Targeting hTERT Research Articles

Downregulation of miR‐138 is associated with overexpression of human telomerase reverse transcriptase protein in human anaplastic thyroid carcinoma cell lines

Alterations of several microRNA (miRNA) have been linked to cancer development and its biology. To search for unique miRNA that might play a role in the development of anaplastic thyroid carcinoma (ATC), we examined the expression of multiple miRNA and their functional effects on target genes in human thyroid carcinoma cell lines. We quantitatively evaluated the expression of multiple miRNA in 10 ATC and five papillary thyroid carcinoma (PTC) cell lines, as well as primary tumors from 11 thyroid carcinoma patients (three ATC and eight PTC), using the stem-loop-mediated reverse transcription real-time polymerase chain reaction method. We also examined the target gene specificity of unique miRNA that showed differences in expression between ATC and PTC cell lines. One miRNA, miR-138, was significantly downregulated in ATC cell lines in comparison with PTC (P < 0.01). Eleven miRNA (including miR-138) potentially targeting the human telomerase reverse transcriptase (hTERT) gene were totally downregulated in both ATC and PTC cell lines in comparison with normal thyroid tissues. A tendency for an inverse correlation between miR-138 and hTERT protein expression was observed in the thyroid cancer cell lines, although this failed to reach significance (r = -0.392, P = 0.148). We demonstrated that overexpression of miR-138 induced a reduction in hTERT protein expression, and confirmed target specificity between miR-138 and the hTERT 3'-untranslated region by luciferase reporter assay. These results suggest that loss of miR-138 expression may partially contribute to the gain of hTERT protein expression in ATC, and that further multiple miRNA targeting hTERT mRNA might be involved in the development of thyroid carcinoma.

Noninvasive Imaging of Human Telomerase Reverse Transcriptase (hTERT) Messenger RNA with 99mTc-Radiolabeled Antisense Probes in Malignant Tumors

The expression of human telomerase reverse transcriptase (hTERT) is present in most malignant cells (>85%) but is undetectable in most normal somatic cells. Visualization of hTERT expression using radionuclide targeting can provide important diagnostic information in malignant tumors. The overall aim of this study was to evaluate whether (99m)Tc-radiolabeled antisense oligonucleotide (ASON) targeting hTERT messenger RNA (mRNA) can be used for imaging of hTERT expression in vivo. One 18-mer antisense or sense uniformly phosphorothioate-modified oligonucleotide targeting hTERT mRNA was radiolabeled with (99m)Tc through the bifunctional chelator N-hydroxysuccinimidyl derivative of S-acetylmercaptoacetyltriglycine (S-acetyl NHS-MAG3). Then the radiolabeled probe was characterized in vitro. Reverse transcription-polymerase chain reaction (RT-PCR) was performed to assay the mRNA level after proliferating cells had been incubated with the antisense and sense probes. (99m)Tc-MAG3-ASON or (99m)Tc-MAG3-SON was injected intravenously in mammary tumor-bearing BALB/c nude mice. Biodistribution and in vivo imaging was performed periodically. All data were analyzed by statistical software. The labeling efficiencies of (99m)Tc-MAG3-ASON/SON reached 76% +/- 5% (n = 5) within 15-30 min at room temperature, the specific activity was up to 1,850 kBq/mug, and the radiochemical purity was >96% after purification. (99m)Tc-MAG3-ASON showed complete stability at room temperature and in fresh 37 degrees C human serum. In comparison with (99m)Tc-MAG3-SON, (99m)Tc-MAG3-ASON preserved the capacity to bind living hTERT-expressing cells specifically and to inhibit the expression of hTERT mRNA significantly as well as ASON. In nude mice bearing hTERT-expressing MCF-7 xenografts, tumor radioactivity uptake of (99m)Tc-MAG3-ASON after injection was significantly higher than that of (99m)Tc-MAG3-SON after injection (P < 0.05). The hTERT-expressing xenografts were clearly imaged at 4-8 h noninvasively after injection of (99m)Tc-MAG3-ASON, whereas the xenografts were not imaged at any time after injection of (99m)Tc-MAG3-SON. This in vivo study provides evidence that (99m)Tc-MAG3-ASON targeting hTERT mRNA can be used as a potential candidate for visualization of hTERT expression in carcinomas.

Inhibition of Proliferation and Induction of Apoptosis in Human Renal Carcinoma Cells by Anti-telomerase Small Interfering RNAs

Telomerase is an attractive molecular target for cancer therapy because it is present in most malignant cells but is undetectable in most normal somatic cells. Human telomerase consists of two subunits, an RNA component (hTR) and a human telomerase reverse transcriptase component (hTERT). Small interfering RNA (siRNA), one kind of RNA interferences, has been demonstrated to be an effective method to inhibit target gene expression in human cells. We investigated the effects of siRNA targeting at both hTR and hTERT mRNA on the inhibition of telomerase activity in human renal carcinoma cells (HRCCs). The proliferation and apoptosis of HRCCs were examined. The treatment of HRCCs using hTR and hTERT siRNAs resulted in significant decrease of hTR mRNA, hTERT mRNA and hTERT protein. The siRNA can also inhibit the telomerase activity and the proliferation of HRCCs. Moreover, they can induce apoptotic cell death in a dose-dependent manner. From these findings, we propose that the inhibition of telomerase activity using siRNA targeting hTR and hTERT might be a rational approach in renal cancer therapy.

Long-term suppression of telomerase expression in HeLa cell clones, transfected with an expression vector carrying siRNA targeting hTERT mRNA

HeLa cell cultures were used as model systems for small interfering RNA (siRNA) induced knockdown of mRNA expression of the human telomerase catalytic subunit, telomerase reverse transcriptase (hTERT). Four 21-bp siRNAs targeting different sites of the hTERT mRNA were designed, and the siRNA molecules produced by a T7 transcription system in vitro. In transient transfection assays on HeLa cells, only one of the tested siRNAs produced a potent knockdown effect on hTERT mRNA expression, associated with the suppression of telomerase activity (both reduced by approximately 50%). An expression vector encoding a hairpin siRNA against the effective hTERT mRNA target site was generated, and HeLa clones stably expressing hTERT-specific siRNA were created. Two clones showed extremely reduced hTERT mRNA expression, associated with unusually short telomeres, the inhibition of cell growth and the induction of senescence and apoptosis. Thus, there was obvious loss of viability in cells lacking hTERT expression and carrying short telomeres. This was most prominent in the clone that showed prolonged reductions (over two months) in both hTERT expression and telomerase activity. Thus, our data clearly show that long-term suppression of telomerase expression by siRNA is an attainable goal, at least in a HeLa cell model system.

Microarray analyses in bladder cancer cells: Inhibition of hTERT expression down‐regulates EGFR

The human telomerase reverse transcriptase (hTERT) contributes to the immortal phenotype of the majority of cancers. Targeting hTERT by transfection with antisense oligonucleotides (AS-ODNs) induced immediate growth inhibition in human bladder cancer (BCa) cells. The molecular basis of the antiproliferative capacity of hTERT AS-ODNs was investigated by oligonucleotide microarray analyses and was compared to effects caused by siRNA-mediated knock-down of hTERT in EJ28 BCa cells. Two different AS-ODNs -- both down-regulated the expression of hTERT -- changed the expression of different genes mainly involved in stress response (including EGR1, ATF3 and GDF15), but without an association to telomerase function. This indicates that the immediate growth inhibition was caused, at least in part, by off-target effects. In comparison to that the blockade of the expression of hTERT using 2 different siRNAs was accompanied by the down-regulation of the oncogenes FOS-like antigen 1 (FOSL1) and epidermal growth factor receptor (EGFR), known to be overexpressed in BCa. We show here for the first time that repression of the hTERT transcript number decreased the expression of EGFR both at the mRNA and protein levels, suggesting a potential new function of hTERT in the regulation of EGFR-stimulated proliferation. Furthermore, the suppression of hTERT by siRNAs caused an enhancement of the antiproliferative capacity of the chemotherapeutics mitomycin C and cisplatin. The results presented herein may support the hypothesis that hTERT promotes the growth of tumor cells by mechanisms independent from telomere lengthening. The detailed clarification of these processes will shed light on the question, whether telomerase inhibitors might constitute suitable anticancer tools.

Inhibition of tumor growth and metastasis with antisense oligonucleotides (Cantide) targeting hTERT in an in situ human hepatocellular carcinoma model

To evaluate the in vivo antitumor effects of Cantide and the combined effect with 5-fluorouracil. An in situ human hepatocellular carcinoma model was established in mice livers orthotopically. Drugs were administered intravenously and tumor sizes were monitored with calipers. Plasma alpha-fetoprotein(AFP) were detected by radiation immunoassay. Morphology of tumors was evaluated by hematoxylin-eosin (H and E) staining of histological sections. Human telomerase reverse transcriptase (hTERT) protein levels were detected by Western blotting. Cantide significantly inhibit in situ human hepatocellular carcinoma growth in mice with a 75 and 50 mg.kg(-1).d(-1) administration of Cantide compared to the saline group in a dose-dependent manner, which included injecting Cantide 25 mg.kg(-1).d(-1) by iv for 20 d after surgically removing the tumor in liver. Cantide was also found to prevent tumor recurrence in the liver and metastasis in the lung, showing a dose-dependent response. When Cantide was administered by iv combined with 5-fluorouracil, it resulted in a significant reduction in tumor growth compared to either agent alone treatment group. After the treatment with Cantide alone or combined with 5-fluorouracil, plasma AFP concentration decreased in a dose-dependent manner. These results demonstrated that Cantide was an effective antitumor antisense oligonucleotide in vivo and has the potential to be developed into a clinical anti-cancer drug.

615. Feasibility of hTERT-Targeting Trans-Splicing Ribozyme in Tumor-Targeted Gene Therapy

Cancer gene therapy without toxicity is hampered by lack of efficient tumor-directed therapeutic gene expression system. In the present study, we describe a new tumor-targeted gene therapy tool that is based upon trans-splicing ribozymes. Trans-splicing ribozymes are RNA-based catalysts capable of performing both cleavage and ligation reaction in target RNA. Thus a chimeric mRNA can be produced and new gene activities triggered in cells dependent on the presence of the target RNA. We chose hTERT-targeting trans-splicing ribozyme for efficient tumor-directed expression of therapeutic gene by transcriptional targeting and RNA replacement because human telomerase reverse transcriptase (hTERT) is expressed at high levels in 90% of malignant tumors and tumor cell lines but is absent in normal postmitotic tissues. After developing a group I intron-based ribozyme targeting hTERT with high fidelity and specificity, we constructed novel adenoviral vectors containing hTERT-targeting trans-splicing ribozymes with downstream lacZ gene (Ad-ribolacZ) or herpes simplex virus thymidine kinase gene (Ad-ribotk). This hTERT-targeting trans-splicing ribozyme could simultaneously reduce hTERT expression and trigger expression of a new gene, such as lacZ or HSVtk, in cells expressing hTERT. To determine whether new gene activity could be selectively induced in hTERT expressing cells by trans-splicing ribozyme, we transfected Ad-ribolacZ into hTERT-positive and -negative cell lines. Then lacZ expression was analyzed by X-gal staining and β-galactosidase enzyme activity assays. We observed 4 to 9-fold increase of lacZ expression in hTERT-positive cells but not in h-TERT-negative ones. In contrast, induction of lacZ expression by adenovirus harboring lacZ gene driven by CMV promoter (Ad-CMVlacZ) could be observed in both hTERT-negative and-positive cells. Those results suggest that selective induction of transgene activity in hTERT-dependent manner by Ad-ribolacZ was mediated by correct targeting of hTERT with trans-splicing ribozyme in hTERT-positive cells. Given the observation that the trans-splicing ribozyme induced transgene expression selectively in target RNA-expressing cells, we determined whether the ribozyme would trigger selective cytotoxicity in hTERT-expressing cells. After transfecting Ad-ribotk into hTERT-positive and -negative cell lines, selective cytotoxicity was monitored by MTT assays. Ad-CMVtk was used as control. We could observe that Ad-ribotk revealed more than 50% reduction in survival rate of hTERT-positive cells, while not in hTERT-negative ones. In control, reductions of cell survival were observed regardless of hTERT expression. Those observation of highly selective cytotoxicity rendered by Ad-ribotk suggest that hTERT-targeting trans-splicing ribozyme may be one of powerful tools in tumor-targeted gene therapy.