Human Telomerase Reverse Transcriptase (hTERT) Transcription Requires Sp1/Sp3 Binding to the Promoter and a Permissive Chromatin Environment

Induction of the human CDC45 gene promoter activity by natural compound trans‑resveratrol

GGAA motifs in the human TP53 and HELB gene promoters play a part in responding to trans‑resveratrol (Rsv) in HeLa S3 cells. This sequence is also present in the 5'‑upstream region of the human CDC45 gene, which encodes a component of CMG DNA helicase protein complex. The cells were treated with Rsv (20 µM), then transcripts and the translated protein were analyzed by quantitative RT‑PCR and western blotting, respectively. The results showed that the CDC45 gene and protein expression levels were induced after the treatment. To examine whether they were due to the activation of transcription, a 5'‑upstream 556‑bp of the CDC45 gene was cloned and inserted into a multi‑cloning site of the Luciferase (Luc) expression vector. In the present study, various deletion/point mutation‑introduced Luc expression plasmids were constructed and they were used for the transient transfection assay. The results showed that the GGAA motif, which is included in a putative RELB protein recognizing sequence, plays a part in the promoter activity with response to Rsv in HeLa S3 cells.

Interaction and Collaboration of SP1, HIF-1, and MYC in Regulating the Expression of Cancer-Related Genes to Further Enhance Anticancer Drug Development

Specificity protein 1 (SP1), hypoxia-inducible factor 1 (HIF-1), and MYC are important transcription factors (TFs). SP1, a constitutively expressed housekeeping gene, regulates diverse yet distinct biological activities; MYC is a master regulator of all key cellular activities including cell metabolism and proliferation; and HIF-1, whose protein level is rapidly increased when the local tissue oxygen concentration decreases, functions as a mediator of hypoxic signals. Systems analyses of the regulatory networks in cancer have shown that SP1, HIF-1, and MYC belong to a group of TFs that function as master regulators of cancer. Therefore, the contributions of these TFs are crucial to the development of cancer. SP1, HIF-1, and MYC are often overexpressed in tumors, which indicates the importance of their roles in the development of cancer. Thus, proper manipulation of SP1, HIF-1, and MYC by appropriate agents could have a strong negative impact on cancer development. Under these circumstances, these TFs have naturally become major targets for anticancer drug development. Accordingly, there are currently many SP1 or HIF-1 inhibitors available; however, designing efficient MYC inhibitors has been extremely difficult. Studies have shown that SP1, HIF-1, and MYC modulate the expression of each other and collaborate to regulate the expression of numerous genes. In this review, we provide an overview of the interactions and collaborations of SP1, HIF1A, and MYC in the regulation of various cancer-related genes, and their potential implications in the development of anticancer therapy.

The Relevance of Telomerase and Telomere-Associated Proteins in B-Acute Lymphoblastic Leukemia

Telomeres and telomerase are closely linked to uncontrolled cellular proliferation, immortalization and carcinogenesis. Telomerase has been largely studied in the context of cancer, including leukemias. Deregulation of human telomerase gene hTERT is a well-established step in leukemia development. B-acute lymphoblastic leukemia (B-ALL) recovery rates exceed 90% in children; however, the relapse rate is around 20% among treated patients, and 10% of these are still incurable. This review highlights the biological and clinical relevance of telomerase for B-ALL and the implications of its canonical and non-canonical action on signaling pathways in the context of disease and treatment. The physiological role of telomerase in lymphocytes makes the study of its biomarker potential a great challenge. Nevertheless, many works have demonstrated that high telomerase activity or hTERT expression, as well as short telomeres, correlate with poor prognosis in B-ALL. Telomerase and related proteins have been proven to be promising pharmacological targets. Likewise, combined therapy with telomerase inhibitors may turn out to be an alternative strategy for B-ALL.

The TERT Promoter: A Key Player in the Fight for Cancer Cell Immortality

The review describes the role of telomeres and telomerase in tumor progression, as well as various mechanisms of the activation of telomerase reverse transcriptase (TERT) expression in CNS tumors and other cancers. The main mechanism of TERT activation involves acquisition of somatic mutations by the TERT gene promoter (TERTp). The article presents information on the TERTp structure and transcription factors directly interacting with TERTp and regulating its transcription. The prospects of using the mutational status of TERTp as a prognostic marker of CNS malignancies and other tumors with a common profile of TERTp mutations are discussed.

SMAR1 suppresses the cancer stem cell population via hTERT repression in colorectal cancer cells

One of the hallmarks of a cancer cell is the ability for indefinite proliferation leading to the immortalization of the cell. Activation of several signaling pathways leads to the immortalization of cancer cells via the reactivation of enzyme telomerase (hTERT). hTERT is active in germ cells, stem cells and also cancer cells. An earlier report from our lab suggests that SMAR1, a tumor suppressor protein, is significantly downregulated in the higher grades of colorectal cancers. Our study identifies SMAR1 as a transcriptional repressor of hTERT. We find that SMAR1 interacts with HDAC1/mSin3a co-repressor complex at the hTERT promoter and brings about HDAC1-mediated transcriptional repression of the promoter. Most solid tumors including colorectal cancer reactivate hTERT expression as it confers several advantages to the cancer cells like increased proliferation and angiogenesis. One of these non-canonical functions of hTERT is inducing the pool of cancer stem cell population. We find that in the CD133^HighCD44^High cancer stem cells population, SMAR1 expression is highly diminished leading to elevated hTERT expression. We also find that knockdown of SMAR1 promotes total CD133⁺CD44⁺ population and impart enhanced sphere-forming ability to the colorectal cancer cells. SMAR1 also inhibits invasion and metastasis in colorectal cancer cell lines via repression of hTERT. Our study provides evidence that downregulation of SMAR1 causes activation of hTERT leading to an increase in the cancer stem cell phenotype in colorectal cancer cells.

ETS variant transcription factor 5 and c-Myc cooperate in derepressing the human telomerase gene promoter via composite ETS/E-box motifs

The human telomerase gene (hTERT) is repressed in most somatic cells. How transcription factors activate the hTERT promoter in its repressive chromatin environment is unknown. Here, we report that the ETS family protein ETS variant transcription factor 5 (ETV5) mediates epidermal growth factor (EGF)-induced hTERT expression in MCF10A cells. This activation required MYC proto-oncogene bHLH transcription factor (c-Myc) and depended on the chromatin state of the hTERT promoter. Using chromatinized bacterial artificial chromosome (BAC) reporters in human fibroblasts, we found that ETV5 and c-Myc/MYC-associated factor X (MAX) synergistically activate the hTERT promoter via two identical, but inverted, composite Ets/E-box motifs enclosing the core promoter. Mutations of Ets or E-box sites in either DNA motif abolished the activation and reduced or eliminated the synergism. ETV5 and c-Myc facilitated each other's binding to the hTERT promoter. ETV5 bound to the hTERT promoter in both telomerase-negative and -positive cells, but it activated the repressed hTERT promoter and altered histone modifications only in telomerase-negative cells. The synergistic ETV5/c-Myc activation disappeared when hTERT promoter repression became relieved because of the loss of distal regulatory elements in chimeric human/mouse BAC reporters. Our results suggest that the binding of c-Myc and ETS family proteins to the Ets/E-box motifs derepresses the hTERT promoter by inducing an active promoter configuration, providing a mechanistic insight into hTERT activation during tumorigenesis.

Telomere Maintenance in Pediatric Cancer

Telomere length has been proposed as a biomarker of biological age and a risk factor for age-related diseases and cancer. Substantial progress has been made in recent decades in understanding the complex molecular relationships in this research field. However, the majority of telomere studies have been conducted in adults. The data on telomere dynamics in pediatric cancers is limited, and interpretation can be challenging, especially in cases where results are contrasting to those in adult entities. This review describes recent advances in the molecular characterization of structure and function of telomeres, regulation of telomerase activity in cancer pathogenesis in general, and highlights the key advances that have expanded our views on telomere biology in pediatric cancer, with special emphasis on the central role of telomere maintenance in neuroblastoma. Furthermore, open questions in the field of telomere maintenance research are discussed in the context of recently published literature.

The mechanism of SP1/p300 complex promotes proliferation of multiple myeloma cells through regulating IQGAP1 transcription

Our previous research had firstly shown that MM cells overexpressed IQGAP1 gene and activated Ras/Raf/MEK/ERK pathway. But the mechanism of IQGAP1 overexpression and IQGAP1 gene transcription regulation remains uncertain. The mechanism of IQGAP1 overexpression and transcriptional regulation of IQGAP1 gene in myeloma cells was explored in the study. Through bioinformatics analysis and prediction we predicted and screened transcription factor Sp1 as a possible upstream regulator of IQGAP1.The proliferation, cell cycle and downstream ERK1/2 and p-ERK1/2 proteins were detected after siRNA-IQGAP1 was transfected to myeloma cells. The expression of Sp1, p300, IQGAP1, p-ERK1/2 and ERK1/2 were detected after Sp1 and p300 were inhibited or overexpressed respectively. The dual-luciferase reporter system was used to detect the activity of IQGAP1 gene promoter. CHIP was used to detect the binding of the Sp1 and IQGAP1 promoter regions.CO-IP was used to explore the interaction between Sp1 and p300.The mRNA expression levels of Sp1,p300 and IQGAP1 of the myeloma patients were detected, and the correlation analysis of their mRNA expression levels were carried out. The results showed IQGAP1-siRNA inhibits cell proliferation, cell cycle, IQGAP1 expression and phosphorylation of ERK1/2 protein. Inhibition of Sp1 or p300 down-regulated ERK1/2 and IQGAP1 expression; overexpression of Sp1 or p300 up-regulated ERK1/2 and IQGAP1 expression; Sp1 and p300 had a positive regulation effect on IQGAP1.Over expression of Sp1 or p300 significantly increased activity of IQGAP1 gene promoter. The transcription factor Sp1 plays a regulatory role in the IQGAP1 promoter region. There is an interaction between Sp1 and p300 in myeloma cells. The mRNA expression levels of Sp1, IQGAP1 and p300 in MM samples showed a positive correlation. In summary IQGAP1 is required for cell proliferation in MM cells, and the transcription of Sp1/p300 complex regulates expression of IQGAP1 gene.

Sp1 contributes to overexpression of stanniocalcin 2 through regulation of promoter activity in colon adenocarcinoma

BACKGROUND

Aberrant expression of stanniocalcin 2 (STC2) is implicated in colon adenocarcinoma (COAD). A previous study identified that STC2 functions as a tumor promoter to drive development of some cancers, but the role of its overexpression in the development of COAD remains unclear.

AIM

To evaluate the regulation mechanism of STC2 overexpression in COAD.

METHODS

The expression of STC2 in COAD was assessed by TCGA COAD database and GEO (GSE50760). Methylation level of the STC2 promoter was evaluated with beta value in UALCAN platform, and the correlation between STC2 expression and survival rate was investigated with TCGA COAD. Transcription binding site prediction was conducted by TRANSFAC and LASAGNA, and a luciferase reporter system was used to identify STC2 promoter activity in several cell lines, including HEK293T, NCM460, HT29, SW480, and HCT116. Western blotting was performed to evaluate the role of Sp1 on the expression of STC2.

RESULTS

The central finding of this work is that STC2 is overexpressed in COAD tissues and positively correlated with poor prognosis. Importantly, the binding site of the transcription factor Sp1 is widely located in the promoter region of STC2. A luciferase reporter system was successfully constructed to analyze the transcription activity of STC2, and knocking down the expression of Sp1 significantly inhibited the transcription activity of STC2. Furthermore, inhibition of Sp1 remarkably decreased protein levels of STC2.

CONCLUSION

Our data provide evidence that the transcription factor Sp1 is essential for the overexpression of STC2 in COAD through activation of promoter activity. Taken together, our finding provides new insights into the mechanism of oncogenic function of COAD by STC2.

Camptothecin induces c-Myc- and Sp1-mediated hTERT expression in LNCaP cells: Involvement of reactive oxygen species and PI3K/Akt

Camptothecin (CPT), a quinoline alkaloid isolated from Camptotheca acuminate, targets topoisomerase I, which is continuously expressed in cancer cells. However, the molecular mechanisms responsible for CPT-induced telomerase inhibition remain unclear. Unexpectedly, we found that CPT upregulates hTERT expression and concomitantly increases telomerase activity. However, transfection of hTERT-targeting siRNA had no effect on CPT-induced G₂/M phase arrest, suggesting that CPT-induced telomerase activation was not related to G₂/M phase arrest. CPT simultaneously increased Nrf2 expression and the level of intracellular reactive oxygen species (ROS), whereas pretreatment with the antioxidants N-acetyl-cysteine (NAC) or glutathione (GSH) strongly attenuated ROS production, which was accompanied by hTERT downregulation. Additionally, transient Nrf2 knockdown enhanced CPT-induced ROS production and hTERT promoter activity. CPT also upregulated hTERT expression and telomerase activity by inducing c-Myc and Sp1 expression and activity. Moreover, c-Myc stimulated ROS production in response to CPT, leading to Sp1 activation, which promoted hTERT expression and telomerase activity. CPT treatment enhanced the phosphorylation of PI3K and Akt, which led to hTERT phosphorylation into the nucleus. These findings demonstrate that CPT positively regulates telomerase activity by upregulating hTERT expression and phosphorylation via the c-Myc/ROS/Sp1 and PI3K/Akt axis.

The cellular senescence of leukemia-initiating cells from acute lymphoblastic leukemia is postponed by β-Arrestin1 binding with P300-Sp1 to regulate hTERT transcription

Although we previously reported that the self-renewal of leukemia-initiating cells of B-lineage acute lymphoblastic leukemia (B-ALL LICs) was regulated by β-Arrestin1, a multiple-function protein, the cellular senescence is critical for LICs fate and leukemia progress, and worthy for further investigation. Here we found that depletion of β-Arrestin1 extended the population doubling time and the percentage of senile cells, the signatures of cellular senescence, of B-ALL LICs. Moreover, lack of β-Arrestin1 enhanced the expression of proteins (CBX, HIRA) and genes (P53, P16) related to senescence in leukemic Reh cells and B-ALL-LICs-derived leukemic mice. Further results showed that loss of β-Arrestin1 induced senescence of Reh cells through mediating hTERT-telomerase-telomere axis, which was reversed by BIBR1532, the telomerase activity inhibitor. Importantly, depletion of β-Arrestin1 decreased the binding of Sp1 to hTERT promoter at the region of −28 to −36 bp. The anti-sense oligonucleotide of this key region downregulated the transcription of hTERT and aggravated the senescence of Reh cells. Further data demonstrated that the depleted β-Arrestin1 reduced the interaction of P300 with Sp1, thus to reduce Sp1 binding to hTERT promoter, downregulate hTERT transcription, decrease telomerase activity, shorten telomere length, and promote Reh cell senescence. Interestingly, the percentage of senile cells in B-ALL LICs was decreased, which was negatively correlated to good prognosis and β-Arrestin1 mRNA expression in childhood B-ALL patients. Our study shed a light on the senescence of B-ALL LICs and is regulated by β-Arrestin1, providing the potential therapeutic target of leukemia by promoting cellular senescence with a key region of hTERT promoter.

Repression of telomerase gene promoter requires human-specific genomic context and is mediated by multiple HDAC1-containing corepressor complexes

The human telomerase reverse transcriptase (hTERT) gene is repressed in most somatic cells, whereas the expression of the mouse mTert gene is widely detected. To understand the mechanisms of this human-specific repression, we constructed bacterial artificial chromosome (BAC) reporters using human and mouse genomic DNAs encompassing the TERT genes and neighboring loci. Upon chromosomal integration, the hTERT, but not the mTert, reporter was stringently repressed in telomerase-negative human cells in a histone deacetylase (HDAC)-dependent manner, replicating the expression of their respective endogenous genes. In chimeric BACs, the mTert promoter became strongly repressed in the human genomic context, but the hTERT promoter was highly active in the mouse genomic context. Furthermore, an unrelated herpes simplex virus-thymidine kinase (HSV-TK) promoter was strongly repressed in the human, but not in the mouse, genomic context. These results demonstrated that the repression of hTERT gene was dictated by distal elements and its chromatin environment. This repression depended on class I HDACs and involved multiple corepressor complexes, including HDAC1/2-containing Sin3B, nucleosome remodeling and histone deacetylase (NuRD), and corepressor of RE1 silencing transcription factor (CoREST) complexes. Together, our data indicate that the lack of telomerase expression in most human somatic cells results from its repressive genomic environment, providing new insight into the mechanism of long-recognized differential telomerase regulation in mammalian species.-Cheng, D., Zhao, Y., Wang, S., Zhang, F., Russo, M., McMahon, S. B., Zhu, J. Repression of telomerase gene promoter requires human-specific genomic context and is mediated by multiple HDAC1-containing corepressor complexes.

TERT promoter mutations in telomere biology

Telomere repeats at chromosomal ends, critical to genome integrity, are maintained through an elaborate network of proteins and pathways. Shelterin complex proteins shield telomeres from induction of DNA damage response to overcome end protection problem. A specialized ribonucleic protein, telomerase, maintains telomere homeostasis through repeat addition to counter intrinsic shortcomings of DNA replication that leads to gradual sequence shortening in successive mitoses. The biogenesis and recruitment of telomerase composed of telomerase reverse transcriptase (TERT) subunit and an RNA component, takes place through the intricate machinery that involves an elaborate number of molecules. The synthesis of telomeres remains a controlled and limited process. Inherited mutations in the molecules involved in the process directly or indirectly cause telomeropathies. Telomerase, while present in stem cells, is deactivated due to epigenetic silencing of the rate-limiting TERT upon differentiation in most of somatic cells with a few exceptions. However, in most of the cancer cells telomerase reactivation remains a ubiquitous process and constitutes one of the major hallmarks. Discovery of mutations within the core promoter of the TERT gene that create de novo binding sites for E-twenty-six (ETS) transcription factors provided a mechanism for cancer-specific telomerase reactivation. The TERT promoter mutations occur mainly in tumors from tissues with low rates of self-renewal. In melanoma, glioma, hepatocellular carcinoma, urothelial carcinoma and others, the promoter mutations have been shown to define subsets of patients with adverse disease outcomes, associate with increased transcription of TERT, telomerase reactivation and affect telomere length; in stem cells the mutations inhibit TERT silencing following differentiation into adult cells. The TERT promoter mutations cause an epigenetic switch on the mutant allele along with recruitment of pol II following the binding of GABPA/B1 complex that leads to mono-allelic expression. Thus, the TERT promoter mutations hold potential as biomarkers as well as future therapeutic targets.

TGF-beta receptor mediated telomerase inhibition, telomere shortening and breast cancer cell senescence

Human telomerase reverse transcriptase (hTERT) plays a central role in telomere lengthening for continuous cell proliferation, but it remains unclear how extracellular cues regulate telomerase lengthening of telomeres. Here we report that the cytokine bone morphogenetic protein-7 (BMP7) induces the hTERT gene repression in a BMPRII receptor- and Smad3-dependent manner in human breast cancer cells. Chonic exposure of human breast cancer cells to BMP7 results in short telomeres, cell senescence and apoptosis. Mutation of the BMPRII receptor, but not TGFbRII, ACTRIIA or ACTRIIB receptor, inhibits BMP7-induced repression of the hTERT gene promoter activity, leading to increased telomerase activity, lengthened telomeres and continued cell proliferation. Expression of hTERT prevents BMP7-induced breast cancer cell senescence and apoptosis. Thus, our data suggest that BMP7 induces breast cancer cell aging by a mechanism involving BMPRII receptor- and Smad3-mediated repression of the hTERT gene.

Telomerase Regulation from Beginning to the End

The vast body of literature regarding human telomere maintenance is a true testament to the importance of understanding telomere regulation in both normal and diseased states. In this review, our goal was simple: tell the telomerase story from the biogenesis of its parts to its maturity as a complex and function at its site of action, emphasizing new developments and how they contribute to the foundational knowledge of telomerase and telomere biology.

Transcription Regulation of the Human Telomerase Reverse Transcriptase (hTERT) Gene

Embryonic stem cells and induced pluripotent stem cells have the ability to maintain their telomere length via expression of an enzymatic complex called telomerase. Similarly, more than 85%–90% of cancer cells are found to upregulate the expression of telomerase, conferring them with the potential to proliferate indefinitely. Telomerase Reverse Transcriptase (TERT), the catalytic subunit of telomerase holoenzyme, is the rate-limiting factor in reconstituting telomerase activity in vivo. To date, the expression and function of the human Telomerase Reverse Transcriptase (hTERT) gene are known to be regulated at various molecular levels (including genetic, mRNA, protein and subcellular localization) by a number of diverse factors. Among these means of regulation, transcription modulation is the most important, as evident in its tight regulation in cancer cell survival as well as pluripotent stem cell maintenance and differentiation. Here, we discuss how hTERT gene transcription is regulated, mainly focusing on the contribution of trans-acting factors such as transcription factors and epigenetic modifiers, as well as genetic alterations in hTERT proximal promoter.

Human Specific Regulation of the Telomerase Reverse Transcriptase Gene

Telomerase, regulated primarily by the transcription of its catalytic subunit telomerase reverse transcriptase (TERT), is critical for controlling cell proliferation and tissue homeostasis by maintaining telomere length. Although there is a high conservation between human and mouse TERT genes, the regulation of their transcription is significantly different in these two species. Whereas mTERT expression is widely detected in adult mice, hTERT is expressed at extremely low levels in most adult human tissues and cells. As a result, mice do not exhibit telomere-mediated replicative aging, but telomere shortening is a critical factor of human aging and its stabilization is essential for cancer development in humans. The chromatin environment and epigenetic modifications of the hTERT locus, the binding of transcriptional factors to its promoter, and recruitment of nucleosome modifying complexes all play essential roles in restricting its transcription in different cell types. In this review, we will discuss recent progress in understanding the molecular mechanisms of TERT regulation in human and mouse tissues and cells, and during cancer development.

Vascular Actions of Angiotensin 1-7 in the Human Microcirculation: Novel Role for Telomerase

OBJECTIVE

This study examined vascular actions of angiotensin 1-7 (ANG 1-7) in human atrial and adipose arterioles.

APPROACH AND RESULTS

The endothelium-derived hyperpolarizing factor of flow-mediated dilation (FMD) switches from antiproliferative nitric oxide (NO) to proatherosclerotic hydrogen peroxide in arterioles from humans with coronary artery disease (CAD). Given the known vasoprotective properties of ANG 1-7, we tested the hypothesis that overnight ANG 1-7 treatment restores the NO component of FMD in arterioles from patients with CAD. Endothelial telomerase activity is essential for preserving the NO component of vasodilation in the human microcirculation; thus, we also tested whether telomerase activity was necessary for ANG 1-7-mediated vasoprotection by treating separate arterioles with ANG 1-7±the telomerase inhibitor 2-[[(2E)-3-(2-naphthalenyl)-1-oxo-2-butenyl1-yl]amino]benzoic acid. ANG 1-7 dilated arterioles from patients without CAD, whereas dilation was significantly reduced in arterioles from patients with CAD. In atrial arterioles from patients with CAD incubated with ANG 1-7 overnight, the NO synthase inhibitor NG-nitro-l-arginine methyl ester abolished FMD, whereas the hydrogen peroxide scavenger polyethylene glycol catalase had no effect. Conversely, in vessels incubated with ANG 1-7+2-[[(2E)-3-(2-naphthalenyl)-1-oxo-2-butenyl1-yl]amino]benzoic acid, NG-nitro-l-arginine methyl ester had no effect on FMD, but polyethylene glycol catalase abolished dilation. In cultured human coronary artery endothelial cells, ANG 1-7 significantly increased telomerase activity. These results indicate that ANG 1-7 dilates human microvessels, and dilation is abrogated in the presence of CAD. Furthermore, ANG 1-7 treatment is sufficient to restore the NO component of FMD in arterioles from patients with CAD in a telomerase-dependent manner.

CONCLUSIONS

ANG 1-7 exerts vasoprotection in the human microvasculature via modulation of telomerase activity.