Telomerase, immortality and cancer

Telomerase and hallmarks of cancer: An intricate interplay governing cancer cell evolution

Transformed cells must acquire specific characteristics to be malignant. Weinberg and Hanahan characterize these characteristics as cancer hallmarks. Though these features are independently driven, substantial signaling crosstalk in transformed cells efficiently promotes these feature acquisitions. Telomerase is an enzyme complex that maintains telomere length. However, its main component, Telomere reverse transcriptase (TERT), has been found to interact with various signaling molecules like cMYC, NF-kB, BRG1 and cooperate in transcription and metabolic reprogramming, acting as a strong proponent of malignant features such as cell death resistance, sustained proliferation, angiogenesis activation, and metastasis, among others. It allows cells to avoid replicative senescence and achieve endless replicative potential. This review summarizes both the canonical and noncanonical functions of TERT and discusses how they promote cancer hallmarks. Understanding the role of Telomerase in promoting cancer hallmarks provides vital insight into the underlying mechanism of cancer genesis and progression and telomerase intervention as a possible therapeutic target for cancer treatment. More investigation into the precise molecular mechanisms of telomerase-mediated impacts on cancer hallmarks will contribute to developing more focused and customized cancer treatment methods.

Mitochondrial homeostasis: a potential target for delaying renal aging

Mitochondria, which are the energy factories of the cell, participate in many life activities, and the kidney is a high metabolic organ that contains abundant mitochondria. Renal aging is a degenerative process associated with the accumulation of harmful processes. Increasing attention has been given to the role of abnormal mitochondrial homeostasis in renal aging. However, the role of mitochondrial homeostasis in renal aging has not been reviewed in detail. Here, we summarize the current biochemical markers associated with aging and review the changes in renal structure and function during aging. Moreover, we also review in detail the role of mitochondrial homeostasis abnormalities, including mitochondrial function, mitophagy and mitochondria-mediated oxidative stress and inflammation, in renal aging. Finally, we describe some of the current antiaging compounds that target mitochondria and note that maintaining mitochondrial homeostasis is a potential strategy against renal aging.

Zeolitic Imidazolate Framework-8 (ZIF-8) as a Drug Delivery Vehicle for the Transport and Release of Telomerase Inhibitor BIBR 1532

Telomerase is constitutively overexpressed in the majority of human cancers and telomerase inhibition provides a promising broad-spectrum anticancer therapeutic strategy. BIBR 1532 is a well-known synthetic telomerase inhibitor that blocks the enzymatic activity of hTERT, the catalytic subunit of telomerase. However, water insolubility of BIBR 1532 leads to low cellular uptake and inadequate delivery and thus, limits its anti-tumor effects. Zeolitic imidazolate framework-8 (ZIF-8) is considered as an attractive drug delivery vehicle for improved transport, release and anti-tumor effects of BIBR 1532. Herein, ZIF-8 and BIBR 1532@ZIF-8 were synthesized, respectively, and the physicochemical characterizations confirmed the successful encapsulation of BIBR 1532 in ZIF-8 coupled with an improved stability of BIBR 1532. ZIF-8 could alter the permeability of lysosomal membrane probably by the imidazole ring-dependent protonation. Moreover, ZIF-8 encapsulation facilitated the cellular uptake and release of BIBR 1532 with more accumulation in the nucleus. BIBR 1532 encapsulation with ZIF-8 triggered a more obvious growth inhibition of cancer cells as compared with free BIBR 1532. A more potent inhibition on hTERT mRNA expression, aggravated G0/G1 arrest accompanied with an increased cellular senescence were detected in BIBR 1532@ZIF-8-treated cancer cells. Our work has provided preliminary information on improving the transport, release and efficacy of water-insoluble small molecule drugs by using ZIF-8 as a delivery vehicle.

Telomeres and Cancer

Telomeres cap the ends of eukaryotic chromosomes and are indispensable chromatin structures for genome protection and replication. Telomere length maintenance has been attributed to several functional modulators, including telomerase, the shelterin complex, and the CST complex, synergizing with DNA replication, repair, and the RNA metabolism pathway components. As dysfunctional telomere maintenance and telomerase activation are associated with several human diseases, including cancer, the molecular mechanisms behind telomere length regulation and protection need particular emphasis. Cancer cells exhibit telomerase activation, enabling replicative immortality. Telomerase reverse transcriptase (TERT) activation is involved in cancer development through diverse activities other than mediating telomere elongation. This review describes the telomere functions, the role of functional modulators, the implications in cancer development, and the future therapeutic opportunities.

Prognostic features of the tumour microenvironment in oesophageal adenocarcinoma

Oesophageal adenocarcinoma (OAC) is a disease with an incredibly poor survival rate and a complex makeup. The growth and spread of OAC tumours are profoundly influenced by their surrounding microenvironment and the properties of the tumour itself. Constant crosstalk between the tumour and its microenvironment is key to the survival of the tumour and ultimately the death of the patient. The tumour microenvironment (TME) is composed of a complex milieu of cell types including cancer associated fibroblasts (CAFs) which make up the tumour stroma, endothelial cells which line blood and lymphatic vessels and infiltrating immune cell populations. These various cell types and the tumour constantly communicate through environmental cues including fluctuations in pH, hypoxia and the release of mitogens such as cytokines, chemokines and growth factors, many of which help promote malignant progression. Eventually clusters of tumour cells such as tumour buds break away and spread through the lymphatic system to nearby lymph nodes or enter the circulation forming secondary metastasis. Collectively, these factors need to be considered when assessing and treating patients clinically. This review aims to summarise the ways in which these various factors are currently assessed and how they relate to patient treatment and outcome at an individual level.

Telomere dynamic in humans and animals: Review and perspectives in environmental toxicology

Telomeres (TLs) play major roles in stabilizing the genome and are usually shortened with ageing. The maintenance of TLs is ensured by two mechanisms involving telomerase (TA) enzyme and alternative lengthening telomeres (ALT). TL shortening and/or TA inhibition have been related to health effects on organisms (leading to reduced reproductive lifespan and survival), suggesting that they could be key processes in toxicity mechanisms (at molecular and cellular levels) and relevant as an early warning of exposure and effect of chemicals on human health and animal population dynamics. Consequently, a critical analysis of knowledge about relationships between TL dynamic and environmental pollution is essential to highlight the relevance of TL measurement in environmental toxicology. The first objective of this review is to provide a survey on the basic knowledge about TL structure, roles, maintenance mechanisms and causes of shortening in both vertebrates (including humans) and invertebrates. Overall, TL length decreases with ageing but some unexpected exceptions are reported (e.g., in species with different lifespans, such as the nematode Caenorhabditis elegans or the crustacean Homarus americanus). Inconsistent results reported in various biological groups or even between species of the same genus (e.g., the microcrustacean Daphnia sp.) indicate that the relation usually proposed between TL shortening and a decrease in TA activity cannot be generalized and depends on the species, stage of development or lifespan. Although the scientific literature provides evidence of the effect of ageing on TL shortening, much less information on the relationships between shortening, maintenance of TLs, influence of other endogenous and environmental drivers, including exposure to chemical pollutants, is available, especially in invertebrates. The second objective of this review is to connect knowledge on TL dynamic and exposure to contaminants. Most of the studies published on humans rely on correlative epidemiological approaches and few in vitro experiments. They have shown TL attrition when exposed to contaminants, such as polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), pesticides and metallic elements (ME). In other vertebrates, the studies we found deals mainly with birds and, overall, report a disturbance of TL dynamic consecutively to exposure to chemicals, including metals and organic compounds. In invertebrates, no data are available and the potential of TL dynamic in environmental risk assessment remains to be explored. On the basis of the main gaps identified some research perspectives (e.g., impact of endogenous and environmental drivers, dose response effects, link between TL length, TA activity, longevity and ageing) are proposed to better understand the potential of TL and TA measurements in humans and animals in environmental toxicology.

TTAGG-repeat telomeres and characterization of telomerase in the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae)

Telomeres are maintained usually by telomerase, a specialized reverse transcriptase that adds this sequence to chromosome ends. In this study, telomerase activity was detected in the in different somatic tissues, such as midgut and fat bodies, by the telomeric repeat amplification protocol (TRAP) in Spodoptera exigua. The structure of the telomeres of S. exigua was evaluated by sequence analysis of the TRAP products, revealing that the telomerase synthesized a (TTAGG)n repeat. The presence of a telomerase reverse transcriptase (TERT) subunit coding gene has been cloned, sequenced and expressed in vitro successively. Notably, the S. exigua telomerase (SpexTERT) gene structure lacks the N-terminal GQ motif. Telomerase contains a large RNA subunit, TER, and a protein catalytic subunit, TERT. Here we report an in vitro system that was reconstructed by all components of the telomerase complex, a purified recombinant SpexTERT without a N-terminal GQ motif and a mutant human telomerase RNA (TER), showed telomerase activity. Together, these results suggest the GQ motif is not essential for telomerase catalysis.

PGC-1alpha/beta induced expression partially compensates for respiratory chain defects in cells from patients with mitochondrial disorders

Members of the peroxisome proliferator-activated receptor gamma coactivator (PGC) family are potent inducers of mitochondrial biogenesis. We have tested the potential effect of increased mitochondrial biogenesis in cells derived from patients harboring oxidative phosphorylation defects due to either nuclear or mitochondrial DNA mutations. We found that the PGC-1alpha and/or PGC-1beta expression improved mitochondrial respiration in cells harboring a complex III or IV deficiency as well as in transmitochondrial cybrids harboring mitochondrial encephalomyopathy lactic acidosis and stroke A3243G tRNA((Leu)UUR) gene mutation. The respiratory function improvement was found to be associated with increased levels of mitochondrial components per cell, although this increase was not homogeneous. These results reinforce the concept that increased mitochondrial biogenesis is a promising venue for the treatment of mitochondrial diseases.

Telomerase: is it the future diagnostic and prognostic tool in human cancer?

A number of methods exist to detect levels of telomerase activity and the presence of telomerase subunits in a variety of tissues. As telomerase activation seems to be an important step in tumorigenesis, accurate detection of the presence and activity of the enzyme and its subunits is vital. The original method of detecting telomerase activity was developed by Kim and coworkers in 1994, and was termed the telomeric repeat amplification protocol. This assay led to a staggering increase in the number of telomerase-associated publications in scientific journals (85 publications from 1974-1994, 5063 publications from 1994-2004). A number of methods have been described to detect telomeres and to measure their length, with the standard measurement of telomere length performed using a modification of the Southern blot protocol. RNA in situ hybridization can be performed to detect levels of the RNA component of telomerase, and standard in situ hybridization and immunohistochemistry can be applied to examine expression levels and localization of the catalytic subunit of the enzyme. Reverse transcriptase PCR has also been applied to assess expression levels of the telomerase components in various tissues. This review provides a synopsis of telomeres, telomerase, telomerase and cancer, and finally, methods for the detection of telomerase in cancer.

Hypoxia induces telomerase reverse transcriptase (TERT) gene expression in non-tumor fish tissues in vivo: the marine medaka (Oryzias melastigma) model

Background

Current understanding on the relationships between hypoxia, hypoxia-inducible factor-1 (HIF-1) and telomerase reverse transcriptase (TERT) gene expression are largely based on in vitro studies in human cancer cells. Although several reports demonstrated HIF-1- mediated upregulation of the human TERT gene under hypoxia, conflicting findings have also been reported. Thus far, it remains uncertain whether these findings can be directly extrapolated to non-tumor tissues in other whole animal systems in vivo. While fish often encounter environmental hypoxia, the in vivo regulation of TERT by hypoxia in non-neoplastic tissues of fish remains virtually unknown.

Results

The adult marine medaka (Oryzias melastigma) was employed as a model fish in this study. We have cloned and characterized a 3261-bp full-length TERT cDNA, omTERT, which encodes a protein of 1086 amino acids. It contains all of the functional motifs that are conserved in other vertebrate TERTs. Motif E is the most highly conserved showing 90.9–100% overall identity among the fish TERTs and 63.6% overall identity among vertebrates. Analysis of the 5'-flanking sequence of the omTERT gene identified two HRE (hypoxia-responsive element; nt. – 283 and – 892) cores. Overexpression of the HIF-1α induced omTERT promoter activity as demonstrated using transient transfection assays. The omTERT gene is ubiquitously expressed in fish under normoxia, albeit at varying levels, where highest expression was observed in gonads and the lowest in liver. In vivo expression of omTERT was significantly upregulated in testis and liver in response to hypoxia (at 96 h and 48 h, respectively), where concomitant induction of the omHIF-1α and erythropoietin (omEpo) genes was also observed. In situ hybridization analysis showed that hypoxic induction of omTERT mRNA was clearly evident in hepatocytes in the caudal region of liver and in spermatogonia-containing cysts in testis.

Conclusion

This study demonstrates for the first time, hypoxic regulation of TERT expression in vivo in a whole fish system. Our findings support the notion that hypoxia upregulates omTERT expression via omHIF-1 in non-neoplastic fish liver and testis in vivo. Overall, the structure and regulation of the TERT gene is highly conserved in vertebrates from fish to human.

Protein tyrosine phosphatase PRL-3 in malignant cells and endothelial cells: expression and function

Protein tyrosine phosphatase PRL-3 mRNA was found highly expressed in colon cancer endothelium and metastases. We sought to associate a function with PRL-3 expression in both endothelial cells and malignant cells using in vitro models. PRL-3 mRNA levels were determined in several normal human endothelial cells exposed or unexposed to the phorbol ester phorbol 12-myristate 13-acetate (PMA) and in 27 human tumor cell lines. In endothelial cells, PRL-3 mRNA expression was increased in human umbilical vascular endothelial cells and human microvascular endothelial cells (HMVEC) exposed to PMA. An oligonucleotide microarray analysis revealed that PRL-3 was among the 10 genes with the largest increase in expression on PMA stimulation. Phenotypically, PMA-treated HMVEC showed increased invasion, tube formation, and growth factor-stimulated proliferation. A flow cytometric analysis of cell surface markers showed that PMA-treated HMVEC retained endothelial characteristics. Infection of HMVEC with an adenovirus expressing PRL-3 resulted in increased tube formation. In tumor cells, PRL-3 mRNA levels varied markedly with high expression in SKNAS neuroblastoma, MCF-7 and BT474 breast carcinoma, Hep3B hepatocellular carcinoma, and HCT116 colon carcinoma. Western blotting analysis of a subset of cell line lysates showed a positive correlation between PRL-3 mRNA and protein levels. PRL-3 was stably transfected into DLD-1 colon cancer cells. PRL-3-overexpressing DLD-1 subclones were assessed for doubling time and invasion. Although doubling time was similar among parental, empty vector, and PRL-3 subclones, invasion was increased in PRL-3-expressing subclones. In models of endogenous expression, we observed that the MCF-7 cell line, which expresses high levels of PRL-3, was more invasive than the SKBR3 cell line, which expresses low levels of PRL-3. However, the MDA-MB-231 cell line was highly invasive with low levels of PRL-3, suggesting that in some models invasion is PRL-3 independent. Transfection of a PRL-3 small interfering RNA into MCF-7 cells inhibited PRL-3 expression and cell invasion. These results indicate that PRL-3 is functional in both endothelial cells and malignant cells and further validate PRL-3 as a potentially important molecular target for anticancer therapy.

[Proliferation and hTERT expression in neuroblastoma]

BACKGROUND

Transcription of the catalytic subunit of telomerase, human Telomerase Reverse Transcriptase (hTERT), and increased tumor cell proliferation are powerful prognostic factors in neuroblastoma. We therefore investigated their relationship in a large group of neuroblastomas.

METHODS

RT-PCR analysis was used to discriminate between the various hTERT transcripts. Tumor cell proliferation was assessed immunohistochemically using two different cell-cycle specific antibodies and the results were compared by statistical analysis.

RESULTS AND CONCLUSIONS

54 out of 115 neuroblastomas showed hTERT transcripts, 25 of which also possessed full-length transcripts. Full-length hTERT transcripts were correlated with MYCN-amplification, with a Ki67-proliferation index > or = 25% and a repp86-proliferation index > or = 10% (p<0,0001), but only a Ki67-proliferation index > or = 25% was associated with general hTERT transcription (p=0,001). Our data confirm the close relationship between hTERT transcription and tumor cell proliferation and further strengthen the exceptional prognostic power of the repp86-proliferation index.

Repp86 expression and outcome in patients with neuroblastoma

PURPOSE

Given the well-known challenges of neuroblastoma prognosis, we investigated whether the expression of restrictedly expressed proliferation-associated protein of 86 kDa theoretical molecular mass (repp86), a proliferation-associated protein expressed in S, G2, and M phases of the cell cycle, correlates with the clinical outcome in patients with neuroblastoma.

PATIENTS AND METHODS

161 children with different stages of neuroblastoma were studied; the median follow-up time was 72.8 months. The patients were staged according to the International Neuroblastoma Staging System, and histologic grading of the tumors was performed according to the criteria of Hughes and those of the International Neuroblastoma Pathology Classification. The MYCN gene copy number was determined by Southern blot analysis or fluorescence in situ-hybridization, and repp86 expression was assessed immunohistochemically by means of monoclonal antibody Ki-S2 on paraffin sections from archival tumor samples.

RESULTS

A repp86 labeling index (RI) of more than 10% positive tumor cells significantly predicted a shortened disease-free interval and an increased tumor mortality (both P <.0001). Moreover, the RI allowed the identification of patients with favorable and adverse prognosis in subsets defined by stage, grade, age, and MYCN status. In a multivariate analysis, the RI emerged as the most important predictor of event-free and disease-specific survival with hazard ratios of 11.7 and 10.5, respectively (both P <.0001).

CONCLUSION

It seems that repp86 expression is closely associated with the biologic behavior of neuroblastoma. Assessment of the RI might, therefore, considerably refine prognostic models.

The relationship between aging and carcinogenesis: a critical appraisal

The incidence of cancer increases with age in humans and in laboratory animals alike. There are different patterns of age-related distribution of tumors in different organs and tissues. Aging may increase or decrease the susceptibility of various tissues to initiation of carcinogenesis and usually facilitates promotion and progression of carcinogenesis. Aging may predispose to cancer by several mechanisms: (1) tissue accumulation of cells in late stages of carcinogenesis; (2) alterations in homeostasis, in particular, alterations in immune and endocrine system and (3) telomere instability linking aging and increased cancer risk. Increased susceptibility to the effects of tumor promoters is found both in aged animals and aged humans, as predicted by the multistage model of carcinogenesis. Available evidence supporting the relevance of replicative senescence of human cells and telomere biology to human cancer seems quite strong, however, the evidence linking cellular senescence to human aging is controversial and required additional studies. Data on the acceleration of aging by carcinogenic agents as well as on increased cancer risk in patients with premature aging are critically discussed. In genetically modified mouse models (transgenic, knockout or mutant) characterized by the aging delay, the incidence of tumors usually similar to those in controls, whereas the latent period of tumor development is increased. Practically all models of accelerated of aging in genetically modified animals show the increase in the incidence and the reduction in the latency of tumors. Strategies for cancer prevention must include not only measures to minimize exposure to exogenous carcinogenic agents, but also measures to normalize the age-related alterations in internal milieu. Life-span prolonging drugs (geroprotectors) may either postpone population aging and increase of tumor latency or decrease the mortality in long-living individuals in populations and inhibit carcinogenesis. At least some geroprotectors may increase the survival of a short-living individuals in populations but increase the incidence of malignancy.

Review article: Pathogenesis of sporadic and familial gastric cancer--implications for clinical management and cancer prevention

Gastric cancer remains a great clinical challenge despite its decreasing incidence. While major progress has been achieved in the understanding of the pathogenesis and molecular biology of sporadic gastric cancer, only recently has the role of familial aggregation of gastric cancers been rediscovered. The genetic changes underlying sporadic and familial gastric cancer have been revealed, and recent studies indicate that this familial aggregation combines genetic and microbiological aspects. Thus, for the prevention of gastric cancers these findings might be helpful for the early diagnosis and for the screening of risk groups and family members.