Semi-quantitative Detection of RNA-dependent RNA Polymerase Activity of Human Telomerase Reverse Transcriptase Protein

Human telomerase reverse transcriptase (TERT) is the catalytic subunit of telomerase, and it elongates telomere through RNA-dependent DNA polymerase ...

National Cancer Center Research Institute

The human endometrium is a dynamic tissue, the proliferative activity of which dramatically changes throughout the menstrual cycle, with exquisite regulation by sex-steroid hormones. Primary endometrial epithelial cells fall into senescence within 2 weeks when cultured on plastic dishes, and more complete understanding of endometrial biology has been delayed because of, in part, a lack of an in vitro culture model for endometrial epithelial cells. Our goal was to establish immortalized human endometrial glandular cells that retain the normal functions and characteristics of the primary cells. Because the Rb/p16 and p53 pathways are known to be critical elements of epithelial senescence in early passages, we used human papillomavirus E6/E7 to target these pathways. The combination of human papillomavirus-16 E6/E7 expression and telomerase activation by the introduction of human telomerase reverse transcriptase (hTERT) led to successful immortalization of the endometrial glandular cells. E6/E7 expression alone was sufficient to extend their life span more than 20 population doublings, but the telomerase activation was further required to enable the cells to pass through the subsequent replicative senescence at 40 population doublings. Isolated immortalized cells contained no chromosomal abnormalities or only nonclonal aberrations, retained responsiveness to sex-steroid hormones, exhibited glandular structure on three-dimensional culture, and lacked transformed phenotypes on soft agar or in nude mice. These findings support the notion that both Rb inactivation/p53 inactivation and telomerase activation are necessary to immortalize endometrial epithelial cells, but additional factors are required for endometrial carcinogenesis. Our established cell lines show great promise for investigation of hormone functions, endometrial biology, and endometrial carcinogenesis.

Successful immortalization of endometrial glandular cells with normal structural and functional characteristics.

Constitutive expression of telomerase in human cells prevents the onset of senescence and crisis by maintaining telomere homeostasis. However, accumulating evidence suggests that the human telomerase reverse transcriptase catalytic subunit (TERT) contributes to cell physiology independently of its ability to elongate telomeres. Here we show that TERT interacts with the RNA component of mitochondrial RNA processing endoribonuclease (RMRP), a gene that is mutated in the inherited pleiotropic syndrome cartilage-hair hypoplasia. Human TERT and RMRP form a distinct ribonucleoprotein complex that has RNA-dependent RNA polymerase (RdRP) activity and produces double-stranded RNAs that can be processed into small interfering RNA in a Dicer (also known as DICER1)-dependent manner. These observations identify a mammalian RdRP composed of TERT in complex with RMRP.

An RNA-dependent RNA polymerase formed by TERT and the RMRP RNA.

RNA-dependent RNA polymerases (RdRPs) synthesize double-stranded RNAs that are processed into small RNAs and mediate gene silencing. Viral RdRPs and cellular RdRPs show little structural homology to each other. Cellular RdRPs play key roles in RNA silencing by producing complementary strands for target RNAs via Dicer-dependent and -independent mechanisms. Although the existence of a functional mammalian homolog of RdRP has long been predicted, traditional approaches to identify such enzymes were unsuccessful. Recently, human telomerase reverse transcriptase, a polymerase closely related to viral RdRPs, has been shown to function as an RdRP and contributes to RNA silencing in vivo. These findings suggest that endogenous small interfering RNAs are produced by several mechanisms in eukaryotes.

RNA-dependent RNA polymerases in RNA silencing.

DICER is an RNase III family endoribonuclease that processes precursor microRNAs (pre-miRNAs) and long double-stranded RNAs, generating microRNA (miRNA) duplexes and short interfering RNA duplexes with 20~23 nucleotides (nts) in length. The typical form of pre-miRNA processed by the Drosha protein is a hairpin RNA with 2-nt 3' overhangs. On the other hand, production of mature miRNA from an endogenous hairpin RNA with 5' overhangs has also been reported, although the mechanism for this process is unknown.

Two-step cleavage of hairpin RNA with 5' overhangs by human DICER.

Indicators of aging such as disruption of telomeric function due to shortening may be more frequent in dysfunctional lacrimal gland. The aims of this study were to 1) determine the viability of quantitative fluorescence in situ hybridization of telomeres (telo-FISH) for the assessment of telomere length in lacrimal gland in Sjögren and non- Sjögren syndrome patients; and 2) investigate the relationship between progenitor cell markers and telomere length in both groups.

Comparison of telomere length and association with progenitor cell markers in lacrimal gland between Sjögren syndrome and non-Sjögren syndrome dry eye patients.

Recent work has identified a subset of cells resident in tumors that exhibit properties similar to those found in normal stem cells. Such cells are highly tumorigenic and may be involved in resistance to treatment. However, the genes that regulate the tumor initiating cell (TIC) state are unknown. Here, we show that overexpression of either of the nucleolar GTP-binding proteins nucleostemin (NS) or GNL3L drives the fraction of genetically defined tumor cells that exhibit markers and tumorigenic properties of TICs. Specifically, cells that constitutively express elevated levels of NS or GNL3L exhibit increased TWIST expression, phosphorylation of STAT3, expression of genes that induce pluripotent stem cells, and enhanced radioresistance; in addition, they form tumors even when small numbers of cells are implanted and exhibit an increased propensity to metastasize. GNL3L/NS forms a complex with the telomerase catalytic subunit [human telomerase reverse transcriptase (hTERT)] and the SWItch-Sucrose NonFermentable (SWI-SNF) complex protein brahma-related gene 1 (BRG1), and the expression of each of these components is necessary to facilitate the cancer stem cell state. Together, these observations define a complex composed of TERT, BRG1, and NS/GNL3L that maintains the function of TICs.

Maintenance of tumor initiating cells of defined genetic composition by nucleostemin.

Endogenous siRNAs (endo-siRNAs) are key regulators of RNA silencing in plants and worms; however, the biogenesis and function of endogenous siRNAs in mammals remain largely unknown. We previously demonstrated that human telomerase reverse transcriptase produces a self-targeting endogenous siRNA from non-coding RMRP RNA via RNA-dependent RNA polymerase (RdRP) activity. Here, we investigated whether the endo-siRNA derived from RMRP targets other genes in addition to RMRP. Four algorithms for microRNA target prediction were used to identify possible targets of the endo-siRNA, and the phytanoyl-CoA hydroxylase-interacting protein-like gene (PHYHIPL) was identified as the most promising candidate. The 3' UTR of PHYHIPL was found to contain three possible target sites with perfect seed pairing; deletion of each of these sites resulted in recovery of upstream luciferase expression. In addition, sequence-specific inhibition of the RMRP-derived endo-siRNA increased expression of PHYHIPL mRNA. The results described here suggest that the endo-siRNA uses silencing mechanisms that are similar to those used by microRNAs for gene silencing. To our knowledge, this study is the first confirmation of the off-target effect of human endogenous siRNA produced by RdRP activity.

Off-target effect of endogenous siRNA derived from RMRP in human cells.

In the fission yeast Schizosaccharomyces pombe, centromeric heterochromatin is maintained by an RNA-directed RNA polymerase complex (RDRC) and the RNA-induced transcriptional silencing (RITS) complex in a manner that depends on the generation of short interfering RNA. In association with the telomerase RNA component (TERC), the telomerase reverse transcriptase (TERT) forms telomerase and counteracts telomere attrition, and without TERC, TERT has been implicated in the regulation of heterochromatin at locations distinct from telomeres. Here, we describe a complex composed of human TERT (hTERT), Brahma-related gene 1 (BRG1), and nucleostemin (NS) that contributes to heterochromatin maintenance at centromeres and transposons. This complex produced double-stranded RNAs homologous to centromeric alpha-satellite (alphoid) repeat elements and transposons that were processed into small interfering RNAs targeted to these heterochromatic regions. These small interfering RNAs promoted heterochromatin assembly and mitotic progression in a manner dependent on the RNA interference machinery. These observations implicate the hTERT/BRG1/NS (TBN) complex in heterochromatin assembly at particular sites in the mammalian genome. 

Involvement of telomerase reverse transcriptase in heterochromatin maintenance.

To investigate the impact of estrogen contained in oral contraceptives (OCs) on the action of progestin on ovarian endometrioma epithelial cells.

Concurrent estrogen action was essential for maximal progestin effect in oral contraceptives.

Treatment of advanced ovarian cancer involves platinum-based chemotherapy. However, chemoresistance is a major obstacle. Cancer stem cells (CSCs) are thought to be one of the causes of chemoresistance, but the underlying mechanism remains elusive. Recently, human telomerase reverse transcriptase (hTERT) has been reported to promote CSC-like traits. In this study, we found that a mitotic inhibitor, eribulin mesylate (eribulin), effectively inhibited growth of platinum-resistant ovarian cancer cell lines. Eribulin-sensitive cells showed a higher efficiency for sphere formation, suggesting that these cells possess an enhanced CSC-like phenotype. Moreover, these cells expressed a higher level of hTERT, and suppression of hTERT expression by siRNA resulted in decreased sensitivity to eribulin, suggesting that hTERT may be a target for eribulin. Indeed, we found that eribulin directly inhibited RNA-dependent RNA polymerase (RdRP) activity, but not telomerase activity of hTERT in vitro. We propose that eribulin targets the RdRP activity of hTERT and may be an effective therapeutic option for CSCs. Furthermore, hTERT may be a useful biomarker to predict clinical responses to eribulin. 

Eribulin mesylate targets human telomerase reverse transcriptase in ovarian cancer cells.

MicroRNAs are small non-coding RNAs that inhibit the translation of target mRNAs. In humans, most microRNAs are transcribed by RNA polymerase II as long primary transcripts and processed by sequential cleavage of the two RNase III enzymes, DROSHA and DICER, into precursor and mature microRNAs, respectively. Although the fundamental functions of microRNAs in RNA silencing have been gradually uncovered, less is known about the regulatory mechanisms of microRNA expression. Here, we report that telomerase reverse transcriptase (TERT) extensively affects the expression levels of mature microRNAs. Deep sequencing-based screens of short RNA populations revealed that the suppression of TERT resulted in the downregulation of microRNAs expressed in THP-1 cells and HeLa cells. Primary and precursor microRNA levels were also reduced under the suppression of TERT. Similar results were obtained with the suppression of either BRG1 (also called SMARCA4) or nucleostemin, which are proteins interacting with TERT and functioning beyond telomeres. These results suggest that TERT regulates microRNAs at the very early phases in their biogenesis, presumably through non-telomerase mechanism(s). 

Telomerase reverse transcriptase regulates microRNAs.

Telomeres, the repetitive sequences at chromosomal ends, protect intact chromosomes. Telomeres progressively shorten through successive rounds of cell divisions, and critically shortened telomeres trigger senescence and apoptosis. The enzyme that elongates telomeres and maintains their structure is known as telomerase. The catalytic subunit of this enzyme (telomerase reverse transcriptase [TERT]) is expressed at a high level in malignant cells, but at a very low level in normal cells. Although telomerase activity was long believed to be the only function of TERT, emerging evidence indicates that TERT plays roles beyond telomeres. For example, TERT contributes to stem cell maintenance and cell reprogramming processes in a manner independent of its canonical function. Even some types of splice variants that lack the telomerase catalytic domains exhibit the functions in a manner that does not depend on telomerase activity. We recently demonstrated that the RNA-dependent RNA polymerase (RdRP) activity of TERT is involved in regulation of gene silencing and heterochromatic transcription. Moreover, TERT RdRP activity is mediated by a newly identified complex, distinct from the authentic telomerase complex, that plays a role in cancer stem cells in a telomere maintenance independent manner. TERT has attracted interest as a molecular target for anticancer treatment, but previous efforts aimed at developing novel therapeutic strategies focused only on the canonical function of TERT. However, accumulating evidence about the non-canonical functions of TERT led us to speculate that the functions other than telomerase might be therapeutic targets as well. In this review, we discuss the non-canonical functions of TERT and their potential applications for anticancer treatment. 

Telomerase reverse transcriptase moonlights: Therapeutic targets beyond telomerase.

RNA-dependent RNA polymerase (RdRP) plays key roles in RNA silencing to generate double-stranded RNAs. In model organisms, such as Caenorhabditis elegans and Neurospora crassa, two types of small interfering RNAs (siRNAs), primary siRNAs and secondary siRNAs, are expressed; RdRP produces secondary siRNAs de novo, without using either Dicer or primers, while primary siRNAs are processed by Dicer. We reported that human telomerase reverse transcriptase (TERT) has RdRP activity and produces endogenous siRNAs in a Dicer-dependent manner. However, de novo synthesis of siRNAs by human TERT has not been elucidated. Here we show that the TERT RdRP generates short RNAs that are complementary to template RNAs and have 5'-triphosphorylated ends, which indicates de novo synthesis of the RNAs. In addition, we confirmed short RNA synthesis by TERT in several human carcinoma cell lines and found that TERT protein levels are positively correlated with RdRP activity.

Yoshiko Maida

Division of Cancer Stem Cell

National Cancer Center Research Institute

Semi-quantitative Detection of RNA-dependent RNA Polymerase Activity of Human Telomerase Reverse Transcriptase Protein

Yoshiko Maida

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Division of Cancer Stem Cell

National Cancer Center Research Institute

Semi-quantitative Detection of RNA-dependent RNA Polymerase Activity of Human Telomerase Reverse Transcriptase Protein

Division of Cancer Stem Cell