Telomere length maintenance in aging and carcinogenesis

Telomerase and Anticancer Treatment

Current chemotherapy uses compounds of organometallic nature that act with different mechanisms of action. Many pharmacological studies are directed toward the creation of compounds with more specific and selective activity toward tumor targets, including telomerase. The design and synthesis of such compounds with specific antitelomerase activity must consider the mechanism of action of the enzyme and its structure. The discovery of a close correlation between telomerase activation, cell immortalization and oncogenesis has suggested that telomerase inhibitors could be potent therapeutic agents, capable of selectively killing cancer cells. Inhibition of telomerase is expected to lead toward shortening of telomeres to a critical length, such that replicative senescence and cell death due to irreparable chromosomal damage can result. It has been observed that cancer cells generally have shorter telomeres than the normal replicative cell population, probably because the malignant cells have undergone more divisions. Therefore, the inhibition telomeres of cancer cells after a few cycles of cell division, without the normal cells suffering harmful consequences during therapy. Telomerase is certainly an interesting target on which to continue to study molecules that inhibit its function to obtain a specificity of therapeutic intervention and a reduction of the nonspecific cytotoxicity of chemotherapy.

In utero exposure to endocrine-disrupting chemicals and telomere length at birth

Telomere length correlates with morbidity and mortality. While telomere length appears to be influenced by hormone levels, the potential impact of exposure to endocrine-disrupting chemicals (EDCs) has not been studied. We examined the association between maternal gestational concentrations of biomarkers of EDC exposure and telomere length at birth in the Harvard Epigenetic Birth Cohort. EDC (phenols and phthalates) biomarker concentrations were measured in maternal spot urine samples during the first trimester and telomere length in maternal and cord blood collected at delivery among 181 mother-newborn singleton dyads. Maternal and newborn telomere length exhibited a positive correlation (Spearman ρ = 0.20 (p-value< 0.01). Infant telomere length was associated with maternal biomarker concentrations of specific EDCs, and most of these associations were observed to be infant sex-specific. Prenatal exposure to triclosan, a non-paraben phenol with antimicrobial properties, was one of the most strongly associated EDCs with telomere length; telomere length was 20% (95% CI 5%-33%) shorter among boys in the highest quartile of maternal biomarker concentrations compared to the lowest quartile. In contrast, we observed longer telomere length associated with increased gestational concentrations of mono-isobutyl phthalate, and among boys, with increased concentrations of mono-2-ethylhexyl phthalate. In this birth cohort, we observed associations between maternal gestational exposure to select EDC biomarkers and telomere length, most of which were sex-specific. These findings need to be confirmed in future studies.

The Relationship between Telomere Length and Cancer Mortality: Data from the 1999-2002 National Healthy and Nutrition Examination Survey (NHANES)

OBJECTIVES

The association between telomeres length (TL) and cancer mortality is uncertain. We tested the hypotheses that long TL are associated with reduced cancer mortality.

DESIGN

Prospective cohort study.

SETTING

the National Health and Nutrition Survey (NHANES, 1999-2002).

PARTICIPANTS

The analytic sample included adults (n = 7183) who had TL measurements.

MEASUREMENTS

DNA was obtained via blood samples. Telomere length was assessed using the quantitative polymerase chain reaction method.

RESULTS

During follow-up (0.08-12.7 person-years, median = 9.5 years), we observed 195 participants had cancer as causes of death. TL was negatively corelated with age, body mass index (BMI), systolic blood pressure (SBP), C-reactive protein (CRP), race, diabetes, hypertension, cardiovascular diseases (CVD) and cancer mortality, conversely, positively corelated with alcohol use, but not related to diastolic blood pressure (DBP) and smoking. Kaplan-Meier analysis revealed that TL was significantly associated with cancer mortality (log-rank, P <0.001).

CONCLUSIONS

Our study expands upon previous evidence of a relationship between TL and cancer mortality. TL may be a useful tool for evaluating risk of cancer mortality in American adults.

Nutrigenomics at the Interface of Aging, Lifespan, and Cancer Prevention

The percentage of elderly people with associated age-related health deterioration, including cancer, has been increasing for decades. Among age-related diseases, the incidence of cancer has grown substantially, in part because of the overlap of some molecular pathways between cancer and aging. Studies with model organisms suggest that aging and age-related conditions are manipulable processes that can be modified by both genetic and environmental factors, including dietary habits. Variations in genetic backgrounds likely lead to differential responses to dietary changes and account for some of the inconsistencies found in the literature. The intricacies of the aging process, coupled with the interrelational role of bioactive food components on gene expression, make this review a complex undertaking. Nevertheless, intriguing evidence suggests that dietary habits can manipulate the aging process and/or its consequences and potentially may have unprecedented health benefits. The present review focuses on 4 cellular events: telomerase activity, bioenergetics, DNA repair, and oxidative stress. These processes are linked to both aging and cancer risk, and their alteration in animal models by selected food components is evident.

Dysfunctional telomeres in primary cells from Fanconi anemia FANCD2 patients

BACKGROUND

Fanconi anemia (FA) is characterized by sensitivity to DNA cross-linking agents, mild cellular, and marked clinical radio sensitivity. In this study we investigated telomeric abnormalities of non-immortalized primary cells (lymphocytes and fibroblasts) derived from FA patients of the FA-D2 complementation group, which provides a more accurate physiological assessment than is possible with transformed cells or animal models.

RESULTS

We analyzed telomere length, telomere dysfunction-induced foci (TIFs), sister chromatid exchanges (SCE), telomere sister chromatid exchanges (T-SCE), apoptosis and expression of shelterin components TRF1 and TRF2. FANCD2 lymphocytes exhibited multiple types of telomeric abnormalities, including premature telomere shortening, increase in telomeric recombination and aberrant telomeric structures ranging from fragile to long-string extended telomeres. The baseline incidence of SCE in FANCD2 lymphocytes was reduced when compared to control, but in response to diepoxybutane (DEB) the 2-fold higher rate of SCE was observed. In contrast, control lymphocytes showed decreased SCE incidence in response to DEB treatment. FANCD2 fibroblasts revealed a high percentage of TIFs, decreased expression of TRF1 and invariable expression of TRF2. The percentage of TIFs inversely correlated with telomere length, emphasizing that telomere shortening is the major reason for the loss of telomere capping function. Upon irradiation, a significant decrease of TIFs was observed at all recovery times. Surprisingly, a considerable percentage of TIF positive cells disappeared at the same time when incidence of γ-H2AX foci was maximal. Both FANCD2 leucocytes and fibroblasts appeared to die spontaneously at higher rate than control. This trend was more evident upon irradiation; the percentage of leucocytes underwent apoptosis was 2.59- fold higher than that in control, while fibroblasts exhibited a 2- h delay before entering apoptosis.

CONCLUSION

The results of our study showed that primary cells originating from FA-D2 patients display shorten telomeres, elevated incidence of T-SCEs and high frequency of TIFs. Disappearance of TIFs in early response to irradiation represent distinctive feature of FANCD2 cells that should be examined further.

The Epstein-Barr virus nuclear antigen-1 promotes telomere dysfunction via induction of oxidative stress

The Epstein–Barr virus (EBV) nuclear antigen (EBNA)-1 promotes the accumulation of chromosomal aberrations in malignant B cells by inducing oxidative stress. Here we report that this phenotype is associated with telomere dysfunction. Stable or conditional expression of EBNA1 induced telomere abnormalities including loss or gain of telomere signals, telomere fusion and heterogeneous length of telomeres. This was accompanied by the accumulation of extrachromosomal telomeres, telomere dysfunction-induced foci (TIFs) containing phosphorylated histone H2AX and the DNA damage response protein 53BP1, telomere-associated promyelocytic leukemia nuclear bodies (APBs), telomeric-sister chromatid exchanges and displacement of the shelterin protein TRF2. The induction of TIFs and APBs was inhibited by treatment with scavengers of reactive oxygen species (ROS) that also promoted the relocalization of TRF2 at telomeres. These findings highlight a novel mechanism by which EBNA1 may promote malignant transformation and tumor progression.

DNA methylation, an epigenetic mechanism connecting folate to healthy embryonic development and aging

Experimental studies demonstrated that maternal exposure to certain environmental and dietary factors during early embryonic development can influence the phenotype of offspring as well as the risk of disease development at the later life. DNA methylation, an epigenetic phenomenon, has been suggested as a mechanism by which maternal nutrients affect the phenotype of their offspring in both honeybee and agouti mouse models. Phenotypic changes through DNA methylation can be linked to folate metabolism by the knowledge that folate, a coenzyme of one-carbon metabolism, is directly involved in methyl group transfer for DNA methylation. During the fetal period, organ-specific DNA methylation patterns are established through epigenetic reprogramming. However, established DNA methylation patterns are not immutable and can be modified during our lifetime by the environment. Aberrant changes in DNA methylation with diet may lead to the development of age-associated diseases including cancer. It is also known that the aging process by itself is accompanied by alterations in DNA methylation. Diminished activity of DNA methyltransferases (Dnmts) can be a potential mechanism for the decreased genomic DNA methylation during aging, along with reduced folate intake and altered folate metabolism. Progressive hypermethylation in promoter regions of certain genes is observed throughout aging, and repression of tumor suppressors induced by this epigenetic mechanism appears to be associated with cancer development. In this review, we address the effect of folate on early development and aging through an epigenetic mechanism, DNA methylation.

Hepatitis B virus X gene induces human telomerase reverse transcriptase mRNA expression in cultured normal human cholangiocytes

AIM: To study the transcriptional regulation of human telomerase reverse transcriptase (hTERT) mRNA in normal human cholangiocytes (HBECs) after hepatitis B virus X (HBx) gene transfection and to elucidate the possible mechanism of HBV infection underlying cholangiocarcinoma.

METHODS: HBECs were cultured in vitro and co-transfected with a eukaryotic expression vector containing the HBx coding region and a cloning vector containing coding sequences of enhanced green fluorescent protein (EGFP) using lipid-mediated gene transfer. The transfection efficiency was determined by the expression of EGFP. The expressions of hTERT mRNA and HBx protein in HBECs were detected by RT-PCR and immunocytochemical stain, respectively.

RESULTS: The transfection efficiencies were about 15% for both HBx gene expression plasmid and empty vector. No hTERT mRNA was expressed in HBECs when transfected with OPTI-MEM medium and empty vector, but a dramatic increase was observed for hTERT mRNA expression in HBECs when transfected with HBx expression vector. HBx protein was only expressed in HBECs when transfected with HBx expression vector.

CONCLUSION: HBx transfection can activate the transcriptional expression of hTERT mRNA. Cis-activation of hTERT mRNA by HBx gene is the primary mechanism underlying the proliferation, differentiation and tumorigenesis of biliary epithelia.

Redundancy, antiredundancy, and the robustness of genomes

Genetic mutations that lead to undetectable or minimal changes in phenotypes are said to reveal redundant functions. Redundancy is common among phenotypes of higher organisms that experience low mutation rates and small population sizes. Redundancy is less common among organisms with high mutation rates and large populations, or among the rapidly dividing cells of multicellular organisms. In these cases, one even observes the opposite tendency: a hypersensitivity to mutation, which we refer to as antiredundancy. In this paper we analyze the evolutionary dynamics of redundancy and antiredundancy. Assuming a cost of redundancy, we find that large populations will evolve antiredundant mechanisms for removing mutants and thereby bolster the robustness of wild-type genomes; whereas small populations will evolve redundancy to ensure that all individuals have a high chance of survival. We propose that antiredundancy is as important for developmental robustness as redundancy, and is an essential mechanism for ensuring tissue-level stability in complex multicellular organisms. We suggest that antiredundancy deserves greater attention in relation to cancer, mitochondrial disease, and virus infection.

Regulative function of telomerase and extracellular regulated protein kinases to leukemic cell apoptosis

In order to investigate the regulative function of telomerase and phosphorylated (activated) extracellular regulated protein kinase (ERK) 1 and 2 in the leukemic cell lines HL-60 and K562 proliferation inhibition and apoptosis, three chemotherapeutic drugs Harringtonine (HRT), Vincristine (VCR) and Etoposide (Vp16) were selected as inducers. The proliferation inhibition rate was detected by MTT method, the cell cycle and cell apoptosis was analyzed by flow cytometry and the telomerase activity was detected by the telomeric repeat amplification protocol (TRAP) assay and bioluminescence analysis method. The phosphorylated ERK1/2 protein expression was detected by western blot method. The results showed that HRT, VCR and Vp16 could inhibit cell proliferation, induce apoptosis, inhibit telomerase activity and down-regulate the protein expression of phosphorylated ERK. It was suggested that ERK signal transduction pathway was involved in the down-regulation of telomerase activity and the onset of apoptosis in the leukemic cells treated by HRT, VCR and Vp16.

Long-term cultivation of colorectal carcinoma cells with anti-cancer drugs induces drug resistance and telomere elongation: an in vitro study

BACKGROUND

The role of telomerase activation in the expression and/or maintenance of drug resistance is not clearly understood. Therefore, we investigated the relationships, among the telomerase activity, telomere length and the expression of multidrug resistance genes in colorectal cancer cell lines cultivated with anti-cancer drugs.

METHODS

LoVo and DLD-1 cells were continuously grown in the presence of both CDDP and 5-FU for up to 100 days. Cell proliferation, telomerase activity, telomere length and the expression of multidrug resistance genes were serially monitored as the PDL increased.

RESULTS

The expression of multidrug resistance genes tended to increase as the PDL increased. However, an abnormal aneuploid clone was not detected as far as the cells were monitored by a DNA histogram analysis. Tumor cells showing resistance to anti-cancer drugs revealed a higher cell proliferation rate. The telomere length gradually increased with a progressive PDL. The telomerase activity reached a maximum level at 15 PDL in LoVo cells and at 27 PDL in DLD-1 cells. An increase in the mRNA expression of the telomerase components, especially in hTERT and in hTR, was observed at the same PDLs.

CONCLUSIONS

These results suggest that a high telomerase activity and an elongation of telomeres both appear to help maintain and/or increase drug resistance in colorectal cancer cells. Cancer cells with long telomeres and a high proliferative activity may thus be able to better survive exposure to anti-cancer drugs. This is presumably due to an increased chromosome stability and a strong expression of both mdr-1 and MRP genes.