记法通过流式细胞仪检测hTERT蛋白含量；利用多聚酶链反应－酶联免疫反应（PCR－ELISA）方法检测As2O3处理HL－60前后端粒酶活性的改变。结果：2umol/L As2O3诱导HL－60细胞凋亡的过程中，hTERT mRNA表达水平显著下降，细胞内hTERT蛋白含量也相应降低，其端粒酶活性明显受到抑制，结论：As2O3可通过下调hTERT基因的表达而抑制HL－6细胞的端粒酶活性。     

Epigenetic regulation of telomerase in retinoid-induced differentiation of human leukemia cells

Changes in the promoter methylation of hTERT, the gene that encodes telomerase, a ribonucleoprotein responsible for replacing telomeric repeats, have been demonstrated in differentiating cells where hTERT is inhibited, suggesting epigenetic regulation of hTERT. All-trans retinoic acid (ATRA) induces differentiation in human leukemia cells and has had significant clinical success treating promyelocytic leukemia in what is termed 'differentiation therapy'. It is thought that the inhibition of telomerase is a target of retinoids and is closely tied to the differentiated phenotype. This study demonstrates the epigenetic changes associated with ATRA-induced inhibition of telomerase activity, including the hypoacetylation and hypermethylation of the hTERT promoter. Further, we have found changes in the differential expression of the three DNA methyltransferases during ATRA-induced differentiation of HL60 human leukemia cells. These results suggest that alteration of DNA methylation may play a role in the activation of telomerase in cancer cells and that epigenetic mechanisms may represent a target for differentiation therapy mechanisms. We propose that epigenetic changes in the hTERT promoter represent a stable locking mechanism in the retionoid-induced suppression of telomerase activity.     

Simultaneous targeting of telomeres and telomerase as a cancer therapeutic approach

Telomeres, which are important for maintaining chromosome integrity and functions, shorten with each cell division. Telomerase, responsible for telomere synthesis, is expressed in approximately 90% of human tumor cells but seldom in normal somatic cells. This study evaluated the hypothesis that simultaneous shortening of telomeres and inhibition of telomerase results in synergistic and tumor-selective cytotoxicity. In telomerase-positive human pharynx FaDu tumor cells, paclitaxel caused telomere erosion (first detected at 1 h) and apoptosis. Expression of antisense to the RNA component of human telomerase (hTR) inhibited telomerase activity, shortened telomere length, reduced cell growth rate, and resulted in a significant higher sensitivity to paclitaxel. Another telomerase inhibitor, 3'-azido-3'-deoxythymidine (AZT), at a concentration that produced little or no cell detachment or apoptosis, inhibited the telomerase activity and enhanced the paclitaxel-induced cell detachment and apoptosis. AZT also enhanced the activity of paclitaxel in mice bearing well-established s.c. FaDu xenograft tumors (i.e., reduced residual tumor size, enhanced apoptotic cell fraction, and prolonged survival time), without enhancing host toxicity. In contrast, AZT did not enhance the paclitaxel activity in the telomerase-negative osteosarcoma Saos-2 cells nor in FaDu cells where telomerase was already suppressed by antisense hTR, confirming that the AZT effect in parent FaDu cells is mediated through telomerase inhibition. These results demonstrate that combined use of agents targeting both telomere and telomerase yielded synergistic activity selective for tumors that depend on telomerase for telomere maintenance.     

Interferon-alpha repressed telomerase along with G1-accumulation of Daudi cells.

The implications of telomerase on senescence and human carcinogenesis are widely accepted, but the changes of telomerase activity along with cell cycle modulation by anticancer treatment still remain obscure. In this paper, we issued whether the telomerase activity fluctuated along with cell cycle of cultured cancer cells using the antiproliferative effect of interferon-alpha (IFN-alpha). Daudi Burkitt lymphoma cells, treated with IFN-alpha, showed proliferation inhibition and cell cycle arrest at G1. The telomerase activity at 72 h was repressed to about 20% of control cells. Furthermore, after 72 h IFN-alpha treatment, the cells in G1 phase showed the marked decrease of telomerase activity, while cells in S and G2/M still possessed it. Among expressions of telomerase-related genes, only the catalytic subunit of telomerase (hTERT) decreased from 48 h, while the template RNA component (hTERC) and telomerase-associated protein 1 (TEP-1) were not affected. The downregulation of c-Myc preceded the change of hTERT. Moreover, the analysis of cells treated with IFN-alpha for 24 h revealed that cells in G1-to-S transition mainly expressed high hTERT, while S and G2/M cells had higher level of telomerase activity than that of G1 cells. These results indicate that (i) the expression of hTERT precedes the telomerase activity which is higher in S and G2/M phases than G1 phase, (ii) IFN-alpha repressed the telomerase activity in a cell cycle-dependent manner with the downregulation of hTERT.     

Attenuation of telomerase activity by hammerhead ribozyme targeting human telomerase RNA induces growth retardation and apoptosis in human breast tumor cells

Ribozyme possesses specific endoribonuclease activity and catalyzes the hydrolysis of specific phosphodiester bonds, which results in the cleavage of target RNA sequences. Here, we evaluated the ability of hammerhead ribozymes targeting human telomerase RNA (hTR) to inhibit the catalytic activity of telomerase and the proliferation of cancer cells. Hammerhead ribozymes were designed against 7 NUX sequences located in open loops of the hTR secondary structure. We verified the ribozyme specificity by in vitro cleavage assay by using a synthetic RNA substrate. Subsequently, we introduced ribozyme expression vector into human breast tumor MCF-7 cells and assessed the biologic effects of ribozyme. Hammerhead ribozyme R1 targeting the template region of hTR efficiently cleaved hTR in vitro, and stable transfectants of this ribozyme induced the degradation of target hTR RNA and attenuated telomerase activity in MCF-7 cells. Moreover, the ribozyme R1 transfectant displayed a significant telomere shortening and a lower proliferation rate than parental cells. Clones with reduced proliferation capacity showed enlarged senescence-like shapes or highly differentiated dendritic morphologies of apoptosis. In conclusion, the inhibition of telomerase activity by hammerhead ribozyme targeting the template region of the hTR presents a promising strategy for inhibiting the growth of human breast cancer cells.     

Attenuation of Telomerase Activity by siRNA Targeted Telomerase RNA Leads to Apoptosis and Inhibition of Proliferation in Human Renal Carcinoma Cells

OBJECTIVE Telomerase is an attractive molecular target for cancer therapy because the activation of telomerase is one of the key steps in cell immortalization and carcinogenesis. RNA interference using small-interfering RNA (siRNA) has been demonstrated to be an effective method for inhibiting the expression of a given gene in human cells. The aim of the present study was to investigate whether inhibition of telomerase activity by siRNA targeted against human telomerase RNA (hTR) can inhibit proliferation and induce apoptotic cell death in human renal carcinoma cells (HRCCs). METHODS The siRNA duplexes for hTR were synthesized and 786-0 HRCCs were transfected with different concentrations of hTR-siRNA. The influence on the hTR mRNA level, telomerase activity, as well as the effect on cell proliferation and apoptosis was examined. RESULTS Anti-hTR siRNA treatment of HRCCs resulted in specific reduction of hTR mRNA and inhibition of telomerase activity. Additionally, significant inhibition of proliferation and induction of apoptosis were observed. CONCLUSION siRNA against the hTR gene can inhibit proliferation and induce apoptosis by blocking telomerase activity of HRCCs. Specific hTR inhibition by siRNA represents a promising new option for renal cancer treatment.