Date Published: August 12, 2019
Publisher: Public Library of Science
Author(s): Takahito Ohira, Hirotada Kojima, Yuko Kuroda, Sayaka Aoki, Daigo Inaoka, Mitsuhiko Osaki, Hideki Wanibuchi, Futoshi Okada, Mitsuo Oshimura, Hiroyuki Kugoh, Klaus Roemer.
Telomerase is a ribonucleoprotein ribonucleic enzyme that is essential for cellular immortalization via elongation of telomere repeat sequences at the end of chromosomes. Human telomerase reverse transcriptase (hTERT), the catalytic subunit of telomerase holoenzyme, is a key regulator of telomerase activity. Telomerase activity, which has been detected in the majority of cancer cells, is accompanied by hTERT expression, suggesting that this enzyme activity contributes to an unlimited replication potential of cancer cells via regulation of telomere length. Thus, hTERT is an attractive target for cancer-specific treatments. We previously reported that pared-like homeodomain 1 (PITX1) is a negative regulator of hTERT through direct binding to the hTERT promoter. However, the mechanism by which the function of PITX1 contributes to transcriptional silencing of the hTERT gene remains to be clarified. Here, we show that PITX1 and zinc finger CCHC-type containing 10 (ZCCHC10) proteins cooperate to facilitate the transcriptional regulation of the hTERT gene by functional studies via FLAG pull-down assay. Co-expression of PITX1 and ZCCHC10 resulted in inhibition of hTERT transcription, in melanoma cell lines, whereas mutate-deletion of homeodomain in PITX1 that interact with ZCCHC10 did not induce similar phenotypes. In addition, ZCCHC10 expression levels showed marked decrease in the majority of melanoma cell lines and tissues. Taken together, these results suggest that ZCCHC10-PITX1 complex is the functional unit that suppresses hTERT transcription, and may play a crucial role as a novel tumor suppressor complex.
Telomerase ribonucleic enzyme is associated with extend cell life span by elongation of telomere repeat sequences on the end of chromosomes, and sustain cell proliferation in cancer cells [1,2]. Human telomerase consists of essential enzyme subunits; the protein catalytic subunit human telomerase reverse transcriptase (hTERT) and the RNA subunit, human telomerase RNA component (hTERC) and the accessory proteins dyskerin, NOP10, NHP2 and GAR1 [3,4]. It is important to note that hTERT molecules are very low rather than other telomerase components in cancer cell lines. This is suggested that hTERT transcription is tightly regulated more than other telomerase components . The expression of hTERT is critical for telomerase enzyme activity. Indeed, ectopic hTERT expression in telomerase-negative normal cells can extend lifespan and establish immortalized cell lines via elongation of telomeres [6,7]. Expression of hTERT is down-regulated in most human adult somatic cells, except in germ cells and some stem cells. On the other hand, its expression was detected in the majority of human cancer cells (around 85–90%) [8,9]. This is consistent with telomerase conferring a strong selective advantage for continued growth of malignant cells, suggesting that telomerase activity is essential for most cancer cell immortalization and it may be possible to inhibit of cancer development by the control of hTERT expression. Furthermore, hTERT has noncanonical functions in addition to that of maintaining telomere length. It was reported that hTERT acts as a transcriptional modulator of Wnt/beta-catenin and NF-kappa B signaling pathways, resulting in the enhanced expression of Wnt and NF-kappa B target genes that facilitate cancer promoting functions such as proliferation and resistance to apoptosis [10,11]. Additionally, hTERT protein directly associates with the RNA polymerase III subunit RPC32, which restore tRNA levels and promote cell metabolism and proliferation in cancer cells . Although it is known that expression of hTERT is regulated by various activating and repressing transcription factors and epigenetic modification [13,14], the underlying molecular mechanisms that are involved in regulation of hTERT transcription during cellular differentiation and cancer development remains unclear.
TERT, which plays a crucial role in the regulation of telomerase activity, contributing to stem cell self-renewal and immortalization of malignant cells, is regulated by many different genes in response to a wide variety of oncogenic and suppressive signaling pathways. We previously identified PITX1 as one of the hTERT suppressor, which directly binds to its promoter region . In this study, we found that ZCCHC10 is one of the components in PITX1 complexes that significantly regulate hTERT transcription. ZCCHC10 is also located in the same human chromosome region 5q31.1 as the PITX1 gene. Recently, another group reported that a long noncoding RNA esophagus epithelial intergenic specific transcript (Epist) can down-regulate hTERT mRNA transcription . Interestingly, the Epist RNA transcript is located 4.5-kbp downstream of the PITX1 coding region. Furthermore, it has been reported that chromosome rearrangements that contain interstitial deletions and chromosome breakpoints were present on the long arm of chromosome 5 in acute myeloid leukemia (AML) and myelodysplasia (MDS) . In particular, the most commonly deleted segment of 5q in MDS is within the 5q31 region . This circumstantial evidence suggests that the human chromosome 5q31 region, which contains a negative regulator of hTERT, plays a significant role as the hTERT regulatory center.