Mice without telomerase: what can they teach us about human cancer?

Cellular senescence in cancers: relationship between bone marrow cancer and cellular senescence

INTRODUCTION

There are many factors and conditions that lead to cellular senescence. Replicative senescence and Hayflick phenomenon are the most important causes of cellular senescence. Senescent cells also lead to wound healing conditions resulting from injury and toxic conditions.

MATERIAL AND METHODS

When a cell becomes senescent, it stops replication and begins to leak inflammatory signals before growth. It also alters the extracellular matrix and behavior of neighbor cells and even motivates them. This review was conducted to determine the association between senescence and bone marrow cancer.

RESULTS

The results showed that senescent cells have a short life span due to their self-destructive nature or natural removal from the body by the immune system. These signals are effective to a certain extent in regenerating the damaged cells when present in a transient state. Cellular senescence can decrease the risk of all cancers, including bone marrow cancer, ensuring that cells with significant DNA injury are prevented from replication.

CONCLUSION

However, senescent cells increase in number as they age, which is very harmful over time. These cells extend into an older tissue for longer periods of time and form longer clusters in older tissues. Therefore, cellular senescence significantly contributes to aging.

Aging of the progenitor cells that initiate prostate cancer

Many organs experience a loss of tissue mass and a decline in regenerative capacity during aging. In contrast, the prostate continues to grow in volume. In fact, age is the most important risk factor for prostate cancer. However, the age-related factors that influence the composition, morphology and molecular features of prostate epithelial progenitor cells, the cells-of-origin for prostate cancer, are poorly understood. Here, we review the evidence that prostate luminal progenitor cells are expanded with age. We explore the age-related changes to the microenvironment that may influence prostate epithelial cells and risk of transformation. Finally, we raise a series of questions about models of aging and regulators of prostate aging which need to be addressed. A fundamental understanding of aging in the prostate will yield critical insights into mechanisms that promote the development of age-related prostatic disease.

Characterization of tree shrew telomeres and telomerase

The use of tree shrews as experimental animals for biomedical research is a new practice. Several recent studies suggest that tree shrews are suitable for studying cancers, including breast cancer, glioblastoma, lung cancer, and hepatocellular carcinoma. However, the telomeres and the telomerase of tree shrews have not been studied to date. Here, we characterize telomeres and telomerase in tree shrews. The telomere length of tree shrews is approximately 23 kb, which is longer than that of primates and shorter than that of mice, and it is extended in breast tumor tissues according to Southern blot and flow-fluorescence in situ hybridization (FISH) analyses. Tree shrew spleen, bone marrow, testis, ovary, and uterus show high telomerase activities, which are increased in breast tumor tissues by telomeric repeat amplification protocol assays. The telomere length becomes shorter, and telomerase activity decreases with age. The tree shrew TERT and TERC are more highly similar to primates than to rodents. These findings lay a solid foundation for using tree shrews to study aging and cancers.

mRNA-Enhanced Cell Therapy and Cardiovascular Regeneration

mRNA has emerged as an important biomolecule in the global call for the development of therapies during the COVID-19 pandemic. Synthetic in vitro-transcribed (IVT) mRNA can be engineered to mimic naturally occurring mRNA and can be used as a tool to target "undruggable" diseases. Recent advancement in the field of RNA therapeutics have addressed the challenges inherent to this drug molecule and this approach is now being applied to several therapeutic modalities, from cancer immunotherapy to vaccine development. In this review, we discussed the use of mRNA for stem cell generation or enhancement for the purpose of cardiovascular regeneration.

TINF2 is a haploinsufficient tumor suppressor that limits telomere length

Telomere shortening is a presumed tumor suppressor pathway that imposes a proliferative barrier (the Hayflick limit) during tumorigenesis. This model predicts that excessively long somatic telomeres predispose to cancer. Here, we describe cancer-prone families with two unique TINF2 mutations that truncate TIN2, a shelterin subunit that controls telomere length. Patient lymphocyte telomeres were unusually long. We show that the truncated TIN2 proteins do not localize to telomeres, suggesting that the mutations create loss-of-function alleles. Heterozygous knock-in of the mutations or deletion of one copy of TINF2 resulted in excessive telomere elongation in clonal lines, indicating that TINF2 is haploinsufficient for telomere length control. In contrast, telomere protection and genome stability were maintained in all heterozygous clones. The data establish that the TINF2 truncations predispose to a tumor syndrome. We conclude that TINF2 acts as a haploinsufficient tumor suppressor that limits telomere length to ensure a timely Hayflick limit.

The secrets of telomerase: Retrospective analysis and future prospects

Telomerase plays a significant role to maintain and regulate the telomere length, cellular immortality and senescence by the addition of guanine-rich repetitive sequences. Chronic inflammation or oxidative stress-induced infection downregulates TERT gene modifying telomerase activity thus contributing to the early steps of gastric carcinogenesis process. Furthermore, telomere-telomerase system performs fundamental role in the pathogenesis and progression of diabetes mellitus as well as in its vascular intricacy. The cessation of cell proliferation in cultured cells by inhibiting the telomerase activity of transformed cells renders the rationale for culling of telomerase as a target therapy for the treatment of metabolic disorders and various types of cancers. In this article, we have briefly described the role of immune system and malignant cells in the expression of telomerase with critical analysis on the gaps and potential for future studies. The key findings regarding the secrets of the telomerase summarized in this article will help in future treatment modalities for the prevention of various types of cancers and metabolic disorders notably diabetes mellitus.

Withania somnifera (L.) Dunal: A potential therapeutic adjuvant in cancer

ETHNOPHARMACOLOGICAL RELEVANCE

Withania somnifera (L.) Dunal (WS) is one of the moststudied Rasayana botanicals used in Ayurveda practice for its immunomodulatory, anti-aging, adaptogenic, and rejuvenating effects. The botanical is being used for various clinical indications, including cancer. Several studies exploring molecular mechanisms of WS suggest its possible role in improving clinical outcomes in cancer management. Therefore, research on WS may offer new insights in rational development of therapeutic adjuvants for cancer.

AIM OF THIS REVIEW

The review aims at providing a detailed analysis of in silico, in vitro, in vivo, and clinical studies related to WS and cancer. It suggests possible role of WS in regulating molecular mechanisms associated with carcinogenesis. The review discusses potential of WS in cancer management in terms of cancer prevention, anti-cancer activity, and enhancing efficacy of cancer therapeutics.

MATERIAL AND METHODS

The present narrative review offers a critical analysis of published literature on WS studies in cancer. The reported studies were analysed in the context of pathophysiology of cancer, commonly referred as 'cancer hallmarks'. The review attempts to bridge Ayurveda knowledge with biological insights into molecular mechanisms of cancer.

RESULTS

Critical analysisof the published literature suggests an anti-cancer potential of WS with a key role in cancer prevention. The possible mechanisms for these effects are associated with the modulation of apoptotic, proliferative, and metastatic markers in cancer. WS can attenuate inflammatory responses and enzymes involved in invasion and metastatic progression of cancer.The properties of WS are likely to be mediated through withanolides, which may activate tumor suppressor proteins to restrict proliferation of cancer cells. Withanolides also regulate the genomic instability, and energy metabolism of cancer cells. The reported studies indicate the need for deeper understanding of molecular mechanisms of WS in inhibiting angiogenesis and promoting immunosurveillance. Additionally, WS can augment efficacy and safety of cancer therapeutics.

CONCLUSION

The experimentally-supported evidence of immunomodulatory, anti-cancer, adaptogenic, and regenerative attributes of WS suggest its therapeutic adjuvant potential in cancer management. The adjuvant properties of withanolides can modulate multidrug resistance and reverse chemotherapy-induced myelosuppression. These mechanisms need to be further explored in systematically designed translational and clinical studies that will pave the way for integration of WS as a therapeutic adjuvant in cancer management.

Mitochondrial dysfunction and oxidative stress in aging and cancer

Aging and cancer are the most important issues to research. The population in the world is growing older, and the incidence of cancer increases with age. There is no doubt about the linkage between aging and cancer. However, the molecular mechanisms underlying this association are still unknown. Several lines of evidence suggest that the oxidative stress as a cause and/or consequence of the mitochondrial dysfunction is one of the main drivers of these processes. Increasing ROS levels and products of the oxidative stress, which occur in aging and age-related disorders, were also found in cancer. This review focuses on the similarities between ageing-associated and cancer-associated oxidative stress and mitochondrial dysfunction as their common phenotype.

Meta-analysis of biomarkers predicting risk of malignant progression in Barrett's oesophagus

BACKGROUND

Barrett's oesophagus is a precursor to the development of oesophageal adenocarcinoma. This study sought to clarify the role of genetic, chromosomal and proliferation biomarkers that have been the subjects of multiple studies through meta-analysis.

METHODS

MEDLINE, Embase, PubMed and the Cochrane Library were searched for clinical studies assessing the value of p53, p16, Ki-67 and DNA content abnormalities in Barrett's oesophagus. The main outcome measure was the risk of development of high-grade dysplasia (HGD) or oesophageal adenocarcinoma.

RESULTS

Some 102 studies, with 12 353 samples, were identified. Mutation (diagnostic odds ratio (DOR) 10·91, sensitivity 47 per cent, specificity 92 per cent, positive likelihood ratio (PLR) 4·71, negative likelihood ratio (NLR) 0·65, area under the curve (AUC) 0·792) and loss (DOR 16·16, sensitivity 31 per cent, specificity 98 per cent, PLR 6·66, NLR 0·41, AUC 0·923) of p53 were found to be superior to the other p53 abnormalities (loss of heterozygosity (LOH) and overexpression). Ki-67 had high sensitivity in identifying high-risk patients (DOR 5·54, sensitivity 82 per cent, specificity 48 per cent, PLR 1·59, NLR 0·42, AUC 0·761). Aneuploidy (DOR 12·08, sensitivity 53 per cent, specificity 87 per cent, PLR 4·26, NLR 0·42, AUC 0·846), tetraploidy (DOR 5·87, sensitivity 46 per cent, specificity 85 per cent, PLR 3·47, NLR 0·65, AUC 0·793) and loss of Y chromosome (DOR 9·23, sensitivity 68 per cent, specificity 80 per cent, PLR 2·67, NLR 0·49, AUC 0·807) also predicted malignant development, but p16 aberrations (hypermethylation, LOH, mutation and loss) failed to demonstrate any advantage over the other biomarkers studied.

CONCLUSION

Loss and mutation of p53, and raised level of Ki-67 predicted malignant progression in Barrett's oesophagus.

Pot1 OB-fold mutations unleash telomere instability to initiate tumorigenesis

Chromosomal aberrations are a hallmark of human cancers, with complex cytogenetic rearrangements leading to genetic changes permissive for cancer initiation and progression. Protection of Telomere 1 (POT1) is an essential component of the shelterin complex and functions to maintain chromosome stability by repressing the activation of aberrant DNA damage and repair responses at telomeres. Sporadic and familial mutations in the oligosaccharide-oligonucleotide (OB) folds of POT1 have been identified in many human cancers, but the mechanism underlying how hPOT1 mutations initiate tumorigenesis has remained unclear. Here we show that the human POT1’s OB-folds are essential for the protection of newly replicated telomeres. Oncogenic mutations in hPOT1 OB-fold fail to bind to ss telomeric DNA, eliciting a DNA damage response at telomeres that promote inappropriate chromosome fusions via the mutagenic alternative non-homologous end joining (A-NHEJ) pathway. hPOT1 mutations also result in telomere elongation and the formation of transplantable hematopoietic malignancies. Strikingly, conditional deletion of both mPot1a and p53 in mouse mammary epithelium resulted in development of highly invasive breast carcinomas and the formation of whole chromosomes containing massive arrays of telomeric fusions reminiscent of chromothripsis. Our results reveal that hPOT1 OB-folds are required to protect and prevent newly replicated telomeres from engaging in A-NHEJ mediated fusions that would otherwise promote genome instability to fuel tumorigenesis.

Promoter hypermethylation of TERT is associated with hepatocellular carcinoma in the Han Chinese population

BACKGROUND

Upstream of the transcription start site (UTSS), hypermethylation of the telomerase reverse transcriptase (TERT) gene has been shown to be associated with tumour progression and a poor prognosis in paediatric brain tumours (Castelo-Branco 2013). It has been inferred that the UTSS region of TERT is a potentially accessible biomarker for various cancers. In this study, we aimed to explore the role of TERT in hepatocellular carcinoma (HCC) and to investigate whether the UTSS region of the TERT promoter shows the same methylation pattern in HCC.

METHODS

We analysed the results of a methylation assay for TERT, including the UTSS region, from 125 paired HCC samples using Mass Array EpiTyper (Sequenom, San Diego, CA, USA). To determine the relationship between TERT promoter methylation status and the TERT expression level, we analysed a validation group of 12 paired HCC samples and acquired the FPKM values for the TERT gene.

RESULTS

Our results showed aberrant methylation of the UTSS region of the TERT promoter in HCC (mean=15.1) compared with the adjacent normal tissues (mean=6.1, P<0.00001). Furthermore, a nearly 56-fold increase in TERT expression from the hypermethylated promoter was found in HCC (P<0.05), indicating a positive relationship between TERT methylation and expression.

CONCLUSIONS

As hypermethylation was positively correlated with high expression of TERT in HCC, TERT is likely to be involved in the aetiology of HCC. Our findings indicate that future studies on TERT might be fruitful.

Role of nicotine dependence on the relationship between variants in the nicotinic receptor genes and risk of lung adenocarcinoma

Several variations in the nicotinic receptor genes have been identified to be associated with both lung cancer risk and smoking in the genome-wide association (GWA) studies. However, the relationships among these three factors (genetic variants, nicotine dependence, and lung cancer) remain unclear. In an attempt to elucidate these relationships, we applied mediation analysis to quantify the impact of nicotine dependence on the association between the nicotinic receptor genetic variants and lung adenocarcinoma risk. We evaluated 23 single nucleotide polymorphisms (SNPs) in the five nicotinic receptor related genes (CHRNB3, CHRNA6, and CHRNA5/A3/B4) previously reported to be associated with lung cancer risk and smoking behavior and 14 SNPs in the four 'control' genes (TERT, CLPTM1L, CYP1A1, and TP53), which were not reported in the smoking GWA studies. A total of 661 lung adenocarcinoma cases and 1,347 controls with a smoking history, obtained from the Environment and Genetics in Lung Cancer Etiology case-control study, were included in the study. Results show that nicotine dependence is a mediator of the association between lung adenocarcinoma and gene variations in the regions of CHRNA5/A3/B4 and accounts for approximately 15% of this relationship. The top two CHRNA3 SNPs associated with the risk for lung adenocarcinoma were rs1051730 and rs12914385 (p-value = 1.9×10(-10) and 1.1×10(-10), respectively). Also, these two SNPs had significant indirect effects on lung adenocarcinoma risk through nicotine dependence (p = 0.003 and 0.007). Gene variations rs2736100 and rs2853676 in TERT and rs401681 and rs31489 in CLPTM1L had significant direct associations on lung adenocarcinoma without indirect effects through nicotine dependence. Our findings suggest that nicotine dependence plays an important role between genetic variants in the CHRNA5/A3/B4 region, especially CHRNA3, and lung adenocarcinoma. This may provide valuable information for understanding the pathogenesis of lung adenocarcinoma and for conducting personalized smoking cessation interventions.

Telomere dysfunction and hematologic disorders

Aplastic anemia is a disease in which the hematopoietic stem cell fails to adequately produce peripheral blood cells, causing pancytopenia. In some cases of acquired aplastic anemia and in inherited type of aplastic anemia, dyskeratosis congenita, telomere biology gene mutations and telomere shortening are etiologic. Telomere erosion hampers the ability of hematopoietic stem and progenitor cells to adequately replicate, clinically resulting in bone marrow failure. Additionally, telomerase mutations and short telomeres are genetic risk factors for the development of some hematologic cancers, including myelodysplastic syndrome, acute myeloid leukemia, and chronic lymphocytic leukemia.

The molecular basis for the pharmacokinetics and pharmacodynamics of curcumin and its metabolites in relation to cancer

This review addresses the oncopharmacological properties of curcumin at the molecular level. First, the interactions between curcumin and its molecular targets are addressed on the basis of curcumin's distinct chemical properties, which include H-bond donating and accepting capacity of the β-dicarbonyl moiety and the phenylic hydroxyl groups, H-bond accepting capacity of the methoxy ethers, multivalent metal and nonmetal cation binding properties, high partition coefficient, rotamerization around multiple C-C bonds, and the ability to act as a Michael acceptor. Next, the in vitro chemical stability of curcumin is elaborated in the context of its susceptibility to photochemical and chemical modification and degradation (e.g., alkaline hydrolysis). Specific modification and degradatory pathways are provided, which mainly entail radical-based intermediates, and the in vitro catabolites are identified. The implications of curcumin's (photo)chemical instability are addressed in light of pharmaceutical curcumin preparations, the use of curcumin analogues, and implementation of nanoparticulate drug delivery systems. Furthermore, the pharmacokinetics of curcumin and its most important degradation products are detailed in light of curcumin's poor bioavailability. Particular emphasis is placed on xenobiotic phase I and II metabolism as well as excretion of curcumin in the intestines (first pass), the liver (second pass), and other organs in addition to the pharmacokinetics of curcumin metabolites and their systemic clearance. Lastly, a summary is provided of the clinical pharmacodynamics of curcumin followed by a detailed account of curcumin's direct molecular targets, whereby the phenotypical/biological changes induced in cancer cells upon completion of the curcumin-triggered signaling cascade(s) are addressed in the framework of the hallmarks of cancer. The direct molecular targets include the ErbB family of receptors, protein kinase C, enzymes involved in prostaglandin synthesis, vitamin D receptor, and DNA.

MITF, the Janus transcription factor of melanoma

Current models of melanoma propose that transition from the proliferative to the invasive stages of tumor development involves a dynamic and reversible switch in cell phenotype. The almost mutually exclusive proliferative and invasive phenotypes are defined by distinct gene expression signatures, which are themselves controlled by the level of functional MITF protein present in the cell. Recently, new signaling pathways and transcription factors that regulate MITF expression have been defined, and high throughput genomics have identified novel MITF target genes. MITF acts both as a transcription activator to promote expression of genes involved in cell cycle, but also as a transcriptional repressor of genes involved in invasion. A novel human germline mutation in MITF has been identified that blocks its sumoylation, thereby altering its transcriptional properties and conferring an increased risk of melanoma. These new studies depict an ever more complex function for MITF in melanoma.

The peculiar biology of mouse mesenchymal stromal cells--oxygen is the key

Because of the ability to manipulate their genome, mice are the experimental tool of choice for many areas of scientific investigation. Moreover, established experimental mouse models of human disease are widely available and offer a valuable resource to obtain proof-of-concept for many cell-based therapies. Nevertheless, efforts to establish reliable methods to isolate mesenchymal stromal cells (MSCs) from mouse bone marrow have been elusive. Indeed, a variety of physical and genetic approaches have been described to fractionate MSCs from other cell lineages in bone marrow, but few have achieved high yields or purity while maintaining the genomic integrity of the cells. We provide a historic overview of published procedures dedicated to the isolation of mouse MSCs from bone marrow and compact bone. We also review current findings indicating that growth-restrictive conditions imposed by atmospheric oxygen promotes immortalization of mouse MSCs and how expansion in a low-oxygen environment enhances cell yields and maintains genomic stability. Finally, we provide basic recommendations for isolating primary mouse MSCs and discuss potential pitfalls associated with these isolation methods.

hTERT, MYC and TP53 deregulation in gastric preneoplastic lesions

Background

Gastric cancer is a serious public health problem in Northern Brazil and in the world due to its high incidence and mortality. Despite the severity of the disease, more research is needed to better understand the molecular events involved in this intestinal-type gastric carcinogenesis process. Since precancerous lesions precede intestinal-type gastric cancer, here, we evaluated the hTERT, MYC, and TP53 mRNA and protein expression, as well as TP33 copy number, in gastric preneoplastic lesions.

Methods

We evaluated 19 superficial gastritis, 18 atrophic gastritis, and 18 intestinal metaplasia from cancer-free individuals of Northern Brazil. Quantitative reverse transcription PCR was used to analyze the mRNA expression and immunohistochemical methods were used to assess protein immunoreactivity in tissue samples. The number of TP53 gene copies was investigated in gastric diseases by quantitative PCR.

Results

We observed hTERT, MYC, and p53 immunoreactivity only in intestinal metaplasia samples. The immunoreactivity of these proteins was strongly associated with each other. A significantly higher MYC mRNA expression was observed in intestinal metaplasia compared to gastritis samples. Loss of TP53 was also only detected in intestinal metaplasia specimens.

Conclusions

We demonstrated that hTERT, MYC, and TP53 are deregulated in intestinal metaplasia of individuals from Northern Brazil and these alterations may facilitate tumor initiation.

Cooperation between p53 and the telomere-protecting shelterin component Pot1a in endometrial carcinogenesis

Type II endometrial cancer (EMCA) represents only 10% of all EMCAs, but accounts for 40% of EMCA-related mortality. Previous studies of human tumors have shown an association between Type II tumors and damaged telomeres. We hypothesized that the lack of murine Type II EMCA models is due to the extremely long telomeres in laboratory mouse strains. We previously showed that telomerase-null mice with critically short telomeres developed endometrial lesions histologically resembling endometrial intraepithelial carcinoma (EIC), the accepted precursor for Type II EMCA. However, these mice did not develop invasive endometrial adenocarcinoma, and instead succumbed prematurely to multi-organ failure. Here, we modeled critical telomere attrition by conditionally inactivating Pot1a, a component of the shelterin complex that stabilizes telomeres, within endometrial epithelium. Inactivation of Pot1a by itself did not stimulate endometrial carcinogenesis, and did not result in detectable DNA damage or apoptosis in endometrium. However, simultaneous inactivation of Pot1a and p53 resulted in EIC-like lesions by 9 months indistinguishable from those seen in late generation telomerase-null mice. These lesions progressed to invasive endometrial adenocarcinomas as early as 9 months of age with metastatic disease in 100% of the animals by 15 months. These tumors were poorly differentiated endometrial adenocarcinomas with prominent nuclear atypia, resembling human Type II cancers. Furthermore, these tumors were aneuploid with double-stranded DNA breaks and end-to-end telomere fusions and most were tetraploid or near-tetraploid. These studies lend further support to the hypothesis that telomeric instability has a critical role in Type II endometrial carcinogenesis and provides an intriguing in-vivo correlate to recent studies implicating telomere-dependent tetraploidization as an important mechanism in carcinogenesis.

Loss of p16(Ink4a) function rescues cellular senescence induced by telomere dysfunction

p16^Ink4a is a tumor suppressor and a marker for cellular senescence. Previous studies have shown that p16^Ink4a plays an important role in the response to DNA damage signals caused by telomere dysfunction. In this study, we crossed Wrn^−/− and p16^Ink4a−/− mice to knock out the p16^Ink4a function in a Wrn null background. Growth curves showed that loss of p16^Ink4a could rescue the growth barriers that are observed in Wrn^−/− mouse embryonic fibroblasts (MEFs). By challenging the MEFs with the global genotoxin doxorubicin, we showed that loss of p16^Ink4a did not dramatically affect the global DNA damage response of Wrn^−/− MEFs induced by doxorubicin. However, in response to telomere dysfunction initiated by the telomere damaging protein TRF2^ΔBΔM, loss of p16^Ink4a could partially overcome the DNA damage response by disabling p16^Ink4a up-regulation and reducing the accumulation of γ-H2AX that is observed in Wrn^−/− MEFs. Furthermore, in response to TRF2^ΔBΔM overexpression, Wrn^−/− MEFs senesced within several passages. In contrast, p16^Ink4a−/− and p16^Ink4a−/−Wrn^−/− MEFs could continuously grow and lose expression of the exogenous TRF2^ΔBΔM in their late passages. In summary, our data suggest that in the context of telomere dysfunction, loss of p16^Ink4a function could prevent cells from senescence. These results shed light on the anti-aging strategy through regulation of p16^Ink4a expression.

Hallmarks of cancer: the next generation

The hallmarks of cancer comprise six biological capabilities acquired during the multistep development of human tumors. The hallmarks constitute an organizing principle for rationalizing the complexities of neoplastic disease. They include sustaining proliferative signaling, evading growth suppressors, resisting cell death, enabling replicative immortality, inducing angiogenesis, and activating invasion and metastasis. Underlying these hallmarks are genome instability, which generates the genetic diversity that expedites their acquisition, and inflammation, which fosters multiple hallmark functions. Conceptual progress in the last decade has added two emerging hallmarks of potential generality to this list-reprogramming of energy metabolism and evading immune destruction. In addition to cancer cells, tumors exhibit another dimension of complexity: they contain a repertoire of recruited, ostensibly normal cells that contribute to the acquisition of hallmark traits by creating the "tumor microenvironment." Recognition of the widespread applicability of these concepts will increasingly affect the development of new means to treat human cancer.

Essential role of microphthalmia transcription factor for DNA replication, mitosis and genomic stability in melanoma

Malignant melanoma is an aggressive cancer known for its notorious resistance to most current therapies. The basic helix-loop-helix microphthalmia transcription factor (MITF) is the master regulator determining the identity and properties of the melanocyte lineage, and is regarded as a lineage-specific 'oncogene' that has a critical role in the pathogenesis of melanoma. MITF promotes melanoma cell proliferation, whereas sustained supression of MITF expression leads to senescence. By combining chromatin immunoprecipitation coupled to high throughput sequencing (ChIP-seq) and RNA sequencing analyses, we show that MITF directly regulates a set of genes required for DNA replication, repair and mitosis. Our results reveal how loss of MITF regulates mitotic fidelity, and through defective replication and repair induces DNA damage, ultimately ending in cellular senescence. These findings reveal a lineage-specific control of DNA replication and mitosis by MITF, providing new avenues for therapeutic intervention in melanoma. The identification of MITF-binding sites and gene-regulatory networks establish a framework for understanding oncogenic basic helix-loop-helix factors such as N-myc or TFE3 in other cancers.

Cellular senescence as a target in cancer control

Somatic cells show a spontaneous decline in growth rate in continuous culture. This is not related to elapsed time but to an increasing number of population doublings, eventually terminating in a quiescent but viable state termed replicative senescence. These cells are commonly multinucleated and do not respond to mitogens or apoptotic stimuli. Cells displaying characteristics of senescent cells can also be observed in response to other stimuli, such as oncogenic stress, DNA damage, or cytotoxic drugs and have been reported to be found in vivo. Most tumors show unlimited replicative potential, leading to the hypothesis that cellular senescence is a natural antitumor program. Recent findings suggest that cellular senescence is a natural mechanism to prevent undesired oncogenic stress in somatic cells that has been lost in malignant tumors. Given that the ultimate goal of cancer research is to find the definitive cure for as many tumor types as possible, exploration of cellular senescence to drive towards antitumor therapies may decisively influence the outcome of new drugs. In the present paper, we will review the potential of cellular senescence to be used as target for anticancer therapy.

Physical exercise prevents cellular senescence in circulating leukocytes and in the vessel wall

BACKGROUND

The underlying molecular mechanisms of the vasculoprotective effects of physical exercise are incompletely understood. Telomere erosion is a central component of aging, and telomere-associated proteins regulate cellular senescence and survival. This study examines the effects of exercising on vascular telomere biology and endothelial apoptosis in mice and the effects of long-term endurance training on telomere biology in humans.

METHODS AND RESULTS

C57/Bl6 mice were randomized to voluntary running or no running wheel conditions for 3 weeks. Exercise upregulated telomerase activity in the thoracic aorta and in circulating mononuclear cells compared with sedentary controls, increased vascular expression of telomere repeat-binding factor 2 and Ku70, and reduced the expression of vascular apoptosis regulators such as cell-cycle-checkpoint kinase 2, p16, and p53. Mice preconditioned by voluntary running exhibited a marked reduction in lipopolysaccharide-induced aortic endothelial apoptosis. Transgenic mouse studies showed that endothelial nitric oxide synthase and telomerase reverse transcriptase synergize to confer endothelial stress resistance after physical activity. To test the significance of these data in humans, telomere biology in circulating leukocytes of young and middle-aged track and field athletes was analyzed. Peripheral blood leukocytes isolated from endurance athletes showed increased telomerase activity, expression of telomere-stabilizing proteins, and downregulation of cell-cycle inhibitors compared with untrained individuals. Long-term endurance training was associated with reduced leukocyte telomere erosion compared with untrained controls.

CONCLUSIONS

Physical activity regulates telomere-stabilizing proteins in mice and in humans and thereby protects from stress-induced vascular apoptosis.

Bladder cancer in the elderly

INTRODUCTION

Age is now widely accepted as the greatest single risk factor for developing bladder cancer, and bladder cancer is considered as primarily a disease of the elderly. Because of the close link between age and incidence of bladder cancer, it can be expected that this disease will become an enormous challenge with the growth of an aging population in the years ahead.

METHODS

Using MEDLINE, a search of the literature between January 1966 and July 2007 was performed to describe normative physiologic changes associated with aging, elucidate genetic and epigenetic alterations that associate aging with bladder cancer and its phenotypes; and to characterize how aging influences efficacies, risks, side effects, and potential complications of the treatments needed for the various stages of bladder cancer.

RESULTS

We discuss influence of aging on host physiology, genetic and epigenetic changes, environmental influences, and host factors in the development and treatment of bladder cancer. Treatments with intravesical bacille Calmette Guerin, radical cystectomy, and perioperative chemotherapy are less well tolerated and have poorer response in elderly patients compared with their younger counterparts. Elderly patients face both clinical and broader institutional barriers to appropriate treatment and may receive less aggressive treatment and sub-therapeutic dosing. However, when appropriately selected, elderly patients tolerate and respond well to cancer treatments.

CONCLUSIONS

The decision to undergo treatment for cancer is a tradeoff between loss of function and/or independence and extension of life, which is complicated by a host of concomitant issues such as comorbid medical conditions, functional declines and "frailty", family dynamics, and social and psychologic issues. Chronological age should not preclude definitive surgical therapy. It is imperative that healthcare practitioners and researchers from disparate disciplines collectively focus efforts towards gaining a better understanding of what the consequences of bladder cancer and its treatments are for older adults and how to appropriately meet the multifaceted medical and psychosocial needs of this growing population.

Constitutive telomere length and gastric cancer risk: case-control analysis in Chinese Han population

The shortening of telomeres may result in chromosome instability and thus promote tumorigenesis. Previous studies have demonstrated clear involvement of telomere shortening in the carcinogenesis of several malignancies. However, the association between constitutive telomere shortening and gastric cancer development has yet to be established. Therefore, in the present study, we measured average telomere length using quantitative real-time PCR in peripheral blood lymphocytes from a gastric cancer (GC) case-control study consisting of 396 cases and 378 controls. The results showed that GC patients had significantly shorter average telomere length than matched controls (mean +/- SD 0.89 +/- 0.19 vs 1.06 +/- 0.25, P < 0.001). We further categorized telomere length using the 50% value in the controls as a cut-off point and assessed the association between telomere length and GC risk using multivariate logistic regression analysis. We found that short telomere length was associated with a significantly increased GC risk (adjusted odds ratio = 2.14, 95% confidence interval = 1.52-2.93). Quartile stratification revealed a dose-response relationship between telomere shortening and GC risk (P for trend < 0.001). Stratified analysis showed that sex, age, and alcohol drinking, but not smoking and Helicobacter pylori infection, seem to have a modulating effect on the average telomere length in both cases and controls. We also found that telomere shortening and smoking had a significant joint effect on GC risk. Collectively, our findings provide the first evidence linking the short telomere length in peripheral blood lymphocytes to elevated GC risk, which warrants further investigation in other populations.

Genetically modified mouse models in cancer studies

Genetically modified animals represent a resource of immense potential for cancer research. Classically, genetic modifications in mice were obtained through selected breeding experiments or treatments with powerful carcinogens capable of inducing random mutagenesis. A new era began in the early 1980s when genetic modifications by inserting foreign DNA genes into the cells of an animal allowed for the development of transgenic mice. Since that moment, genetic modifications have been able to be made in a predetermined way. Gene targeting emerged later as a method of in vivo mutagenesis whereby the sequence of a predetermined gene is selectively modified within an intact cell. In this review we focus on how genetically modified mice can be created to study tumour development, and how these models have contributed to an understanding of the genetic alterations involved in human cancer. We also discuss the strengths and weaknesses of the different mouse models for identifying cancer genes, and understanding the consequences of their alterations in order to obtain the maximum benefit for cancer patients.

Effects of physical exercise on myocardial telomere-regulating proteins, survival pathways, and apoptosis

OBJECTIVES

The purpose of this study was to study the underlying molecular mechanisms of the protective cardiac effects of physical exercise.

BACKGROUND

Telomere-regulating proteins affect cellular senescence, survival, and regeneration.

METHODS

C57/Bl6 wild-type, endothelial nitric oxide synthase (eNOS)-deficient and telomerase reverse transcriptase (TERT)-deficient mice were randomized to voluntary running or no running wheel conditions (n = 8 to 12 per group).

RESULTS

Short-term running (21 days) up-regulated cardiac telomerase activity to >2-fold of sedentary controls, increased protein expression of TERT and telomere repeat binding factor (TRF) 2, and reduced expression of the proapoptotic mediators cell-cycle-checkpoint kinase 2 (Chk2), p53, and p16. Myocardial and leukocyte telomere length did not differ between 3-week- and 6-month-old sedentary or running mice, but telomerase activity, TRF2 and TERT expression were persistently increased after 6 months and the expression of Chk2, p53, and p16 remained down-regulated. The exercise-induced changes were absent in both TERT(-/-) and eNOS(-/-) mice. Running increased cardiac expression of insulin-like growth factor (IGF)-1. Treatment with IGF-1 up-regulated myocardial telomerase activity >14-fold and increased the expression of phosphorylated Akt protein kinase and phosphorylated eNOS. To test the physiologic relevance of these exercise-mediated prosurvival pathways, apoptotic cardiomyopathy was induced by treatment with doxorubicin. Up-regulation of telomere-stabilizing proteins by physical exercise in mice reduced doxorubicin-induced p53 expression and potently prevented cardiomyocyte apoptosis in wild-type, but not in TERT(-/-) mice.

CONCLUSIONS

Long- and short-term voluntary physical exercise up-regulates cardiac telomere-stabilizing proteins and thereby induces antisenescent and protective effects, for example, to prevent doxorubicin-induced cardiomyopathy. These beneficial cardiac effects are mediated by TERT, eNOS, and IGF-1.

Understanding and exploiting hTERT promoter regulation for diagnosis and treatment of human cancers

Telomerase activation is a critical step for human carcinogenesis through the maintenance of telomeres, but the activation mechanism during carcinogenesis remains unclear. Transcriptional regulation of the human telomerase reverse transcriptase (hTERT) gene is the major mechanism for cancer-specific activation of telomerase, and a number of factors have been identified to directly or indirectly regulate the hTERT promoter, including cellular transcriptional activators (c-Myc, Sp1, HIF-1, AP2, ER, Ets, etc.) as well as the repressors, most of which comprise tumor suppressor gene products, such as p53, WT1, and Menin. Nevertheless, none of them can clearly account for the cancer specificity of hTERT expression. The chromatin structure via the DNA methylation or modulation of nucleosome histones has recently been suggested to be important for regulation of the hTERT promoter. DNA unmethylation or histone methylation around the transcription start site of the hTERT promoter triggers the recruitment of histone acetyltransferase (HAT) activity, allowing hTERT transcription. These facts prompted us to apply these regulatory mechanisms to cancer diagnostics and therapeutics. Telomerase-specific replicative adenovirus (Telomelysin, OBP-301), in which E1A and E1B genes are driven by the hTERT promoter, has been developed as an oncolytic virus that replicates specifically in cancer cells and causes cell death via viral toxicity. Direct administration of Telomelysin was proved to effectively eradicate solid tumors in vivo, without apparent adverse effects. Clinical trials using Telomelysin for cancer patients with progressive stages are currently ongoing. Furthermore, we incorporated green fluorescent protein gene (GFP) into Telomelysin (TelomeScan, OBP-401). Administration of TelomeScan into the primary tumor enabled the visualization of cancer cells under the cooled charged-coupled device (CCD) camera, not only in primary tumors but also the metastatic foci. This technology can be applied to intraoperative imaging of metastatic lymphnodes. Thus, we found novel tools for cancer diagnostics and therapeutics by utilizing the hTERT promoter.

Melanomagenesis: overcoming the barrier of melanocyte senescence

Although melanoma ultimately progresses to a highly aggressive and metastatic disease that is typically resistant to currently available therapy, it often begins as a benign nevus consisting of a clonal population of hyperplastic melanocytes that cannot progress because they are locked in a state of cellular senescence. Once senescence is overcome, the nevus can exhibit dysplastic features and readily progress to more lethal stages. Recent advances have convincingly demonstrated that senescence represents a true barrier to the progression of many types of cancer, including melanoma. Thus, understanding the mechanism(s) by which melanoma evades senescence has become a priority in the melanoma research community. Senescence in most cells is regulated through some combination of activities within the RB and p53 pathways. However, differences discovered among various tumor types, some subtle and others quite profound, have revealed that senescence frequently operates in a context-dependent manner. Here we review what is known about melanocyte senescence, and how such knowledge may provide a much-needed edge in our struggles to contain or perhaps vanquish this often-fatal malignancy.

Translating insights from the cancer genome into clinical practice

Cancer cells have diverse biological capabilities that are conferred by numerous genetic aberrations and epigenetic modifications. Today's powerful technologies are enabling these changes to the genome to be catalogued in detail. Tomorrow is likely to bring a complete atlas of the reversible and irreversible alterations that occur in individual cancers. The challenge now is to work out which molecular abnormalities contribute to cancer and which are simply 'noise' at the genomic and epigenomic levels. Distinguishing between these will aid in understanding how the aberrations in a cancer cell collaborate to drive pathophysiology. Past successes in converting information from genomic discoveries into clinical tools provide valuable lessons to guide the translation of emerging insights from the genome into clinical end points that can affect the practice of cancer medicine.

Experimental models of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a common and deadly cancer whose pathogenesis is incompletely understood. Comparative genomic studies from human HCC samples have classified HCCs into different molecular subgroups; yet, the unifying feature of this tumor is its propensity to arise upon a background of inflammation and fibrosis. This review seeks to analyze the available experimental models in HCC research and to correlate data from human populations with them in order to consolidate our efforts to date, as it is increasingly clear that different models will be required to mimic different subclasses of the neoplasm. These models will be instrumental in the evaluation of compounds targeting specific molecular pathways in future preclinical studies.

Telomere maintenance and human bone marrow failure

Acquired and congenital aplastic anemias recently have been linked molecularly and pathophysiologically by abnormal telomere maintenance. Telomeres are repeated nucleotide sequences that cap the ends of chromosomes and protect them from damage. Telomeres are eroded with cell division, but in hematopoietic stem cells, maintenance of their length is mediated by telomerase. Accelerated telomere shortening is virtually universal in dyskeratosis congenita, caused by mutations in genes encoding components of telomerase or telomere-binding protein (TERT, TERC, DKC1, NOP10, or TINF2). About one-third of patients with acquired aplastic anemia also have short telomeres, which in some cases associate with TERT or TERC mutations. These mutations cause low telomerase activity, accelerated telomere shortening, and diminished proliferative capacity of hematopoietic progenitors. As in other genetic diseases, additional environmental, genetic, and epigenetic modifiers must contribute to telomere erosion and ultimately to disease phenotype. Short telomeres also may cause genomic instability and malignant progression in these marrow failure syndromes. Identification of short telomeres has potential clinical implications: it may be useful in dyskeratosis congenita diagnosis, in suggesting mutations in patients with acquired aplastic anemia, and for selection of suitable hematopoietic stem cell family donors for transplantation in telomerase-deficient patients.

Curcumin sensitizes TRAIL-resistant xenografts: molecular mechanisms of apoptosis, metastasis and angiogenesis

BACKGROUND

We have recently shown that curcumin (a diferuloylmethane, the yellow pigment in turmeric) enhances apoptosis-inducing potential of TRAIL in prostate cancer PC-3 cells, and sensitizes TRAIL-resistant LNCaP cells in vitro through multiple mechanisms. The objectives of this study were to investigate the molecular mechanisms by which curcumin sensitized TRAIL-resistant LNCaP xenografts in vivo.

METHODS

Prostate cancer TRAIL-resistant LNCaP cells were implanted in Balb c nude mice to examine the effects of curcumin and/or TRAIL on tumor growth and genes related to apoptosis, metastasis and angiogenesis.

RESULTS

Curcumin inhibited growth of LNCaP xenografts in nude mice by inducing apoptosis (TUNEL staining) and inhibiting proliferation (PCNA and Ki67 staining), and sensitized these tumors to undergo apoptosis by TRAIL. In xenogrfated tumors, curcumin upregulated the expression of TRAIL-R1/DR4, TRAIL-R2/DR5, Bax, Bak, p21/WAF1, and p27/KIP1, and inhibited the activation of NFkappaB and its gene products such as cyclin D1, VEGF, uPA, MMP-2, MMP-9, Bcl-2 and Bcl-XL. The regulation of death receptors and members of Bcl-2 family, and inactivation of NFkappaB may sensitize TRAIL-resistant LNCaP xenografts. Curcumin also inhibited number of blood vessels in tumors, and circulating endothelial growth factor receptor 2-positive endothelial cells in mice.

CONCLUSION

The ability of curcumin to inhibit tumor growth, metastasis and angiogenesis, and enhance the therapeutic potential of TRAIL suggests that curcumin alone or in combination with TRAIL can be used for prostate cancer prevention and/or therapy.

Factors impacting human telomere homeostasis and age-related disease

Loss of telomere length homeostasis has been linked to age-related disease especially cancer. In this review, we discuss two major causes of telomere dysfunction that potentially lead to tumorigenesis: replicative aging and environmental assaults. Aging has long been recognized as a source for telomere dysfunction through increasing numbers of cell divisions in the absence of sufficient telomerase activity. However, environmental assaults that cause telomere dysfunction are only beginning to be identified and recognized. Environmental stressors that influence telomere length may be physical or induced by psychological situations like stress. Knowledge of all factors, including genetic and environmental forces, that moderate telomere length will be critical for understanding basic mechanisms of human telomere maintenance during development and aging as well as for disease prevention and treatment strategies.

Hepatitis B virus X protein overcomes oncogenic RAS-induced senescence in human immortalized cells

Chronic infection with hepatitis B virus (HBV) is a major risk factor for hepatocellular carcinoma. The HBV X protein (HBx) is thought to have oncogenic potential, although the molecular mechanism remains obscure. Pathological roles of HBx in the carcinogenic process have been examined using rodent systems and no report is available on the oncogenic roles of HBx in human cells in vitro. We therefore examined the effect of HBx on immortalization and transformation in human primary cells. We found that HBx could overcome active RAS-induced senescence in human immortalized cells and that these cells could form colonies in soft agar and tumors in nude mice. HBx alone, however, could contribute to neither immortalization nor transformation of these cells. In a population doubling analysis, an N-terminal truncated mutant of HBx, HBx-D1 (amino acids 51-154), which harbors the coactivation domain, could overcome active RAS-induced cellular senescence, but these cells failed to exhibit colonigenic and tumorigenic abilities, probably due to the low expression level of the protein. By scanning a HBx expression library of the clustered-alanine substitution mutants, the N-terminal domain was found to be critical for overcoming active RAS-induced senescence by stabilizing full-length HBx. These results strongly suggest that HBx can contribute to carcinogenesis by overcoming active oncogene-induced senescence.

Science of Cancer and Aging

This review provides an overview of a selection of the most pertinent molecular pathways that link cancer and aging and focuses on those where recent advances were most important. When organizing the bulk of information on this subject, I became aware of the fact that the most evident partition, namely, mechanisms that influence aging and mechanisms that influence cancer occurrence, is difficult to apply. Most mechanisms explaining the aging process are also those that influence carcinogenesis. Mechanisms that are described in tumor suppressor pathways are also contributors to the aging process. From an intuitive point of view, there are phenomena that have traditionally been contributed to aging others to cancer-inducing factors and they are presented herein.

Human stem cells, chromatin, and tissue engineering: Boosting relevancy in developmental toxicity testing

Risk assessment derives its confidence from toxicology research that is based on relevancy to human health. This article focuses on two highly topical areas of current scientific research, stem cells and chromatin biology, which present new avenues for preclinical and clinical applications, and the frontier role of tissue engineering and regeneration. Appreciating the utility and necessity of chromatin and human somatic stem cells as research tools and looking toward tissue engineering may close the uncertainty gaps between animal and human cross-species toxicology evaluations. The focus will be on developmental toxicology applications, but appropriate extrapolation to any other areas of toxicology can be made. We further provide background on basic biology of these three areas and examples of how early life exposure to known and potential environmental toxicants induce malformations, childhood and adult-onset diseases, through aberrant chromatin modification of critical gene expressions (acute lymphocyte leukemia, heavy-metal nickel and cadmium-associated defects, and reproductive tract malformations and carcinomas induced by the synthetic estrogen, diethylstilbestrol).

Molecular functions and biological roles of hepatitis B virus x protein

Chronic infection of hepatitis B virus (HBV) is one of the major causes of hepatocellular carcinoma (HCC) in the world. Hepatitis B virus X protein (HBx) has been long suspected to be involved in hepatocarcinogenesis, although its oncogenic role remains controversial. HBx is a multifunctional regulator that modulates transcription, signal transduction, cell cycle progress, protein degradation pathways, apoptosis, and genetic stability by directly or indirectly interacting with host factors. This review focuses on the biological roles of HBx in HBV replication and cellular transformation in terms of the molecular functions of HBx. Using the transient HBV replication assay, ectopically expressed HBx could stimulate HBV transcription and replication with the X-defective replicon to the level of those with the wild one. The transcription coactivation is mainly contributing to the stimulatory role of HBx on HBV replication although the other functions may affect HBV replication. Effect of HBx on cellular transformation remains controversial and was never addressed with human primary or immortal cells. Using the human immortalized primary cells, HBx was found to retain the ability to overcome active oncogene RAS-induced senescence that requires full-length HBx. At least two functions of HBx, the coactivation function and the ability to overcome oncogene-induced senescence, may be cooperatively involved in HBV-related hepatocarcinogenesis.

Identification and validation of oncogenes in liver cancer using an integrative oncogenomic approach

The heterogeneity and instability of human tumors hamper straightforward identification of cancer-causing mutations through genomic approaches alone. Herein we describe a mouse model of liver cancer initiated from progenitor cells harboring defined cancer-predisposing lesions. Genome-wide analyses of tumors in this mouse model and in human hepatocellular carcinomas revealed a recurrent amplification at mouse chromosome 9qA1, the syntenic region of human chromosome 11q22. Gene-expression analyses delineated cIAP1, a known inhibitor of apoptosis, and Yap, a transcription factor, as candidate oncogenes in the amplicon. In the genetic context of their amplification, both cIAP1 and Yap accelerated tumorigenesis and were required to sustain rapid growth of amplicon-containing tumors. Furthermore, cIAP1 and Yap cooperated to promote tumorigenesis. Our results establish a tractable model of liver cancer, identify two oncogenes that cooperate by virtue of their coamplification in the same genomic locus, and suggest an efficient strategy for the annotation of human cancer genes.

Investigation of the mouse serum proteome

With the rapid assimilation of genomic information and the equally impressive developments in the field of proteomics, there is an unprecedented interest in biomarker discovery. Although human biofluids represent increasingly attractive samples from which new and more accurate disease biomarkers may be found, the intrinsic person-to-person variability in these samples complicates their discovery. One of the most extensively used animal models for studying human disease is mouse because, unlike humans, they represent a highly controllable experimental model system. Unfortunately, very little is known about the proteomic composition of mouse serum. In this study, a multidimensional fractionation approach on both the protein and the peptide level that does not require depletion of highly abundant serum proteins was combined with tandem mass spectrometry to characterize proteins within mouse serum. Over 12 300 unique peptides that originate from 4567 unique proteins-approximately 16% of all known mouse proteins-were identified. The results presented here represent the broadest proteome coverage in mouse serum and provide a foundation from which quantitative comparisons can be made in this important animal model.

Caspase-independent cytochrome c release is a sensitive measure of low-level apoptosis in cell culture models

Age-associated loss of tissue function and several chronic diseases may derive in part from the cumulative effects of subtle changes in the level of apoptotic cell death. Because apoptosis is rapid and undetectable once complete, small changes in its incidence are difficult to detect, even in well-controlled cell cultures. We describe a new apoptosis assay that provides greater sensitivity than conventional assays because it measures the accumulation of apoptotic cells. Human and mouse fibroblasts and human mammary epithelial cells that initiated apoptosis were preserved for 3 days by inhibiting caspase activity using the chemical inhibitor Q-VD-OPH (QVD). Cells suspended in the process of apoptosis were scored by immunostaining for cytochrome c, which redistributed from mitochondria in healthy cells to the cytoplasm in dying cells. This caspase-independent cytochrome c release (CICR) assay was more sensitive than several conventional assays when apoptosis was induced by actinomycin D, and detected cumulative background levels of apoptosis over a 3-day interval. Using this assay, we show that normal fibroblasts undergo very little apoptosis upon X-irradiation, indicating dominance of the senescence response in this cell type. Further, apoptosis increased subtly but measurably when human mammary epithelial and skin fibroblast cells entered crisis, indicating that cell death during crisis is largely non-apoptotic.

Telomere dysfunction in aging and cancer

Telomeres are unique DNA-protein structures that contain noncoding TTAGGG repeats and telomere-associated proteins. These specialized structures are essential for maintaining genomic integrity. Alterations that lead to the disruption of telomere maintenance result in chromosome end-to-end fusions and/or ends being recognized as double-strand breaks. A large body of evidence suggests that the cell responds to dysfunctional telomeres by undergoing senescence, apoptosis, or genomic instability. In conjunction with other predisposing mechanisms, the genomic instability encountered in preimmortal cells due to dysfunctional or uncapped telomeres might lead to cancer. Furthermore, telomere dysfunction has been proposed to play critical roles in aging as well as cancer progression. Conversely, recent evidence has shown that targeting telomere maintenance mechanisms and inducing telomere dysfunction in cancer cells by inhibiting telomerase can lead to catastrophic events including rapid cell death and increased sensitivity to other cancer therapeutics. Thus, given the major role telomeres play during development, it is important to continue our understanding telomere structure, function and maintenance. Herein, we provide an overview of the emerging knowledge of telomere dysfunction and how it relates to possible links between aging and cancer.

Cancer and aging: the importance of telomeres in genome maintenance

Telomeres are the specialized DNA-protein structures that cap the ends of linear chromosomes, thereby protecting them from degradation and fusion by cellular DNA repair processes. In vertebrate cells, telomeres consist of several kilobase pairs of DNA having the sequence TTAGGG, a few hundred base pairs of single-stranded DNA at the 3' end of the telomeric DNA tract, and a host of proteins that organize the telomeric double and single-stranded DNA into a protective structure. Functional telomeres are essential for maintaining the integrity and stability of genomes. When combined with loss of cell cycle checkpoint controls, telomere dysfunction can lead to genomic instability, a common cause and hallmark of cancer. Consequently, normal mammalian cells respond to dysfunctional telomeres by undergoing apoptosis (programmed cell death) or cellular senescence (permanent cell cycle arrest), two cellular tumor suppressor mechanisms. These tumor suppressor mechanisms are potent suppressors of cancer, but recent evidence suggests that they can antagonistically also contribute to aging phenotypes. Here, we review what is known about the structure and function of telomeres in mammalian cells, particularly human cells, and how telomere dysfunction may arise and contribute to cancer and aging phenotypes.

Fate of premalignant clones during the asymptomatic phase preceding lymphoid malignancy

Almost all cancers are preceded by a prolonged period of clinical latency during which a combination of cellular events helps move carcinogen-exposed cells towards a malignant phenotype. Hitherto, investigating the fate of premalignant cells in vivo remained strongly hampered by the fact that these cells are usually indistinguishable from their normal counterparts. Here, for the first time, we have designed a strategy able to reconstitute the replicative history of the bona fide premalignant clone in an animal model, the sheep experimentally infected with the lymphotropic bovine leukemia virus. We have shown that premalignant clones are early and clearly distinguished from other virus-exposed cells on the basis of their degree of clonal expansion and genetic instability. Detectable as early as 0.5 month after the beginning of virus exposure, premalignant cells displayed a two-step pattern of extensive clonal expansion together with a mutation load approximately 6 times higher than that of other virus-exposed cells that remained untransformed during the life span of investigated animals. There was no fixation of somatic mutations over time, suggesting that they regularly lead to cellular death, partly contributing to maintain a normal lymphocyte count during the prolonged premalignant stage. This equilibrium was finally broken after a period of 18.5 to 60 months of clinical latency, when a dramatic decrease in the genetic instability of premalignant cells coincided with a rapid increase in lymphocyte count and lymphoma onset.

The role of p53-mediated apoptosis as a crucial anti-tumor response to genomic instability: lessons from mouse models

Genomic instability is a major force driving human cancer development. A cellular safeguard against such genetic destabilization, which can ensue from defects in telomere maintenance, DNA repair, and checkpoint function, is activation of the p53 tumor suppressor protein, which commonly responds to these DNA damage signals by inducing apoptosis. If, however, p53 becomes inactivated, as is typical of many tumors and pre-cancerous lesions, then cells with compromised genome integrity pathways survive inappropriately, and the accrual of oncogenic lesions can fuel the carcinogenic process. Studies of mouse models have been instrumental in providing support for this idea. Mouse knockouts in genes important for telomere function, DNA damage checkpoint activation and DNA repair - both non-homologous end joining and homologous recombination - are prone to the development of genomic instability. As a consequence of these DNA damage signals, p53 becomes activated in cells of these mutant mice, leading to the induction of apoptosis, sometimes at the expense of organismal viability. This apoptotic response can be rescued through crosses to p53-deficient mice, but has dire consequences: mice predisposed to genomic instability and lacking p53 are susceptible to tumorigenesis. Thus p53-mediated apoptosis provides a crucial tumor suppressive mechanism to eliminate cells succumbing to genomic instability.

Cellular lifespan and senescence signaling in embryonic stem cells

Most mammalian cells when placed in culture will undergo a limited number of cell divisions before entering an unresponsive non-proliferating state termed senescence. However, several pathways that are activated singly or in concert can allow cells to bypass senescence at least for limited periods. These include the telomerase pathway required to maintain telomere ends, the p53 and Rb pathways required to direct senescence in response to DNA damage, telomere shortening and mitogenic signals, and the insulin-like growth factor--Akt pathway that may regulate lifespan and cell proliferation. In this review, we summarize recent findings related to these pathways in embryonic stem (ES) cells and suggest that ES cells are immortal because these pathways are tightly regulated.