Reversible cellular senescence: implications for immortalization of normal human diploid fibroblasts

Differential DNA Methylation of THOR and hTAPAS in the Regulation of hTERT and the Diagnosis of Cancer

Simple Summary

Because of its high prevalence of >45% in 9 out of 11 (82%) cancer types screened, THOR hypermethylation has been suggested to be a frequent telomerase-activating mechanism in hTERT-expressing tumor types, e.g., in cancers of the prostate, breast, blood, colon, lung, bladder, and brain. In this prime example, we present detailed DNA methylation profiles in urothelial cancer that reveal the exact positions of the most differentially methylated CpG dinucleotides within the THOR region in order to design an efficient Methylation-Specific PCR (MSPCR) approach for diagnostic and prognostic purposes. Furthermore, our data suggest an epigenetic mechanism regulating hTERT expression through the methylation status of THOR and lncRNA hTAPAS.

Abstract

Background: Although DNA methylation in the gene promoters usually represses gene expression, the TERT hypermethylated oncological region (THOR) located 5′ of the hTERT gene is hypermethylated when hTERT is expressed in diverse cancer types, including urothelial cancer (UC). Methods: Comprehensive MeDIP and DNA methylation array analyses complemented by the technically independent method of bisulfite genomic sequencing were applied on pathologically reviewed and classified urothelial carcinoma specimens and healthy urothelial tissue samples to reveal the methylation status of THOR in detail. Results: The detailed DNA methylation profiles reveal the exact positions of differentially methylated CpG dinucleotides within THOR in urothelial cancer and provide evidence ofa diverging role of methylation of these CpGs in the regulation of hTERT. In particular, our data suggest a regulating mechanism in which THOR methylation acts on hTERT expression through epigenetic silencing of the lncRNA hTERT antisense promoter-associated (hTAPAS), which represses hTERT. Conclusions: These findings precisely define the most differentially methylated CpGs of THOR in early urothelial cancer, enabling optimal design of Methylation-Specific PCR (MSPCR) primers to reliably probe these methylation differences for diagnostic and prognostic purposes. In addition, this strategy presents a prime example that is also applicable to many other malignancies. Finally, the first evidence for the underlying epigenetic mechanism regulating hTERT expression through the methylation status of THOR is provided.

Overcoming tumor resistance mechanisms in CAR-NK cell therapy

Despite the impressive results of autologous CAR-T cell therapy in refractory B lymphoproliferative diseases, CAR-NK immunotherapy emerges as a safer, faster, and cost-effective approach with no signs of severe toxicities as described for CAR-T cells. Permanently scrutinized for its efficacy, recent promising data in CAR-NK clinical trials point out the achievement of deep, high-quality responses, thus confirming its potential clinical use. Although CAR-NK cell therapy is not significantly affected by the loss or downregulation of its CAR tumor target, as in the case of CAR-T cell, a plethora of common additional tumor intrinsic or extrinsic mechanisms that could also disable NK cell function have been described. Therefore, considering lessons learned from CAR-T cell therapy, the emergence of CAR-NK cell therapy resistance can also be envisioned. In this review we highlight the processes that could be involved in its development, focusing on cytokine addiction and potential fratricide during manufacturing, poor tumor trafficking, exhaustion within the tumor microenvironment (TME), and NK cell short in vivo persistence on account of the limited expansion, replicative senescence, and rejection by patient’s immune system after lymphodepletion recovery. Finally, we outline new actively explored alternatives to overcome these resistance mechanisms, with a special emphasis on CRISPR/Cas9 mediated genetic engineering approaches, a promising platform to optimize CAR-NK cell function to eradicate refractory cancers.

Multiwalled Carbon Nanotubes Induce Fibrosis and Telomere Length Alterations

Telomere shortening can result in cellular senescence and in increased level of genome instability, which are key events in numerous of cancer types. Despite this, few studies have focused on the effect of nanomaterial exposure on telomere length as a possible mechanism involved in nanomaterial-induced carcinogenesis. In this study, effects of exposure to multiwalled carbon nanotubes (MWCNT) on telomere length were investigated in mice exposed by intrapleural injection, as well as in human lung epithelial and mesothelial cell lines. In addition, cell cycle, apoptosis, and regulation of genes involved in DNA damage repair were assessed. Exposure to MWCNT led to severe fibrosis, infiltration of inflammatory cells in pleura, and mesothelial cell hyperplasia. These histological alterations were accompanied by deregulation of genes involved in fibrosis and immune cell recruitment, as well as a significant shortening of telomeres in the pleura and the lung. Assessment of key carcinogenic mechanisms in vitro confirmed that long-term exposure to the long MWCNT led to a prominent telomere shortening in epithelial cells, which coincided with G1-phase arrest and enhanced apoptosis. Altogether, our data show that telomere shortening resulting in cell cycle arrest and apoptosis may be an important mechanism in long MWCNT-induced inflammation and fibrosis.

Targeting BRF2 in Cancer Using Repurposed Drugs

Simple Summary

BRF2, a subunit of the RNA polymerase III transcription complex, is upregulated in a wide variety of cancers and is a potential therapeutic target; however, no effective drugs are available to target BRF2. The upregulation of BRF2 in cancer cells confers survival via the prevention of oxidative stress-induced apoptosis. In this manuscript, we report the identification of potential BRF2 inhibitors through in silico drug repurposing screening. We further characterized bexarotene as a hit compound for the development of selective BRF2 inhibitors and provide experimental validation to support the repurposing of this FDA-approved drug as an agent to reduce the cellular levels of ROS and consequent BRF2 expression in cancers with elevated levels of oxidative stress.

Abstract

The overexpression of BRF2, a selective subunit of RNA polymerase III, has been shown to be crucial in the development of several types of cancers, including breast cancer and lung squamous cell carcinoma. Predominantly, BRF2 acts as a central redox-sensing transcription factor (TF) and is involved in rescuing oxidative stress (OS)-induced apoptosis. Here, we showed a novel link between BRF2 and the DNA damage response. Due to the lack of BRF2-specific inhibitors, through virtual screening and molecular dynamics simulation, we identified potential drug candidates that interfere with BRF2-TATA-binding Protein (TBP)-DNA complex interactions based on binding energy, intermolecular, and torsional energy parameters. We experimentally tested bexarotene as a potential BRF2 inhibitor. We found that bexarotene (Bex) treatment resulted in a dramatic decline in oxidative stress and Tert-butylhydroquinone (tBHQ)-induced levels of BRF2 and consequently led to a decrease in the cellular proliferation of cancer cells which may in part be due to the drug pretreatment-induced reduction of ROS generated by the oxidizing agent. Our data thus provide the first experimental evidence that BRF2 is a novel player in the DNA damage response pathway and that bexarotene can be used as a potential inhibitor to treat cancers with the specific elevation of oxidative stress.

Abnormal function of telomere protein TRF2 induces cell mutation and the effects of environmental tumor‑promoting factors (Review)

Recent studies have found that somatic gene mutations and environmental tumor-promoting factors are both indispensable for tumor formation. Telomeric repeat-binding factor (TRF)2 is the core component of the telomere shelterin complex, which plays an important role in chromosome stability and the maintenance of normal cell physiological states. In recent years, TRF2 and its role in tumor formation have gradually become a research hot topic, which has promoted in-depth discussions into tumorigenesis and treatment strategies, and has achieved promising results. Some cells bypass elimination, due to either aging, apoptosis via mutations or abnormal prolongation of the mitotic cycle, and enter the telomere crisis period, where large-scale DNA reorganization occurs repeatedly, which manifests as the precancerous cell cycle. Finally, at the end of the crisis cycle, the mutation activates either the expression level of telomerase or activates the alternative lengthening of telomere mechanism to extend the local telomeres. Under the protection of TRF2, chromosomes are gradually stabilized, immortal cells are formed and the stagewise mutation-driven transformation of normal cells to cancer cells is completed. In addition, TRF2 also shares the characteristics of environmental tumor-promoting factors. It acts on multiple signal transduction pathway-related proteins associated with cell proliferation, and affects peripheral angiogenesis, inhibits the immune recognition and killing ability of the microenvironment, and maintains the stemness characteristics of tumor cells. TRF2 levels are abnormally elevated by a variety of tumor control proteins, which are more conducive to the protection of telomeres and the survival of tumor cells. In brief, the various regulatory mechanisms which tumor cells rely on to survive are organically integrated around TRF2, forming a regulatory network, which is conducive to the optimization of the survival direction of heterogeneous tumor cells, and promotes their survival and adaptability. In terms of clinical application, TRF2 is expected to become a new type of cancer prognostic marker and a new tumor treatment target. Inhibition of TRF2 overexpression could effectively cut off the core network regulating tumor cell survival, reduce drug resistance, or bypass the mutation under the pressure of tumor treatment selection, which may represent a promising therapeutic strategy for the complete eradication of tumors in the clinical setting. Based on recent research, the aim of the present review was to systematically elaborate on the basic structure and functional characteristics of TRF2 and its role in tumor formation, and to analyze the findings indicating that TRF2 deficiency or overexpression could cause severe damage to telomere function and telomere shortening, and induce DNA damage response and chromosomal instability.

Imaging assay to probe the role of telomere length shortening on telomere-gene interactions in single cells

Telomeres are repetitive non-coding nucleotide sequences (TTAGGGn) capping the ends of chromosomes. Progressive telomere shortening with increasing age has been associated with shifts in gene expression through models such as the telomere position effect (TPE), which suggests reduced interference of the telomere with transcriptional activity of increasingly more distant genes. A modification of the TPE model, referred to as Telomere Position Effects over Long Distance (TPE-OLD), explains why some genes 1-10 MB from a telomere are still affected by TPE, but genes closer to the telomere are not. Here, we describe an imaging approach to systematically examine the occurrence of TPE-OLD at the single cell level. Compared to existing methods, the pipeline allows rapid analysis of hundreds to thousands of cells, which is necessary to establish TPE-OLD as an acceptable mechanism of gene expression regulation. We examined two human genes, ISG15 and TERT, for which TPE-OLD has been described before. For both genes, we found less interaction with the telomere on the same chromosome in old cells compared to young cells; and experimentally elongated telomeres in old cells rescued the level of telomere interaction for both genes. However, the dependency of the interactions on the age progression from young to old cells varied. One model for the differences between ISG15 and TERT may relate to the markedly distinct interstitial telomeric sequence arrangement in the two genes. Overall, this provides a strong rationale for the role of telomere length shortening in the regulation of gene expression.

Senescent T cells within suppressive tumor microenvironments: emerging target for tumor immunotherapy

The functional state of the preexisting T cells in the tumor microenvironment is a key determinant for effective antitumor immunity and immunotherapy. Increasing evidence suggests that immunosenescence is an important state of T cell dysfunction that is distinct from exhaustion, a key strategy used by malignant tumors to evade immune surveillance and sustain the suppressive tumor microenvironment. Here, we discuss the phenotypic and functional characteristics of senescent T cells and their role in human cancers. We also explore the possible mechanisms and signaling pathways responsible for induction of T cell senescence by malignant tumors, and then discuss potential strategies to prevent and/or reverse senescence in tumor-specific T cells. A better understanding of these critical issues should provide novel strategies to enhance cancer immunotherapy.

The Role of Alternative RNA Splicing in the Regulation of hTERT, Telomerase, and Telomeres: Implications for Cancer Therapeutics

Alternative RNA splicing impacts the majority (>90%) of eukaryotic multi-exon genes, expanding the coding capacity and regulating the abundance of gene isoforms. Telomerase (hTERT) is a key example of a gene that is alternatively spliced during human fetal development and becomes dysregulated in nearly all cancers. Approximately 90% of human tumors use telomerase to synthesize de novo telomere repeats and obtain telomere-dependent cellular immortality. Paradigm shifting data indicates that hTERT alternative splicing, in addition to transcription, plays an important role in the regulation of active telomerase in cells. Our group and others are pursuing the basic science studies to progress this emerging area of telomerase biology. Recent evidence demonstrates that switching splicing of hTERT from the telomerase activity producing full-length hTERT isoform to alternatively spliced, non-coding isoforms may be a novel telomerase inhibition strategy to prevent cancer growth and survival. Thus, the goals of this review are to detail the general roles of telomerase in cancer development, explore the emerging regulatory mechanisms of alternative RNA splicing of the hTERT gene in various somatic and cancer cell types, define the known and potential roles of hTERT splice isoforms in cancer cell biology, and provide insight into new treatment strategies targeting hTERT in telomerase-positive cancers.

Structural biology of telomeres and telomerase

Telomeres are protein-DNA complexes that protect chromosome ends from illicit ligation and resection. Telomerase is a ribonucleoprotein enzyme that synthesizes telomeric DNA to counter telomere shortening. Human telomeres are composed of complexes between telomeric DNA and a six-protein complex known as shelterin. The shelterin proteins TRF1 and TRF2 provide the binding affinity and specificity for double-stranded telomeric DNA, while the POT1-TPP1 shelterin subcomplex coats the single-stranded telomeric G-rich overhang that is characteristic of all our chromosome ends. By capping chromosome ends, shelterin protects telomeric DNA from unwanted degradation and end-to-end fusion events. Structures of the human shelterin proteins reveal a network of constitutive and context-specific interactions. The shelterin protein-DNA structures reveal the basis for both the high affinity and DNA sequence specificity of these interactions, and explain how shelterin efficiently protects chromosome ends from genome instability. Several protein-protein interactions, many provided by the shelterin component TIN2, are critical for upholding the end-protection function of shelterin. A survey of these protein-protein interfaces within shelterin reveals a series of "domain-peptide" interactions that allow for efficient binding and adaptability towards new functions. While the modular nature of shelterin has facilitated its part-by-part structural characterization, the interdependence of subunits within telomerase has made its structural solution more challenging. However, the exploitation of several homologs in combination with recent advancements in cryo-EM capabilities has led to an exponential increase in our knowledge of the structural biology underlying telomerase function. Telomerase homologs from a wide range of eukaryotes show a typical retroviral reverse transcriptase-like protein core reinforced with elements that deliver telomerase-specific functions including recruitment to telomeres and high telomere-repeat addition processivity. In addition to providing the template for reverse transcription, the RNA component of telomerase provides a scaffold for the catalytic and accessory protein subunits, defines the limits of the telomeric repeat sequence, and plays a critical role in RNP assembly, stability, and trafficking. While a high-resolution definition of the human telomerase structure is only beginning to emerge, the quick pace of technical progress forecasts imminent breakthroughs in this area. Here, we review the structural biology surrounding telomeres and telomerase to provide a molecular description of mammalian chromosome end protection and end replication.

Alternative Lengthening of Telomeres and Chromatin Status

Telomere length is maintained by either telomerase, a reverse transcriptase, or alternative lengthening of telomeres (ALT), a mechanism that utilizes homologous recombination (HR) proteins. Since access to DNA for HR enzymes is regulated by the chromatin status, it is expected that telomere elongation is linked to epigenetic modifications. The aim of this review is to elucidate the epigenetic features of ALT-positive cells. In order to do this, it is first necessary to understand the telomeric chromatin peculiarities. So far, the epigenetic nature of telomeres is still controversial: some authors describe them as heterochromatic, while for others, they are euchromatic. Similarly, ALT activity should be characterized by the loss (according to most researchers) or formation (as claimed by a minority) of heterochromatin in telomeres. Besides reviewing the main works in this field and the most recent findings, some hypotheses involving the role of telomere non-canonical sequences and the possible spatial heterogeneity of telomeres are given.

Characterization and Establishment of an Immortalized Rabbit Melanocyte Cell Line Using the SV40 Large T Antigen

Melanocytes (MCs) are specialized cells that synthesize melanin within the melanosome. Cultured MCs are useful in order to study their role in relation to pigmentation. However, MC isolation is laborious and the obtained cells have a limited culture time. In this study, we transformed lentivirus-mediated simian virus 40 Large T (SV40-LT) into primary rabbit melanocytes (Pri RMCs) to establish an immortalized cell line. Morphologically, the immortalized RMCs (Im RMC) were indistinguishable from the Pri RMCs, and dendrites were visible following Dopa staining. No significant differences in cell proliferation or growth between immortalized and primary RMCs were observed. Based on melanocyte-specific markers, the expression of MITF, TYR, and TYRP1 were detected by PCR, immunofluorescence staining, and western blot analysis. Through karyotype, soft agar, and tumorigenesis assays, the immortalized RMCs did not undergo malignant transformation. Our results show that Im RMCs can be used as a tool cell for future MC studies on the pigmentation mechanisms of fur animals.

Inhibition of FAK Signaling Elicits Lamin A/C-Associated Nuclear Deformity and Cellular Senescence

Focal adhesion kinase (FAK) is a non-receptor kinase that facilitates tumor aggressiveness. The effects of FAK inhibition include arresting proliferation, limiting metastasis, and inhibiting angiogenesis. PF-573228 is an ATP-competitive inhibitor of FAK. Treating lung cancer cells with PF-573228 resulted in FAK inactivation and changes in the expressions of lamin A/C and nuclear deformity. Since lamin A/C downregulation or deficiency was associated with cellular senescence, the senescence-associated β-galactosidase (SA-β-gal) assay was used to investigate whether PF-573228 treatment drove cellular senescence, which showed more SA-β-gal-positive cells in culture. p53 is known to play a pivotal role in mediating the progression of cellular senescence, and the PF-573228-treated lung cancer cells resulted in a higher p53 expression level. Subsequently, the FAK depletion in lung cancer cells was employed to confirm the role of FAK inhibition on cellular senescence. FAK depletion and pharmacological inhibition of lung cancer cells elicited similar patterns of cellular senescence, lamin A/C downregulation, and p53 upregulation, implying that FAK signaling is associated with the expression of p53 and the maintenance of lamin A/C levels to shape regular nuclear morphology and manage anti-senescence. Conversely, FAK inactivation led to p53 upregulation, disorganization of the nuclear matrix, and consequently cellular senescence. Our data suggest a new FAK signaling pathway, in that abolishing FAK signaling can activate the senescence program in cells. Triggering cellular senescence could be a new therapeutic approach to limit tumor growth.

Telomere Biology and Human Phenotype

Telomeres are nucleoprotein structures that cap the end of each chromosome arm and function to maintain genome stability. The length of telomeres is known to shorten with each cell division and it is well-established that telomere attrition is related to replicative capacity in vitro. Moreover, telomere loss is also correlated with the process of aging in vivo. In this review, we discuss the mechanisms that lead to telomere shortening and summarise telomere homeostasis in humans throughout a lifetime. In addition, we discuss the available evidence that shows that telomere shortening is related to human aging and the onset of age-related disease.

Lifetime extension of humpback whale skin fibroblasts and their response to lipopolysaccharide (LPS) and a mixture of polychlorinated biphenyls (Aroclor)

Marine mammals, such as whales, have a high proportion of body fat and so are susceptible to the accumulation, and associated detrimental health effects, of lipophilic environmental contaminants. Recently, we developed a wild-type cell line from humpback whale fibroblasts (HuWa). Extensive molecular assessments with mammalian wild-type cells are typically constrained by a finite life span, with cells eventually becoming senescent. Thus, the present work explored the possibility of preventing senescence in the HuWa cell line by transfection with plasmids encoding the simian virus large T antigen (SV40T) or telomerase reverse transcriptase (TERT). No stable expression was achieved upon SV40 transfection. Transfection with TERT, on the other hand, activated the expression of telomerase in HuWa cells. At the time of manuscript preparation, the transfected HuWa cells (HuWa_TERT) have been stable for at least 59 passages post-transfection. HuWa_TERT proliferate rapidly and maintain initial cell characteristics, such as morphology and chromosomal stability. The response of HuWa_TERT cells to an immune stimulant (lipopolysaccharide (LPS)) and an immunotoxicant (Aroclor1254) was assessed by measurement of intracellular levels of the pro-inflammatory cytokines interleukin (IL)-6, IL-1β and tumour necrosis factor (TNF)-α. HuWa_TERT cells constitutively express IL-6, IL-1β and TNFα. Exposure to neither LPS nor Aroclor1254 had an effect on the levels of these cytokines. Overall, this work supports the diverse applicability of HuWa cell lines in that they display reliable long-term preservation, susceptibility to exogenous gene transfer and enable the study of humpback whale-specific cellular response mechanisms.

Reconstructing the molecular life history of gliomas

At the time of their clinical manifestation, the heterogeneous group of adult and pediatric gliomas carries a wide range of diverse somatic genomic alterations, ranging from somatic single-nucleotide variants to structural chromosomal rearrangements. Somatic abnormalities may have functional consequences, such as a decrease, increase or change in mRNA transcripts, and cells pay a penalty for maintaining them. These abnormalities, therefore, must provide cells with a competitive advantage to become engrained into the glioma genome. Here, we propose a model of gliomagenesis consisting of the following five consecutive phases that glioma cells have traversed prior to clinical manifestation: (I) initial growth; (II) oncogene-induced senescence; (III) stressed growth; (IV) replicative senescence/crisis; (V) immortal growth. We have integrated the findings from a large number of studies in biology and (neuro)oncology and relate somatic alterations and other results discussed in these papers to each of these five phases. Understanding the story that each glioma tells at presentation may ultimately facilitate the design of novel, more effective therapeutic approaches.

Mechanisms of human telomerase reverse transcriptase (hTERT) regulation: clinical impacts in cancer

Background

Limitless self-renewal is one of the hallmarks of cancer and is attained by telomere maintenance, essentially through telomerase (hTERT) activation. Transcriptional regulation of hTERT is believed to play a major role in telomerase activation in human cancers.

Main body

The dominant interest in telomerase results from its role in cancer. The role of telomeres and telomere maintenance mechanisms is well established as a major driving force in generating chromosomal and genomic instability. Cancer cells have acquired the ability to overcome their fate of senescence via telomere length maintenance mechanisms, mainly by telomerase activation.

hTERT expression is up-regulated in tumors via multiple genetic and epigenetic mechanisms including hTERT amplifications, hTERT structural variants, hTERT promoter mutations and epigenetic modifications through hTERT promoter methylation. Genetic (hTERT promoter mutations) and epigenetic (hTERT promoter methylation and miRNAs) events were shown to have clinical implications in cancers that depend on hTERT activation. Knowing that telomeres are crucial for cellular self-renewal, the mechanisms responsible for telomere maintenance have a crucial role in cancer diseases and might be important oncological biomarkers. Thus, rather than quantifying TERT expression and its correlation with telomerase activation, the discovery and the assessment of the mechanisms responsible for TERT upregulation offers important information that may be used for diagnosis, prognosis, and treatment monitoring in oncology. Furthermore, a better understanding of these mechanisms may promote their translation into effective targeted cancer therapies.

Conclusion

Herein, we reviewed the underlying mechanisms of hTERT regulation, their role in oncogenesis, and the potential clinical applications in telomerase-dependent cancers.

Techniques to Induce and Quantify Cellular Senescence

In response to cellular stress or damage, proliferating cells can induce a specific program that initiates a state of long-term cell-cycle arrest, termed cellular senescence. Accumulation of senescent cells occurs with organismal aging and through continual culturing in vitro. Senescent cells influence many biological processes, including embryonic development, tissue repair and regeneration, tumor suppression, and aging. Hallmarks of senescent cells include, but are not limited to, increased senescence-associated β-galactosidase activity (SA-β-gal); p16^INK4A, p53, and p21 levels; higher levels of DNA damage, including γ-H2AX; the formation of Senescence-associated Heterochromatin Foci (SAHF); and the acquisition of a Senescence-associated Secretory Phenotype (SASP), a phenomenon characterized by the secretion of a number of pro-inflammatory cytokines and signaling molecules. Here, we describe protocols for both replicative and DNA damage-induced senescence in cultured cells. In addition, we highlight techniques to monitor the senescent phenotype using several senescence-associated markers, including SA-β-gal, γ-H2AX and SAHF staining, and to quantify protein and mRNA levels of cell cycle regulators and SASP factors. These methods can be applied to the assessment of senescence in various models and tissues.

Roles of telomeres and telomerase in cancer, and advances in telomerase-targeted therapies

Telomeres maintain genomic integrity in normal cells, and their progressive shortening during successive cell divisions induces chromosomal instability. In the large majority of cancer cells, telomere length is maintained by telomerase. Thus, telomere length and telomerase activity are crucial for cancer initiation and the survival of tumors. Several pathways that regulate telomere length have been identified, and genome-scale studies have helped in mapping genes that are involved in telomere length control. Additionally, genomic screening for recurrent human telomerase gene hTERT promoter mutations and mutations in genes involved in the alternative lengthening of telomeres pathway, such as ATRX and DAXX, has elucidated how these genomic changes contribute to the activation of telomere maintenance mechanisms in cancer cells. Attempts have also been made to develop telomere length- and telomerase-based diagnostic tools and anticancer therapeutics. Recent efforts have revealed key aspects of telomerase assembly, intracellular trafficking and recruitment to telomeres for completing DNA synthesis, which may provide novel targets for the development of anticancer agents. Here, we summarize telomere organization and function and its role in oncogenesis. We also highlight genomic mutations that lead to reactivation of telomerase, and mechanisms of telomerase reconstitution and trafficking that shed light on its function in cancer initiation and tumor development. Additionally, recent advances in the clinical development of telomerase inhibitors, as well as potential novel targets, will be summarized.

Metformin-mediated increase in DICER1 regulates microRNA expression and cellular senescence

Metformin, an oral hypoglycemic agent, has been used for decades to treat type 2 diabetes mellitus. Recent studies indicate that mice treated with metformin live longer and have fewer manifestations of age‐related chronic disease. However, the molecular mechanisms underlying this phenotype are unknown. Here, we show that metformin treatment increases the levels of the microRNA‐processing protein DICER1 in mice and in humans with diabetes mellitus. Our results indicate that metformin upregulates DICER1 through a post‐transcriptional mechanism involving the RNA‐binding protein AUF1. Treatment with metformin altered the subcellular localization of AUF1, disrupting its interaction with DICER1 mRNA and rendering DICER1 mRNA stable, allowing DICER1 to accumulate. Consistent with the role of DICER1 in the biogenesis of microRNAs, we found differential patterns of microRNA expression in mice treated with metformin or caloric restriction, two proven life‐extending interventions. Interestingly, several microRNAs previously associated with senescence and aging, including miR‐20a, miR‐34a, miR‐130a, miR‐106b, miR‐125, and let‐7c, were found elevated. In agreement with these findings, treatment with metformin decreased cellular senescence in several senescence models in a DICER1‐dependent manner. Metformin lowered p16 and p21 protein levels and the abundance of inflammatory cytokines and oncogenes that are hallmarks of the senescence‐associated secretory phenotype (SASP). These data lead us to hypothesize that changes in DICER1 levels may be important for organismal aging and to propose that interventions that upregulate DICER1 expression (e.g., metformin) may offer new pharmacotherapeutic approaches for age‐related disease.

Role of Telomeres and Telomerase in Aging and Cancer

Telomeres progressively shorten throughout life. A hallmark of advanced malignancies is the ability for continuous cell divisions that almost universally correlates with the stabilization of telomere length by the reactivation of telomerase. The repression of telomerase and shorter telomeres in humans may have evolved, in part, as an anticancer protection mechanism. Although there is still much we do not understand about the regulation of telomerase, it remains a very attractive and novel target for cancer therapeutics. This review focuses on the current state of advances in the telomerase area, identifies outstanding questions, and addresses areas and methods that need refinement.

SIGNIFICANCE

Despite many recent advances, telomerase remains a challenging target for cancer therapy. There are few telomerase-directed therapies, and many of the assays used to measure telomeres and telomerase have serious limitations. This review provides an overview of the current state of the field and how recent advances could affect future research and treatment approaches. Cancer Discov; 6(6); 584-93. ©2016 AACR.

Regulation of the Telomerase Reverse Transcriptase Subunit through Epigenetic Mechanisms

Chromosome-shortening is characteristic of normal cells, and is known as the end replication problem. Telomerase is the enzyme responsible for extending the ends of the chromosomes in de novo synthesis, and occurs in germ cells as well as most malignant cancers. There are three subunits of telomerase: human telomerase RNA (hTERC), human telomerase associated protein (hTEP1), or dyskerin, and human telomerase reverse transcriptase (hTERT). hTERC and hTEP1 are constitutively expressed, so the enzymatic activity of telomerase is dependent on the transcription of hTERT. DNA methylation, histone methylation, and histone acetylation are basic epigenetic regulations involved in the expression of hTERT. Non-coding RNA can also serve as a form of epigenetic control of hTERT. This epigenetic-based regulation of hTERT is important in providing a mechanism for reversibility of hTERT control in various biological states. These include embryonic down-regulation of hTERT contributing to aging and the upregulation of hTERT playing a critical role in over 90% of cancers. Normal human somatic cells have a non-methylated/hypomethylated CpG island within the hTERT promoter region, while telomerase-positive cells paradoxically have at least a partially methylated promoter region that is opposite to the normal roles of DNA methylation. Histone acetylation of H3K9 within the promoter region is associated with an open chromatin state such that transcription machinery has the space to form. Histone methylation of hTERT has varied control of the gene, however. Mono- and dimethylation of H3K9 within the promoter region indicate silent euchromatin, while a trimethylated H3K9 enhances gene transcription. Non-coding RNAs can target epigenetic-modifying enzymes, as well as transcription factors involved in the control of hTERT. An epigenetics diet that can affect the epigenome of cancer cells is a recent fascination that has received much attention. By combining portions of this diet with epigenome-altering treatments, it is possible to selectively regulate the epigenetic control of hTERT and its expression.

Understanding TERT Promoter Mutations: A Common Path to Immortality

Telomerase (TERT) activation is a fundamental step in tumorigenesis. By maintaining telomere length, telomerase relieves a main barrier on cellular lifespan, enabling limitless proliferation driven by oncogenes. The recently discovered, highly recurrent mutations in the promoter of TERT are found in over 50 cancer types, and are the most common mutation in many cancers. Transcriptional activation of TERT, via promoter mutation or other mechanisms, is the rate-limiting step in production of active telomerase. Although TERT is expressed in stem cells, it is naturally silenced upon differentiation. Thus, the presence of TERT promoter mutations may shed light on whether a particular tumor arose from a stem cell or more differentiated cell type. It is becoming clear that TERT mutations occur early during cellular transformation, and activate the TERT promoter by recruiting transcription factors that do not normally regulate TERT gene expression. This review highlights the fundamental and widespread role of TERT promoter mutations in tumorigenesis, including recent progress on their mechanism of transcriptional activation. These somatic promoter mutations, along with germline variation in the TERT locus also appear to have significant value as biomarkers of patient outcome. Understanding the precise molecular mechanism of TERT activation by promoter mutation and germline variation may inspire novel cancer cell-specific targeted therapies for a large number of cancer patients.

Cell death during crisis is mediated by mitotic telomere deprotection

Tumour formation is blocked by two barriers, replicative senescence and crisis¹. Senescence is triggered by short telomeres and is bypassed by disruption of tumour suppressive pathways. After senescence bypass, cells undergo crisis, during which almost all of the cells in the population die. Cells that escape crisis harbor unstable genomes and other parameters of transformation. The mechanism of cell death during crisis remained elusive. We show that cells in crisis undergo spontaneous mitotic arrest, resulting in death during mitosis or in the following cell cycle. The phenotype was induced by loss of p53 function, and suppressed by telomerase overexpression. Telomere fusions triggered mitotic arrest in p53-compromised non-crisis cells, indicating such fusions as the underlying cause. Exacerbation of mitotic telomere deprotection by partial TRF2 knockdown² increased the ratio of cells that died during mitotic arrest and sensitized cancer cells to mitotic poisons. We propose a crisis pathway wherein chromosome fusions induce mitotic arrest, resulting in mitotic telomere deprotection and cell death, thereby eliminating precancerous cells from the population.

Establishment of Hertwig's epithelial root sheath/epithelial rests of Malassez cell line from human periodontium

Human Hertwig's epithelial root sheath/epithelial rests of Malassez (HERS/ERM) cells are epithelial remnants of teeth residing in the periodontium. Although the functional roles of HERS/ERM cells have yet to be elucidated, they are a unique epithelial cell population in adult teeth and are reported to have stem cell characteristics. Therefore, HERS/ERM cells might play a role as an epithelial component for the repair or regeneration of dental hard tissues; however, they are very rare population in periodontium and the primary isolation of them is considered to be difficult. To overcome these problems, we immortalized primary HERS/ERM cells isolated from human periodontium using SV40 large T antigen (SV40 LT) and performed a characterization of the immortalized cell line. Primary HERS/ERM cells could not be maintained for more than 6 passages; however, immortalized HERS/ERM cells were maintained for more than 20 passages. There were no differences in the morphological and immunophenotypic characteristics of HERS/ERM cells and immortalized HERS/ERM cells. The expression of epithelial stem cell and embryonic stem cell markers was maintained in immortalized HERS/ERM cells. Moreover, immortalized HERS/ERM cells could acquire mesenchymal phenotypes through the epithelial-mesenchymal transition via TGF-β1. In conclusion, we established an immortalized human HERS/ERM cell line with SV40 LT and expect this cell line to contribute to the understanding of the functional roles of HERS/ERM cells and the tissue engineering of teeth.

let-7-repressesed Shc translation delays replicative senescence

The p66Shc adaptor protein is an important regulator of lifespan in mammals, but the mechanisms responsible are still unclear. Here, we show that expression of p66Shc, p52Shc, and p46Shc is regulated at the post-transcriptional level by the microRNA let-7a. The levels of let-7a correlated inversely with the levels of Shc proteins without affecting Shc mRNA levels. We identified 'seedless' let-7a interaction elements in the coding region of Shc mRNA; mutation of the 'seedless' interaction sites abolished the regulation of Shc by let-7a. Our results further revealed that repression of Shc expression by let-7a delays senescence of human diploid fibroblasts (HDFs). In sum, our findings link let-7a abundance to the expression of p66Shc, which in turn controls the replicative lifespan of HDFs.

The multifaceted effects of omega-3 polyunsaturated Fatty acids on the hallmarks of cancer

Omega-3 polyunsaturated fatty acids, in particular eicosapentaenoic acid, and docosahexaenoic acid have been shown to have multiple beneficial antitumour actions that affect the essential alterations that dictate malignant growth. In this review we explore the putative mechanisms of action of omega-3 polyunsaturated fatty acid in cancer protection in relation to self-sufficiency in growth signals, insensitivity to growth-inhibitory signals, apoptosis, limitless replicative potential, sustained angiogenesis, and tissue invasion, and how these will hopefully translate from bench to bedside.

NF90 coordinately represses the senescence-associated secretory phenotype

A hallmark trait of cellular senescence is the acquisition of a senescence-associated secretory phenotype (SASP). SASP factors include cytokines and their receptors (IL-6, IL-8, osteoprotegerin, GM-CSF), chemokines and their ligands (MCP-1, HCC4), and oncogenes (Gro1 and Gro2), many of them encoded by mRNAs whose stability and translation are tightly regulated. Using two models of human fibroblast senescence (WI-38 and IDH4 cells), we report the identification of RNA-binding protein NF90 as a post-transcriptional repressor of several SASP factors. In ‘young’, proliferating fibroblasts, NF90 was highly abundant, associated with numerous SASP mRNAs, and inhibited their expression. By contrast, senescent cells expressed low levels of NF90, thus allowing SASP factor expression to increase. NF90 elicited these effects mainly by repressing the translation of target SASP mRNAs, since silencing NF90 did not increase the steady-state levels of SASP mRNAs but elevated key SASP factors including MCP-1, GROa, IL-6, and IL-8. Our findings indicate that NF90 contributes to maintaining low levels of SASP factors in non-senescent cells, while NF90 reduction in senescent cells allows SASP factor expression to rise.

Coordinated interactions of multiple POT1-TPP1 proteins with telomere DNA

Telomeres are macromolecular nucleoprotein complexes that protect the ends of eukaryotic chromosomes from degradation, end-to-end fusion events, and from engaging the DNA damage response. However, the assembly of this essential DNA-protein complex is poorly understood. Telomere DNA consists of the repeated double-stranded sequence 5'-TTAGGG-3' in vertebrates, followed by a single-stranded DNA overhang with the same sequence. Both double- and single-stranded regions are coated with high specificity by telomere end-binding proteins, including POT1 and TPP1, that bind as a heterodimer to single-stranded telomeric DNA. Multiple POT1-TPP1 proteins must fully coat the single-stranded telomere DNA to form a functional telomere. To better understand the mechanism of multiple binding, we mutated or deleted the two guanosine nucleotides residing between adjacent POT1-TPP1 recognition sites in single-stranded telomere DNA that are not required for multiple POT1-TPP1 binding events. Circular dichroism demonstrated that spectra from the native telomere sequence are characteristic of a G-quadruplex secondary structure, whereas the altered telomere sequences were devoid of these signatures. The altered telomere strands, however, facilitated more cooperative loading of multiple POT1-TPP1 proteins compared with the wild-type telomere sequence. Finally, we show that a 48-nucleotide DNA with a telomere sequence is more susceptible to nuclease digestion when coated with POT1-TPP1 proteins than when it is left uncoated. Together, these data suggest that POT1-TPP1 binds telomeric DNA in a coordinated manner to facilitate assembly of the nucleoprotein complexes into a state that is more accessible to enzymatic activity.

The Asian-American E6 variant protein of human papillomavirus 16 alone is sufficient to promote immortalization, transformation, and migration of primary human foreskin keratinocytes

We examined how well the human papillomavirus (HPV) E6 oncogene can function in the absence of the E7 oncogene during the carcinogenic process in human keratinocytes using a common HPV variant strongly associated with cervical cancer: the Asian-American E6 variant (AAE6). This E6 variant is 20 times more frequently detected in cervical cancer than the prototype European E6 variant, as evidenced by independent epidemiological data. Using cell culture and cell-based functional assays, we assessed how this variant can perform crucial carcinogenesis steps compared to the prototype E6 variant. The ability to immortalize and transform primary human foreskin keratinocytes (PHFKs) to acquire resilient phenotypes and the ability to promote cell migration were evaluated. The immortalization capability was assayed based on population doublings, number of passages, surpassing mortality stages 1 and 2, human telomerase reverse transcriptase (hTERT) expression, and the ability to overcome G(1) arrest via p53 degradation. Transformation and migration efficiency were analyzed using a combination of functional cell-based assays. We observed that either AAE6 or prototype E6 proteins alone were sufficient to immortalize PHFKs, although AAE6 was more potent in doing so. The AAE6 variant protein alone pushed PHFKs through transformation and significantly increased their migration ability over that of the E6 prototype. Our findings are in line with epidemiological data that the AA variant of HPV16 confers an increased risk over the European prototype for cervical cancer, as evidenced by a superior immortalization, transformation, and metastatic potential.

Cellular senescence and cancer chemotherapy resistance

Innate or acquired resistance to cancer therapeutics remains an important area of biomedical investigation that has clear ramifications for improving cancer specific death rates. Importantly, clues to key resistance mechanisms may lie in the well-orchestrated and highly conserved cellular and systemic responses to injury and stress. Many anti-neoplastic therapies typically rely on DNA damage, which engages potent DNA damage response signaling pathways that culminate in apoptosis or growth arrest at checkpoints to allow for damage repair. However, an alternative cellular response, senescence, can also be initiated when challenged with these internal/external pressures and in ideal situations acts as a self-protecting mechanism. Senescence-induction therapies are an attractive concept in that they represent a normal, highly conserved and commonly invoked tumor-suppressing response to overwhelming genotoxic stress or oncogene activation. Yet, such approaches should ensure that senescence by-pass or senescence re-emergence does not occur, as emergent cells appear to have highly drug resistant phenotypes. Further, cell non-autonomous senescence responses may contribute to therapy-resistance in certain circumstances. Here we provide an overview of mechanisms by which cellular senescence plausibly contributes to therapy resistance and concepts by which senescence responses can be influenced to improve cancer treatment outcomes.

Long telomeres bypass the requirement for telomere maintenance in human tumorigenesis

Despite the importance of telomere maintenance in cancer cell survival via the elongation of telomeres by telomerase reverse transcriptase (TERT) or alternative lengthening of telomeres (ALT), it had not been tested directly whether telomere maintenance is dispensable for human tumorigenesis. We engineered human tumor cells containing loxP-flanked hTERT to enable extensive telomere elongation prior to complete hTERT excision. Despite unabated telomere erosion, hTERT-excised cells formed tumors in mice and proliferated in vitro for up to 1 year. Telomerase reactivation or ALT was not observed, and the eventual loss of telomeric signal coincided with loss of tumorigenic potential and cell viability. Crisis was averted via the reintroduction of active but not inactive hTERT. Thus, telomere maintenance is dispensable for human tumorigenesis when telomere reserves are long. Yet, despite telomere instability and the presence of oncogenic RAS, human tumors remain susceptible to crisis induced by critically short telomeres.

Role of telomeres and telomerase in cancer

There is mounting evidence for the existence of an important relationship between telomeres and telomerase and cellular aging and cancer. Normal human cells progressively lose telomeres with each cell division until a few short telomeres become uncapped leading to a growth arrest known as replicative aging. In the absence of genomic alterations these cells do not die but remain quiescent producing a different constellation of proteins compared to young quiescent cells. Upon specific genetic and epigenetic alterations, normal human cells bypass replicative senescence and continue to proliferate until many telomere ends become uncapped leading to a phenomenon known as crisis. In crisis cells have critically shortened telomeres but continue to attempt to divide leading to significant cell death (apoptosis) and progressive genomic instability. Rarely, a human cell escapes crisis and these cells almost universally express the ribonucleoprotein, telomerase, and maintain stable but short telomeres. The activation of telomerase may be thought of as a mechanism to slow down the rate genomic instability due to dysfunctional telomeres. While telomerase does not drive the oncogenic process, it is permissive and required for the sustain growth of most advanced cancers. Since telomerase is not expressed in most normal human cells, this has led to the development of targeted telomerase cancer therapeutic approaches that are presently in advanced clinical trials.

Is telomerase a viable target in cancer?

The ideal cancer treatment would specifically target cancer cells yet have minimal or no adverse effects on normal somatic cells. Telomerase, the ribonucleoprotein reverse transcriptase that maintains the ends of human chromosome, is an attractive cancer therapeutic target for exactly this reason [1]. Telomerase is expressed in more than 85% of cancer cells, making it a nearly universal cancer marker, while the majority of normal somatic cells are telomerase negative. Telomerase activity confers limitless replicative potential to cancer cells, a hallmark of cancer which must be attained for the continued growth that characterizes almost all advanced neoplasms [2]. In this review we will summarize the role of telomeres and telomerase in cancer cells, and how properties of telomerase are being exploited to create targeted cancer therapies including telomerase inhibitors, telomerase-targeted immunotherapies and telomerase-driven virotherapies. A frank and balanced assessment of the current state of telomerase inhibitors with caveats and potential limitations will be included.

Improvement of a method to reproducibly immortalize human T cells by oncogene transfection

The method to immortalize human T cells efficiently and reproduciblyby oncogene transfection was improved. T cells were first grown selectively from peripheralblood lymphocytes population of healthy donors andatopic asthma patients, and from lymph nodelymphocytes population of lung cancer patients byactivating with mitogens (phytohemagglutinin andconcanavalin A) and recombinant human interleukin-2(rhIL-2) for five days. Plasmids expressingoncogenes, such as c-Ha-ras, c-myc,c-fos, v-myb and v-jun under the controlof human cytomegalovirus promoter, were then introducedinto these stimulated lymphocytes either separately orin various combinations by electropolation. Afterculturing these transfected lymphocytes for recoveryfor 1 day, they were fed every 3-4 days. Although all the control cells died within one month,oncogene-transfected lymphocytes continued toproliferate actively even for more than severalmonths, indicating that oncogene-transfectedlymphocytes were successfully immortalized. Flowcytometric analyses revealed that most of theimmortalized lymphocytes were T cells expressingCD3(+) surface antigen. The ratios of CD4(+)and CD8(+) subpopulations in immortalized T cellsderived from healthy donors varied, depending onthe kinds of oncogenes used. However, CD8(+)subpopulation in immortalized T cells derived fromcancer patients and atopic asthma patients weredominant, independent of the kinds of oncogenes. These immortalized T cells showed differentproliferative responses in the presence or absence ofexogenous human rhIL-2, depending on their origin ofdonors. Furthermore, immortalized T cells derivedfrom healthy donors showed stronger cytotoxicityagainst K562 cells, suggesting that MHC-nonrestrictedkiller T cells in T cell population were alsoimmortalized. Immortalized T cell lines, whichproliferate continuously without stimulation of amitogen or antigen in medium containing a lowconcentration of rhIL-2, have been maintained for morethan 2 years without any growth rate decrease.

Culture, immortalization, and characterization of human meibomian gland epithelial cells

PURPOSE

Meibomian gland epithelial cells are essential in maintaining the health and integrity of the ocular surface. However, very little is known about their physiological regulation. In this study, the cellular control mechanisms were explored, first to establish a defined culture system for the maintenance of primary epithelial cells from human meibomian glands and, second, to immortalize these cells, thereby developing a preclinical model that could be used to identify factors that regulate cell activity.

METHODS

Human meibomian glands were removed from lid segments after surgery, enzymatically digested, and dissociated. Isolated epithelial cells were cultured in media with or without serum and/or 3T3 feeder layers. To attempt immortalization, the cells were exposed to retroviral human telomerase reverse transcriptase (hTERT) and/or SV40 large T antigen cDNA vectors, and antibiotic-resistant cells were selected, expanded, and subcultured. Analyses for possible biomarkers, cell proliferation and differentiation, lipid-related enzyme gene expression, and the cellular response to androgen were performed with biochemical, histologic, and molecular biological techniques.

RESULTS

It was possible to isolate viable human meibomian gland epithelial cells and to culture them in serum-free medium. These cells proliferated, survived through at least the fifth passage, and contained neutral lipids. Infection with hTERT immortalized these cells, which accumulated neutral lipids during differentiation, expressed multiple genes for lipogenic enzymes, responded to androgen, and continued to proliferate.

CONCLUSIONS

The results show that human meibomian gland epithelial cells may be isolated, cultured, and immortalized.

HuR uses AUF1 as a cofactor to promote p16INK4 mRNA decay

In this study, we show that HuR destabilizes p16(INK4) mRNA. Although the knockdown of HuR or AUF1 increased p16 expression, concomitant AUF1 and HuR knockdown had a much weaker effect. The knockdown of Ago2, a component of the RNA-induced silencing complex (RISC), stabilized p16 mRNA. The knockdown of HuR diminished the association of the p16 3' untranslated region (3'UTR) with AUF1 and vice versa. While the knockdown of HuR or AUF1 reduced the association of Ago2 with the p16 3'UTR, Ago2 knockdown had no influence on HuR or AUF1 binding to the p16 3'UTR. The use of EGFP-p16 chimeric reporter transcripts revealed that p16 mRNA decay depended on a stem-loop structure present in the p16 3'UTR, as HuR and AUF1 destabilized EGFP-derived chimeric transcripts bearing wild-type sequences but not transcripts with mutations in the stem-loop structure. In senescent and HuR-silenced IDH4 human diploid fibroblasts, the EGFP-p16 3'UTR transcript was more stable. Our results suggest that HuR destabilizes p16 mRNA by recruiting the RISC, an effect that depends on the secondary structure of the p16 3'UTR and requires AUF1 as a cofactor.

PHF10 is required for cell proliferation in normal and SV40-immortalized human fibroblast cells

Normal human diploid fibroblasts have limited life span in culture and undergo replicative senescence after 50-60 population doublings. On the contrary, cancer cells typically divide indefinitely and are immortal. Expression of SV40 large T and small t antigens in human fibroblasts transiently extends their life span by 20-30 population doublings and facilitates immortalization. We have identified a rearrangement in chromosome 6 shared by SV40-transformed human fibroblasts. Rearrangements involving chromosome 6 are among the most frequent in human carcinogenesis. In this paper, we extend analysis of the 6q26-q27 region, a putative site for a growth suppressor gene designated SEN6 involved in immortalization of SV40-transformed cells. Detailed molecular characterization of the rearranged chromosomes (6q*, normal appearing; and 6q(t), translocated) in the SV40-immortalized cell line HALneo by isolating each of these 2 chromosomes in mouse/HAL somatic cell hybrids is presented. Analysis of these mouse/HAL somatic cell hybrids with polymorphic and nonpolymorphic markers revealed that the 6q* has undergone a chromosomal break in the MLLT4 gene (alias AF6). This result in conjunction with previous published observations leads us to conclude that SEN6 lies between MLLT4 and TBP at chromosomal region 6q27. Examination of different genes (MLLT4, DLL1, FAM120B, PHF10) located within this interval that are expressed in HS74 normal fibroblast cells reveals that overexpression of epitope-tagged truncated PHF10 cDNAs resulted in reduced cell proliferation in multiple cell lines. Paradoxically, down-regulation of PHF10 by RNAi also resulted in loss of cell proliferation in normal fibroblast cells, indicating PHF10 function is required for cell growth. Taken together, these observations suggest that decreased cell proliferation with epitope-tagged truncated PHF10 proteins may be due to dominant negative effects or due to unregulated expression of these mutant proteins. Hence we conclude that PHF10 is not SEN6 but is required for cell growth.

Fusion of short telomeres in human cells is characterized by extensive deletion and microhomology, and can result in complex rearrangements

Telomere fusion is an important mutational event that has the potential to lead to large-scale genomic rearrangements of the types frequently observed in cancer. We have developed single-molecule approaches to detect, isolate and characterize the DNA sequence of telomere fusion events in human cells. Using these assays, we have detected complex fusion events that include fusion with interstitial loci adjacent to fragile sites, intra-molecular rearrangements, and fusion events involving the telomeres of both arms of the same chromosome consistent with ring chromosome formation. All fusion events were characterized by the deletion of at least one of the telomeres extending into the sub-telomeric DNA up to 5.6 kb; close to the limit of our assays. The deletion profile indicates that deletion may extend further into the chromosome. Short patches of DNA sequence homology with a G:C bias were observed at the fusion point in 60% of events. The distinct profile that accompanies telomere fusion may be a characteristic of the end-joining processes involved in the fusion event.

Reduced nuclear export of HuR mRNA by HuR is linked to the loss of HuR in replicative senescence

The RNA-binding protein, HuR, associates with the HuR mRNA, but the consequences of this interaction are unknown. Here, we use human diploid fibroblasts (HDFs) and cervical carcinoma cells to study this regulatory paradigm. Ectopic overexpression of HuR potently enhanced the translation and cytoplasmic levels of endogenous HuR, but did not affect HuR mRNA levels. Inhibition of CRM1 function by Lemptomycin B or by knockdown of CRM1 greatly diminished the cytoplasmic levels of endogenous HuR mRNA and hence blocked the induction of endogenous HuR by exogenous HuR. Further studies showed that HuR interacted with the 3'-untranslated region (UTR) of HuR and that overexpression of HuR increased the cytoplasmic levels of a chimeric luciferase-HuR 3'-UTR reporter transcript, as well as luciferase activity; conversely, HuR knockdown reduced both parameters. Moreover, the loss of HuR in senescent, late-passage HDFs was accompanied by a reduced cytoplasmic presence of endogenous HuR mRNA, ectopic Luc-HuR-3'UTR reporter transcript, and luciferase activity relative to what was observed in young, early-passage cells. Our results reveal a positive feedback mechanism for the regulation of HuR, which may play an important role in the regulation of HuR during replicative senescence.

Role of SV40 integration site at chromosomal interval 1q21.1 in immortalized CRL2504 cells

We have applied a functional gene transfer strategy to show the importance of viral integration site in cellular immortalization. The large tumor antigen of SV40 is capable of extending the cellular life span by sequestering tumor suppressor proteins pRB and p53 in virus-transformed human cells. Although SV40 large T antigen is essential, it is not sufficient for cellular immortalization, suggesting that additional alterations in cellular genes are required to attain infinite proliferation. We show here that the disruption of human chromosomal interval at 1q21.1 by SV40 integration can be an essential step for cellular immortalization. The transfer of a 150-kb bacterial artificial chromosome (BAC) clone, RP364B14, corresponding to viral integration site in CRL2504 cells, reverted their immortal phenotype. Interestingly, the BAC transfer clones of CRL2504 cells displayed characteristics of either senescence as shown by beta-galactosidase activity or apoptosis as revealed by positive staining with M30 CytoDEATH antibody. The SV40 integration at 1q21.1, in the vicinity of epidermal differentiation complex (EDC) genes, resulted in the down-regulation of the filaggrin (FLG) gene that is part of the EDC. FLG gene expression was increased in BAC transfer senescent and apoptotic clones. Our results suggest that the disruption of native genomic sequence by SV40 may alter expression of genes involved in senescence and apoptosis by modulating chromatin structure. These studies imply that identification of genes located in the vicinity of viral integration sites in human cancers may be helpful in developing new diagnostic and therapeutic strategies.

Gene network signaling in hormone responsiveness modifies apoptosis and autophagy in breast cancer cells

Resistance to endocrine therapies, whether de novo or acquired, remains a major limitation in the ability to cure many tumors that express detectable levels of the estrogen receptor alpha protein (ER). While several resistance phenotypes have been described, endocrine unresponsiveness in the context of therapy-induced tumor growth appears to be the most prevalent. The signaling that regulates endocrine resistant phenotypes is poorly understood but it involves a complex signaling network with a topology that includes redundant and degenerative features. To be relevant to clinical outcomes, the most pertinent features of this network are those that ultimately affect the endocrine-regulated components of the cell fate and cell proliferation machineries. We show that autophagy, as supported by the endocrine regulation of monodansylcadaverine staining, increased LC3 cleavage, and reduced expression of p62/SQSTM1, plays an important role in breast cancer cells responding to endocrine therapy. We further show that the cell fate machinery includes both apoptotic and autophagic functions that are potentially regulated through integrated signaling that flows through key members of the BCL2 gene family and beclin-1 (BECN1). This signaling links cellular functions in mitochondria and endoplasmic reticulum, the latter as a consequence of induction of the unfolded protein response. We have taken a seed-gene approach to begin extracting critical nodes and edges that represent central signaling events in the endocrine regulation of apoptosis and autophagy. Three seed nodes were identified from global gene or protein expression analyses and supported by subsequent functional studies that established their abilities to affect cell fate. The seed nodes of nuclear factor kappa B (NFkappaB), interferon regulatory factor-1 (IRF1), and X-box binding protein-1 (XBP1)are linked by directional edges that support signal flow through a preliminary network that is grown to include key regulators of their individual function: NEMO/IKKgamma, nucleophosmin and ER respectively. Signaling proceeds through BCL2 gene family members and BECN1 ultimately to regulate cell fate.

Altered states of telomere deprotection and the two-stage mechanism of replicative aging

The molecular distinctions between mortality stages 1 (M1; senescence) and 2 (M2; crisis) of human replicative aging are ill defined. We demonstrate a qualitative difference between telomeric end associations at M1 and the end fusions that produce dicentric chromosomes and breakage-fusion cycles. Knockdown of ligase IV sufficient to completely inhibit radiation-induced dicentric chromosome formation had no effect on the frequency of telomere associations (TAs), establishing that TAs are not covalent conventional nonhomologous end-joining (NHEJ) products. TAs preceded and were more numerous than dicentric chromosomes. Cells initially tolerated dicentric chromosomes without dying, but eventually, a combination of too many TAs and dicentrics/complex chromosomal rearrangements resulted in apoptosis. We propose a working model in which end associations represent abortive DNA repair intermediates when the number of telomeric repeats is too small to completely inhibit DNA damage signaling but is sufficient to prevent the final covalent ligation step of NHEJ and induces the M1 checkpoint arrest in normal human cells. Rather than being all-or-none, telomere deprotection would thus proceed first through TAs before additional shortening leads to dicentric chromosomes. M2/crisis involves both qualitative changes (a shift from TAs to TAs plus dicentric chromosomes) and quantitative changes (an increase in the number of dysfunctional telomeres).

Human flap endonuclease I is in complex with telomerase and is required for telomerase-mediated telomere maintenance

Studies from budding yeast and ciliates have suggested that telomerase extension of telomeres requires the conventional DNA replication machinery, yet little is known about how DNA replication proteins regulate telomerase action in higher eukaryotic cells. Here we investigate the role of one of the DNA replication factors, flap endonuclease I (FEN1), in regulating telomerase activity in mammalian cells. FEN1 is a nuclease that plays an important role in DNA replication, repair, and recombination. We show that FEN1 is in complex with telomerase in vivo via telomeric DNA. We further demonstrate that FEN1 deficiency in mouse embryonic fibroblasts leads to an increase in telomere end-to-end fusions. In cancer cells, FEN1 deficiency induces gradual shortening of telomeres but does not alter the single-stranded G-overhangs. This is, to our knowledge, the first evidence that FEN1 and telomerase physically co-exist as a complex and that FEN1 can regulate telomerase activity at telomeres in mammalian cells.

Oxidative Stress Increases the Number of Stress Granules in Senescent Cells and Triggers a Rapid Decrease in p21waf1/cip1 Translation

Very little is known as to how the accumulation of senescent cells during aging may affect our ability to cope with various stresses. Here we show that the assembly of stress granules (SGs) is part of the early events used by senescent cells to respond to certain stresses. Although SGs can form in response to stress during senescence activation, their number significantly increases once the cells are fully senescent. This increase correlates with a rapid decrease in the expression levels of the cyclin kinase inhibitor p21, an important activator of senescence. Throughout stress, p21 mRNA is stabilized and localizes to SGs, but only during late senescence does this localization interferes with its translation. Additionally, we observed that when the stress is relieved, senescent cells produce lower levels of p21 protein, which correlates with a small delay in SG disassembly. Therefore, our data suggest that SG formation and the reduction in p21 protein levels represent two main events by which senescent cells respond to stress.

Targeting telomerase for cancer therapeutics

One of the hallmarks of advanced malignancies is continuous cell growth and this almost universally correlates with the reactivation of telomerase. Although there is still much we do not understand about the regulation of telomerase, it remains a very attractive and novel target for cancer therapeutics. Several clinical trials have been initiated, and in this review we highlight some of the most promising approaches and conclude by speculating on the role of telomerase in cancer stem cells.

Genomic Analysis Highlights the Role of the JAK-STAT Signaling in the Anti-proliferative Effects of Dietary Flavonoid-'Ashwagandha' in Prostate Cancer Cells

Phytochemicals are dietary phytoestrogens that may play a role in prostate cancer prevention. Forty percent of Americans use complementary and alternative medicines (CAM) for disease prevention and therapy. Ashwagandha (Withania somnifera) contains flavonoids and active ingredients like alkaloids and steroidal lactones which are called ‘Withanolides’. We hypothesize that the immunomodulatory and anti-inflammatory properties of Ashwagandha might contribute to its overall effectiveness as an anti-carcinogenic agent. The goal of our study was gain insight into the general biological and molecular functions and immunomodulatory processes that are altered as a result of Ashwagandha treatment in prostate cancer cells, and to identify the key signaling mechanisms that are involved in the regulation of these physiological effects using genomic microarray analysis in conjunction with quantitative real-time PCR and western blot analysis. Ashwagandha treatment significantly downregulated the gene and protein expression of proinflammatory cytokines IL-6, IL-1β, chemokine IL-8, Hsp70 and STAT-2, while a reciprocal upregulation was observed in gene and protein expression of p38 MAPK, PI3K, caspase 6, Cyclin D and c-myc. Furthermore, Ashwagandha treatment significantly modulated the JAK-STAT pathway which regulates both the apoptosis process as well as the MAP kinase signaling. These studies outline several functionally important classes of genes, which are associated with immune response, signal transduction, cell signaling, transcriptional regulation, apoptosis and cell cycle regulation and provide insight into the molecular signaling mechanisms that are modulated by Ashwagandha, thereby highlighting the use of this bioflavanoid as effective chemopreventive agent relevant to prostate cancer progression.

3'-Azido-2',3'-dideoxynucleoside 5'-triphosphates inhibit telomerase activity in vitro, and the corresponding nucleosides cause telomere shortening in human HL60 cells

Telomerase adds telomeric DNA repeats to the ends of linear chromosomal DNA. 3′-Azido-3′-deoxythymidine 5′-triphosphate (AZTTP) is a known telomerase inhibitor. To obtain more selective and potent inhibitors that can be employed as tools for studying telomerase, we investigated the telomerase-inhibitory effects of purine nucleosides bearing a 3′-down azido group: 3′-azido-2′,3′-dideoxyguanosine (AZddG) 5′-triphosphate (AZddGTP), 3′-azido-2′,3′-dideoxy-6-thioguanosine (AZddSG) 5′-triphosphate (AZddSGTP), 3′-azido-2′,3′-dideoxyadenosine (AZddA) 5′-triphosphate (AZddATP) and 3′-azido-2′,3′-dideoxy-2-aminoadenosine (AZddAA) 5′-triphosphate (AZddAATP). Of these, AZddGTP showed the most potent inhibitory activity against HeLa cell telomerase. AZddGTP was significantly incorporated into the 3′-terminus of DNA by partially purified telomerase. However, AZddGTP did not exhibit significant inhibitory activity against DNA polymerases α and δ, suggesting that AZddGTP is a selective inhibitor of telomerase.

We also investigated whether long-term treatment with these nucleosides could alter telomere length and growth rates of human HL60 cells in culture. Southern hybridization analysis of genomic DNA prepared from cells cultured in the presence of AZddG and AZddAA revealed reproducible telomere shortening.

Resistance to apoptosis, increased growth potential, and altered gene expression in cells that survived genotoxic hexavalent chromium [Cr(VI)] exposure

Certain hexavalent chromium [Cr(VI)] compounds are known genotoxic respiratory carcinogens, which induce apoptosis as a predominant mode of cell death. Selection of cells that are resistant to apoptosis may be a factor in tumour progression. We developed sub-populations of telomerase-transfected human fibroblasts (BJ-hTERT) that survived a 99% clonogenically lethal exposure to Cr(VI) (B-5Cr). B-5Cr cells were markedly resistant to apoptosis induced by several agents and exhibited increased clonogenic survival, especially at apoptogenic doses. B-5Cr cells did not exhibit altered cellular uptake of Cr(VI) and retained a normal p53 response to Cr(VI) exposure. We conducted large-scale gene expression analysis at different time-points after a secondary genotoxic Cr(VI) insult in B-5Cr and BJ-hTERT cells using Affymetrix Genechip human genome arrays. Cr(VI) exposure led to differential regulation of many genes, which affect a diverse set of cellular activities such as transcription, signal transduction, stress response, cell adhesion, DNA repair, apoptosis and cell cycle modulation. We compared Cr(VI)-induced altered gene expression in the B-5Cr cells to that in the parental cells and identified 223, 147 and 204 genes with at least a two-fold difference in expression at 4, 8 and 18 h after exposure, respectively. Cluster analysis by gene function revealed altered expression of genes involved in apoptosis, cell cycle regulation and DNA repair. Our data suggest an alteration in gene expression that may favor cell survival and/or incomplete DNA repair after genotoxic exposure. Selection of cells with altered expression of these genes may constitute the early stages of tumour progression.

Escape from Therapy-Induced Accelerated Cellular Senescence in p53-Null Lung Cancer Cells and in Human Lung Cancers

Accelerated cellular senescence (ACS) has been described for tumor cells treated with chemotherapy and radiation. Following exposure to genotoxins, tumor cells undergo terminal growth arrest and adopt morphologic and marker features suggestive of cellular senescence. ACS is elicited by a variety of chemotherapeutic agents in the p53-null, p16-deficient human non-small cell H1299 carcinoma cells. After 10 to 21 days, infrequent ACS cells (1 in 10(6)) can bypass replicative arrest and reenter cell cycle. These cells express senescence markers and resemble the parental cells in their transcription profile. We show that these escaped H1299 cells overexpress the cyclin-dependent kinase Cdc2/Cdk1. The escape from ACS can be disrupted by Cdc2/Cdk1 kinase inhibitors or by knockdown of Cdc2/Cdk1 with small interfering RNA and can be promoted by expression of exogenous Cdc2/Cdk1. We also present evidence that ACS occurs in vivo in human lung cancer following induction chemotherapy. Viable tumors following chemotherapy also overexpress Cdc2/Cdk1. We propose that ACS is a mechanism of in vivo tumor response and that mechanisms aberrantly up-regulate Cdc2/Cdk1 promotes escape from the senescence pathway may be involved in a subset of tumors and likely accounts for tumor recurrence/progression.