Telomere loss in cells treated with cisplatin

G-quadruplex-guided cisplatin triggers multiple pathways in targeted chemotherapy and immunotherapy

G-quadruplexes (G4s) are atypical nucleic acid structures involved in basic human biological processes and are regulated by small molecules. To date, pyridostatin and its derivatives [e.g., PyPDS (4-(2-aminoethoxy)-N 2,N 6-bis(4-(2-(pyrrolidin-1-yl) ethoxy) quinolin-2-yl) pyridine-2,6-dicarboxamide)] are the most widely used G4-binding small molecules and considered to have the best G4 specificity, which provides a new option for the development of cisplatin-binding DNA. By combining PyPDS with cisplatin and its analogs, we synthesize three platinum complexes, named PyPDSplatins. We found that cisplatin with PyPDS (CP) exhibits stronger specificity for covalent binding to G4 domains even in the presence of large amounts of dsDNA compared with PyPDS either extracellularly or intracellularly. Multiomics analysis reveals that CP can effectively regulate G4 functions, directly damage G4 structures, activate multiple antitumor signaling pathways, including the typical cGAS-STING pathway and AIM2-ASC pathway, trigger a strong immune response and lead to potent antitumor effects. These findings reflect that cisplatin-conjugated specific G4 targeting groups have antitumor mechanisms different from those of classic cisplatin and provide new strategies for the antitumor immunity of metals.

DNA Conformational Changes Induced by Its Interaction with Binuclear Platinum Complexes in Solution Indicate the Molecular Mechanism of Platinum Binding

Platinum anticancer drugs inhibit the division of cancer cells through a DNA binding mechanism. The bimetallic platinum compounds have a possibility for blocking DNA replication via the cross-linking of DNA functional groups at different distances. Many compounds with metals of the platinum group have been tested for possible antitumor activity. The main target of their biological action is a DNA molecule. A combined approach to the study of the interaction of DNA with biologically active compounds of this type is proposed. The capabilities of various methods (hydrodynamic, spectral, microscopy) in obtaining information on the type of binding of coordination compounds to DNA are compared. The analysis of DNA binding with platinum binuclear compounds containing pyrazine, tetrazole, 5- methyltetrazole, 3-propanediamine as bridging ligands in a solution was carried out with the methods of circular dichroism (CD), luminescent spectroscopy (LS), low gradient viscometry (LGV), flow birefringence (FB) and atomic force microscopy (AFM). The competitive binding of different platinum compounds to DNA and the analysis of platinum attachment to DNA after protonation of its nitrogen bases simply indicates the involvement of N7 guanine in binding. Fluorescent dye DAPI was also used to recognize the location of platinum compounds in DNA grooves. DNA conformational changes recorded by variations in persistent length, polyelectrolyte swelling, DNA secondary structure, and its stability clarify the molecular mechanism of the biological activity of platinum compounds.

Mutagen-induced telomere instability in human cells

Telomere instability is one of the main sources of genome instability and may result from chromosome end loss (due to chromosome breakage at one or both ends) or, more frequently, telomere dysfunction. Dysfunctional telomeres arise when they lose their end-capping function or become critically short, which causes chromosomal termini to behave like a DNA double-strand break. Telomere instability may occur at the chromosomal or at the molecular level, giving rise, respectively, to telomere-related chromosomal aberrations or the loss or modification of any of the components of the telomere (telomere DNA, telomere-associated proteins, or telomere RNA). Since telomeres play a fundamental role in maintaining genome stability, the study of telomere instability in cells exposed to mutagens is of great importance to understand the telomere-driven genomic instability present in those cells. In the present review, we will focus on the current knowledge about telomere instability induced by physical, chemical, and biological mutagens in human cells.

Proteomic Analysis of the Non-genetic Response to Cisplatin in Lung Cancer Cells

BACKGROUND

Drug resistance is the main cause of therapy failure in advanced lung cancer. Although non-genetic mechanisms play important roles in tumor chemoresistance, drug-induced epigenetic reprogramming is still poorly understood.

MATERIALS AND METHODS

The A549 cell line was used to generate cells with non-genetic resistance to cisplatin (CDDP), namely A549/CDDP cells. Bioorthogonal non-canonical amino acid tagging (BONCAT) and mass spectrometry were used to identify proteins modulated by CDDP in A549 and A549/CDDP cells.

RESULTS

Proteins related to proteostasis, telomere maintenance, cell adhesion, cytoskeletal remodeling, and cell redox homeostasis were found enriched in both cell lines upon CDDP exposure. On the other hand, proteins involved in drug response, metabolic pathways and mRNA processing and splicing were up-regulated by CDDP only in A549/CDDP cells.

CONCLUSION

Our study revealed proteome dynamics involved in the non-genetic response to CDDP, pointing out potential targets to monitor and overcome epigenetic resistance in lung cancer.

Adsorption and Diffusion of Cisplatin Molecules in Nanoporous Materials: A Molecular Dynamics Study

Using molecular dynamics simulations, the adsorption and diffusion of cisplatin drug molecules in nanopores is investigated for several inorganic materials. Three different materials are studied with widely-varying properties: metallic gold, covalent silicon, and silica. We found a strong influence of both the van der Waals and the electrostatic interaction on the adsorption behavior on the pore walls, which in turn influence the diffusion coefficients. While van der Waals forces generally lead to a reduction of the diffusion coefficient, the fluctuations in the electrostatic energy induced by orientation changes of the cisplatin molecule were found to help desorb the molecule from the wall.

A high-throughput screen to identify novel small molecule inhibitors of the Werner Syndrome Helicase-Nuclease (WRN)

Werner syndrome (WS), an autosomal recessive genetic disorder, displays accelerated clinical symptoms of aging leading to a mean lifespan less than 50 years. The WS helicase-nuclease (WRN) is involved in many important pathways including DNA replication, recombination and repair. Replicating cells are dependent on helicase activity, leading to the pursuit of human helicases as potential therapeutic targets for cancer treatment. Small molecule inhibitors of DNA helicases can be used to induce synthetic lethality, which attempts to target helicase-dependent compensatory DNA repair pathways in tumor cells that are already genetically deficient in a specific pathway of DNA repair. Alternatively, helicase inhibitors may be useful as tools to study the specialized roles of helicases in replication and DNA repair. In this study, approximately 350,000 small molecules were screened based on their ability to inhibit duplex DNA unwinding by a catalytically active WRN helicase domain fragment in a high-throughput fluorometric assay to discover new non-covalent small molecule inhibitors of the WRN helicase. Select compounds were screened to exclude ones that inhibited DNA unwinding by other helicases in the screen, bound non-specifically to DNA, acted as irreversible inhibitors, or possessed unfavorable chemical properties. Several compounds were tested for their ability to impair proliferation of cultured tumor cells. We observed that two of the newly identified WRN helicase inhibitors inhibited proliferation of cancer cells in a lineage-dependent manner. These studies represent the first high-throughput screen for WRN helicase inhibitors and the results have implications for anti-cancer strategies targeting WRN in different cancer cells and genetic backgrounds.

Selective binding of an organoruthenium complex to G-rich human telomeric sequence by tandem mass spectrometry

RATIONALE

Human telomeric DNA is reported to be a potential target for anticancer organometallic ruthenium(II) complexes, however, the interaction sites were not clearly discriminated and identified.

METHODS

In the current study, tandem mass spectrometry (MS/MS) using collision-induced dissociation (CID) was firstly introduced to identify the interaction sites of an organometallic ruthenium(II) complex [(η⁶ -biphenyl)Ru(en)Cl][PF₆ ] (1; en = ethylenediamine) with 5'-T₁ T₂ A₃ G₄ G₅ G₆ -3' (I), the repeating unit of human telomeric DNA, in both positive- and negative-ion mode at a low reaction molar ratio (1/I = 0.2) which was applied to preserve the site selectivity.

RESULTS

Mass spectrometric results showed that mono-ruthenated I was the main product under the conditions. In positive-ion mode, MS/MS results indicated that ruthenium complex 1 binds to T₂ or G₆ in strand I. However, in negative-ion mode, no efficient information was obtained for exact identification of ruthenation sites which may be attributed to losses of fragment ions due to charge neutralization by the coordination of the positively charged ruthenium complex to the short MS/MS fragments.

CONCLUSIONS

This is the first report of using top-down MS to characterize the interactions of organometallic ruthenium(II) complexes and human telomeric DNA. Thymine can be thermodynamically competitive with guanine for binding to ruthenium complexes even at low reaction molar ratio, which inspired us to explore in greater depth the significance of thymine binding.

Platinum Complexes Can Bind to Telomeres by Coordination

It is suggested that several compounds, including G-quadruplex ligands, can target telomeres, inducing their uncapping and, ultimately, cell death. However, it has never been demonstrated whether such ligands can bind directly and quantitatively to telomeres. Here, we employed the property of platinum and platinum-G-quadruplex complexes to target G-rich sequences to investigate and quantify their covalent binding to telomeres. Using inductively coupled plasma mass spectrometry, surprisingly, we found that, in cellulo, in the presence of cisplatin, a di-functional platinum complex, telomeric DNA was platinated 13-times less than genomic DNA in cellulo, as compared to in vitro data. On the contrary, the amount of mono-functional platinum complexes (Pt-ttpy and Pt-tpy) bound either to telomeric or to genomic DNA was similar and occurred in a G-quadruplex independent-manner. Importantly, the quantification revealed that the low level of cisplatin bound to telomeric DNA could not be the direct physical cause of TRF2 displacement from telomeres. Altogether, our data suggest that platinum complexes can affect telomeres both directly and indirectly.

Dual functional dinuclear platinum complex with selective reactivity towards c-myc G-quadruplex

G-quadruplexes (GQ) folded by the oncogenic G-rich sequences are the promising targets for developing anticancer therapeutic molecules. However, the current drug development mainly focused on non-covalent dynamic binders to stabilize GQ structures, while the covalent targeting from inorganic complexes via chelating principles, as a potent therapeutic strategy was surprisingly lack of exploration. Herein, a series of dinuclear platinum complexes, [(Pt(Dip)Cl)₂(μ-diamine)](NO₃)₂ (Dip: 4,7-diphenyl-1,10-phenanthroline), were designed to contain two dual-functional Pt cores connected by an alkyl linkage. Pt3 with nonanediamine linkage optimized the specific binding towards c-myc G-quadruplex via dual functional clamp on GQ as 1) non-covalently π-stacking of aromatic ligands, and 2) two Pt(II) cores covalently chelated to guanines at both 3'- and 5'-ends.

Common Chemical Inductors of Replication Stress: Focus on Cell-Based Studies

DNA replication is a highly demanding process regarding the energy and material supply and must be precisely regulated, involving multiple cellular feedbacks. The slowing down or stalling of DNA synthesis and/or replication forks is referred to as replication stress (RS). Owing to the complexity and requirements of replication, a plethora of factors may interfere and challenge the genome stability, cell survival or affect the whole organism. This review outlines chemical compounds that are known inducers of RS and commonly used in laboratory research. These compounds act on replication by direct interaction with DNA causing DNA crosslinks and bulky lesions (cisplatin), chemical interference with the metabolism of deoxyribonucleotide triphosphates (hydroxyurea), direct inhibition of the activity of replicative DNA polymerases (aphidicolin) and interference with enzymes dealing with topological DNA stress (camptothecin, etoposide). As a variety of mechanisms can induce RS, the responses of mammalian cells also vary. Here, we review the activity and mechanism of action of these compounds based on recent knowledge, accompanied by examples of induced phenotypes, cellular readouts and commonly used doses.

Heregulin, a new regulator of telomere length in human cells

The growth factor heregulin (HRG) promotes breast cancer (BC) tumorigenesis and metastasis and differentially modulates BC cell responses to DNA-damaging agents via its dual extracellular and nuclear localization. Given the central role of telomere dysfunction to drive carcinogenesis and to alter the chemotherapeutic profile of transformed cells, we hypothesized that an unanticipated nuclear function of HRG might be to regulate telomere length. Engineered overexpression of the HRGβ2 isoform in non-aggressive, HRG-negative MCF-7 BC cells resulted in a significant shortening of telomeres (up to 1.3 kb) as measured by Southern blotting of telomere terminal restriction fragments. Conversely, antisense-mediated suppression of HRGβ2 in highly aggressive, HRG-overexpressing MDA-MB-231 and Hs578T cells increased telomere length up to 3.0 kb. HRGβ2 overexpression promoted a marked upregulation of telomere-binding protein 2 (TRF2) protein expression, whereas its knockdown profoundly decreased TRF2 expression. Double staining of endogenous HRGβ2 with telomere-specific peptide nucleic acid probe/fluorescence in situ hybridization (PNA/FISH) revealed the partial localization of HRG at the chromosome ends. Moreover, a predominantly nucleoplasmic staining pattern of endogenous HRGβ2 appeared to co-localize with TRF2 and, concomitantly with RAP1, a telomere regulator that specifically interacts with TRF2. Small interfering RNA-mediated knockdown of HRG decreased the expression of TRF2 and RAP1, decreased their presence at chromosome ends, and coincidentally resulted in the formation of longer telomeres. This study uncovers a new function for HRGβ2 in controlling telomere length, in part due to its ability to regulate and interact with the telomere-associated proteins TRF2 and RAP1.

Characterisation of the DNA sequence specificity, cellular toxicity and cross-linking properties of novel bispyridine-based dinuclear platinum complexes

Background

The anti-tumour activity of cisplatin is thought to be a result of its capacity to form DNA adducts which prevent cellular processes such as DNA replication and transcription. These DNA adducts can effectively induce cancer cell death, however, there are a range of clinical side effects and drug resistance issues associated with its use. In this study, the biological properties of three novel dinuclear platinum-based compounds (that contain alkane bridging linkers of eight, ten and twelve carbon atoms in length) were characterised to assess their potential as anticancer agents.

Methods

The properties of these compounds were determined using a DNA template containing seven tandem telomeric repeat sequences. A linear amplification reaction was used in combination with capillary electrophoresis to quantify the sequence specificity of DNA adducts formed by these compounds at base pair resolution. The DNA cross-linking ability of these compounds was assessed using denaturing agarose gel electrophoresis and cytotoxicity was determined in HeLa cells using a colorimetric cell viability assay.

Results

The dinuclear compounds were found to preferentially form DNA adducts at guanine bases and they exhibited different damage intensity profiles at the telomeric repeat sequences compared to that of cisplatin. The dinuclear compounds were found to exhibit a low level of cytotoxicity relative to cisplatin and their cytotoxicity increased as the linker length increased. Conversely, the interstrand cross-linking efficiency of the dinuclear compounds increased as the linker length decreased and the compound with the shortest alkane linker was six-fold more effective than cisplatin.

Conclusions

Since the bifunctional compounds exhibit variation in sequence specificity of adduct formation and a greater ability to cross-link DNA relative to cisplatin they warrant further investigation towards the goal of developing new cancer chemotherapeutic agents.

Causes of genome instability: the effect of low dose chemical exposures in modern society

Genome instability is a prerequisite for the development of cancer. It occurs when genome maintenance systems fail to safeguard the genome's integrity, whether as a consequence of inherited defects or induced via exposure to environmental agents (chemicals, biological agents and radiation). Thus, genome instability can be defined as an enhanced tendency for the genome to acquire mutations; ranging from changes to the nucleotide sequence to chromosomal gain, rearrangements or loss. This review raises the hypothesis that in addition to known human carcinogens, exposure to low dose of other chemicals present in our modern society could contribute to carcinogenesis by indirectly affecting genome stability. The selected chemicals with their mechanisms of action proposed to indirectly contribute to genome instability are: heavy metals (DNA repair, epigenetic modification, DNA damage signaling, telomere length), acrylamide (DNA repair, chromosome segregation), bisphenol A (epigenetic modification, DNA damage signaling, mitochondrial function, chromosome segregation), benomyl (chromosome segregation), quinones (epigenetic modification) and nano-sized particles (epigenetic pathways, mitochondrial function, chromosome segregation, telomere length). The purpose of this review is to describe the crucial aspects of genome instability, to outline the ways in which environmental chemicals can affect this cancer hallmark and to identify candidate chemicals for further study. The overall aim is to make scientists aware of the increasing need to unravel the underlying mechanisms via which chemicals at low doses can induce genome instability and thus promote carcinogenesis.

Role of diallyl disulfide-mediated cleavage of c-Myc and Sp-1 in the regulation of telomerase activity in human lymphoma cell line U937

OBJECTIVE

Garlic (Allium sativum) has been considered a wonder herb for years with a reputation of disease prevention. Telomerase, a ribonucleoprotein enzyme responsible for telomere integrity, is strongly up-regulated in different types of cancers. The aim of this study was to reveal the role of diallyl disulfide (DADS), an organosulfur component of garlic, on telomerase activity in human lymphoma with an emphasis on key transcription factors c-Myc and Sp-1.

METHODS

Human lymphoma cell line U937 was used as model cell line. Telomerase activity was measured by telomerase repeat amplification protocol assay, levels of related proteins and mRNAs were measured by Western blot and reverse transcriptase polymerase chain reaction, respectively. Moreover, in vitro binding assay was performed using radiolabeled double-stranded DNA having specific sequences to detect involvement of transcription factors in DADS-dependent modulation of telomerase activity.

RESULTS

The present study demonstrated DADS-mediated decrease in telomerase activity in U937 cells with concomitant transcriptional down-regulation of human telomerase reverse transcriptase (hTERT) that is caused by reduced binding of c-Myc and Sp-1 to their respective binding sites on hTERT promoter. Lowering of DNA-binding activity of c-Myc and Sp-1 due to DADS treatment is caused by the deactivation of these transcription factors due to cleavage. Additionally, Mad1-the repressor protein of hTERT expression-is also overexpressed in DADS-treated U937 cells.

CONCLUSIONS

These findings strongly suggest that DADS down-regulate telomerase activity through c-Myc-, Sp-1-, and Mad1-dependent transcriptional down-regulation of hTERT.

Visualization and quantitative analysis of extrachromosomal telomere-repeat DNA in individual human cells by Halo-FISH

Current methods for characterizing extrachromosomal nuclear DNA in mammalian cells do not permit single-cell analysis, are often semi-quantitative and frequently biased toward the detection of circular species. To overcome these limitations, we developed Halo-FISH to visualize and quantitatively analyze extrachromosomal DNA in single cells. We demonstrate Halo-FISH by using it to analyze extrachromosomal telomere-repeat (ECTR) in human cells that use the Alternative Lengthening of Telomeres (ALT) pathway(s) to maintain telomere lengths. We find that GM847 and VA13 ALT cells average ∼80 detectable G/C-strand ECTR DNA molecules/nucleus, while U2OS ALT cells average ∼18 molecules/nucleus. In comparison, human primary and telomerase-positive cells contain <5 ECTR DNA molecules/nucleus. ECTR DNA in ALT cells exhibit striking cell-to-cell variations in number (<20 to >300), range widely in length (<1 to >200 kb) and are composed of primarily G- or C-strand telomere-repeat DNA. Halo-FISH enables, for the first time, the simultaneous analysis of ECTR DNA and chromosomal telomeres in a single cell. We find that ECTR DNA comprises ∼15% of telomere-repeat DNA in GM847 and VA13 cells, but <4% in U2OS cells. In addition to its use in ALT cell analysis, Halo-FISH can facilitate the study of a wide variety of extrachromosomal DNA in mammalian cells.

DNA interstrand cross-link repair requires replication-fork convergence

DNA interstrand cross-links (ICLs) prevent strand separation during DNA replication and transcription and therefore are extremely cytotoxic. In metazoans, a major pathway of ICL repair is coupled to DNA replication, and it requires the Fanconi anemia pathway. In most current models, collision of a single DNA replication fork with an ICL is sufficient to initiate repair. In contrast, we show here that in Xenopus egg extracts two DNA replication forks must converge on an ICL to trigger repair. When only one fork reaches the ICL, the replicative CMG helicase fails to unload from the stalled fork, and repair is blocked. Arrival of a second fork, even when substantially delayed, rescues repair. We conclude that ICL repair requires a replication-induced X-shaped DNA structure surrounding the lesion, and we speculate on how this requirement helps maintain genomic stability in S phase.

DNA fragment conformations in adducts with Kiteplatin

The anticancer-active platinum complex with cis-1,4-diaminocyclohexane has proved to be very valuable in detecting multiple conformers in adducts with oligonucleotides.

Effects of four chemotherapeutic agents, bleomycin, etoposide, cisplatin, and cyclophosphamide, on DNA damage and telomeres in a mouse spermatogonial cell line

Treatment with chemotherapeutics agents may induce persistent DNA damage in male germ cells with the possibility of long-term consequences on fertility and progeny outcome. Telomeres, specialized structures at the physical ends of chromosomes, play an important role in the maintenance of genetic stability and in the response of somatic cells to anticancer drugs. Our objective was to test the hypothesis that exposure to bleomycin, etoposide, or cisplatin (the drugs used to treat testicular cancer) or cyclophosphamide (an anticancer agent and immunosuppressant) targets telomeres in the male germ line. C18-4 spermatogonial cells were exposed to bleomycin, etoposide, cisplatin, or 4-hydroperoxycyclophosphamide (4OOH-CPA, a preactivated analog of cyclophosphamide). All four anticancer drugs induced a significant increase in DNA damage in C18-4 cells, as assessed by gamma-H2AX immunofluorescence. Interestingly, the gamma-H2AX signal was localized to telomeres after treatment with bleomycin, cisplatin, and 4OOH-CPA, but not etoposide. Mean telomere lengths, the intensity of the telomere fluorescence in situ hybridization signal, telomerase activity, and the expression of the telomerase enzyme mRNA components, Tert and Terc, were reduced by exposure to cisplatin and 4OOH-CPA, but not by bleomycin or etoposide. Thus, although all four anticancer drugs induced DNA damage in this spermatogonial cell line, telomeres were not specifically affected by etoposide and only the two alkylating agents, cisplatin and 4OOH-CPA, induced telomere dysfunction. This telomere dysfunction may contribute to infertility and developmental defects in the offspring.

The interaction of cisplatin with a human telomeric DNA sequence containing seventeen tandem repeats

The anti-tumour drug, cisplatin, preferentially forms adducts at G-rich DNA sequences. Telomeres are found at the ends of chromosomes and, in humans, contain the repeated DNA sequence (GGGTTA)(n) that is expected to be targeted by cisplatin. Using a plasmid clone with 17 tandem telomeric repeats, (GGGTTA)(17), the DNA sequence specificity of cisplatin was investigated utilising the linear amplification procedure that pin-pointed the precise sites of cisplatin adduct formation. This procedure used a fluorescently labelled primer and capillary electrophoresis with laser-induced fluorescence detection to determine the DNA sequence specificity of cisplatin. This technique provided a very accurate analysis of cisplatin-DNA adduct formation in a long telomeric repeat DNA sequence. The DNA sequence specificity of cisplatin in a long telomeric tandem repeat has not been previously reported. The results indicated that the 3'-end of the G-rich strand of the telomeric repeat was preferentially damaged by cisplatin and this suggests that the telomeric DNA repeat has an unusual conformation.

Telomere and microtubule targeting in treatment-sensitive and treatment-resistant human prostate cancer cells

Modulating telomere dynamics may be a useful strategy for targeting prostate cancer cells, because they generally have short telomeres. Because a plateau has been reached in the development of taxane-based treatments for prostate cancer, this study was undertaken to evaluate the relative efficacy of targeting telomeres and microtubules in taxane-sensitive, taxane-resistant, androgen-sensitive, and androgen-insensitive prostate cancer cells. Paclitaxel- and docetaxel-resistant DU145 cells were developed and their underlying adaptive responses were evaluated. Telomere dynamics and the effects of targeting telomeres with sodium meta-arsenite (KML001) (an agent undergoing early clinical trials), including combinations with paclitaxel and docetaxel, were evaluated in parental and drug-resistant cells. The studies were extended to androgen-sensitive LNCaP cells and androgen-insensitive LNCaP/C81 cells. Both P-glycoprotein (Pgp)-dependent and non-Pgp-dependent mechanisms of resistance were recruited within the same population of DU145 cells with selection for drug resistance. Wild-type DU145 cells have a small side population (SP) (0.4-1.2%). The SP fraction increased with increasing drug resistance, which was correlated with enhanced expression of Pgp but not breast cancer resistance protein. Telomere dynamics remained unchanged in taxane-resistant cells, which retained sensitivity to KML001. Furthermore, KML001 targeted SP and non-SP fractions, inducing DNA damage signaling in both fractions. KML001 induced telomere erosion, decreased telomerase gene expression, and was highly synergistic with the taxanes in wild-type and drug-resistant DU145 cells. This synergism extended to androgen-sensitive and androgen-insensitive LNCaP cells under basal and androgen-deprived conditions. These studies demonstrate that KML001 plus docetaxel and KML001 plus paclitaxel represent highly synergistic drug combinations that should be explored further in the different disease states of prostate cancer.

Telomere length and genetic variation in telomere maintenance genes in relation to ovarian cancer risk

BACKGROUND

Telomeres protect chromosomal ends, shorten with cellular division, and signal cellular senescence, but unchecked telomere attrition can lead to telomere dysfunction, upregulation of telomerase, and carcinogenesis. Shorter telomeres in peripheral blood leukocytes (PBL) have been associated with elevated cancer risk. Furthermore, genetic variants in and around the TERT gene have been implicated in carcinogenesis.

METHODS

We measured relative telomere length (RTL) in PBLs of 911 cases and 948 controls from the New England case-control (NECC) study, a population-based study of ovarian cancer. In addition, we assessed germ line genetic variation in five telomere maintenance genes among 2,112 cases and 2,456 controls from the NECC study and the Nurses' Health Study, a prospective cohort study. ORs and 95% CIs were estimated by logistic regression.

RESULTS

Overall, we observed no differences in telomere length between cases and controls. Compared with women with RTL in the longest tertile, women with RTL in the shortest tertile had no increase in risk (OR = 1.01, 95% CI: 0.80-1.28). However, several SNPs in the TERT gene, including rs2736122, rs4246742, rs4975605, rs10069690, rs2736100, rs2853676, and rs7726159, were significantly associated with ovarian cancer risk. We observed a significant gene-level association between TERT and ovarian cancer risk (P = 0.00008).

CONCLUSION

Our observations suggest that genetic variation in the TERT gene may influence ovarian cancer risk, but the association between average telomere length in PBLs and ovarian cancer remains unclear.

IMPACT

The role of telomeres in ovarian carcinogenesis remains unsettled and warrants further investigation.

Tandem mass spectrometry of platinated quadruplex DNA

Quadruplexes are higher-order structures formed by G-rich DNA strands that are involved in various processes of cell cycle regulation, such as control of telomere length and participation in gene regulation. Because of these central biological functions, quadruplex DNA represents a promising target for cancer therapy, e.g. by applying organometallic drugs, such as cisplatin. High-resolution electrospray tandem mass spectrometry is evaluated as a technique for exploring structural features of unplatinated and platinated quadruplexes. Results of experiments on tetramolecular, bimolecular and monomolecular quadruplexes provide information about the extent of platination and the binding sites of the drug. The dissociation behavior of the different types of quadruplexes is compared. Tetramolecular quadruplexes were found to weave out a strand end in order to provide a platination site, and their fragmentation is characterized by the release of an unplatinated strand and the formation of a platinated triplex. Partial opening of the structure in combination with the loss of small fragments leads to truncated quadruplex ions. For the bimolecular quadruplexes studied, strand separation is the predominant dissociation pathway. Depending on the loop sequence, cross-linking of the loops by cisplatin is demonstrated. Distinct differences in the product ion spectra of unannealed and annealed monomolecular sequences provide proof of quadruplex formation and show that platination preferentially occurs at the terminal regions.

Targeting human telomerase for cancer therapeutics

The enzyme telomerase is involved in the replication of telomeres, specialized structures that cap and protect the ends of chromosomes. Its activity is required for maintenance of telomeres and for unlimited lifespan, a hallmark of cancer cells. Telomerase is overexpressed in the vast majority of human cancer cells and therefore represents an attractive target for therapy. Several approaches have been developed to inhibit this enzyme through the targeting of its RNA or catalytic components as well as its DNA substrate, the single-stranded 3'-telomeric overhang. Telomerase inhibitors are chemically diverse and include modified oligonucleotides as well as small diffusable molecules, both natural and synthetic. This review presents an update of recent investigations pertaining to these agents and discusses their biological properties in the context of the initial paradigm that the exposure of cancer cells to these agents should lead to progressive telomere shortening followed by a delayed growth arrest response.

Use of an automated capillary DNA sequencer to investigate the interaction of cisplatin with telomeric DNA sequences

The determination of the sequence selectivity of DNA-damaging agents is very important in elucidating the mechanism of action of anti-tumour drugs. The development of automated capillary DNA sequencers with fluorescent labelling has enabled a more precise method for DNA sequence specificity analysis. In this work we utilized the ABI 3730 capillary sequencer with laser-induced fluorescence to examine the sequence selectivity of cisplatin with purified DNA sequences. The use of this automated machine enabled a higher degree of precision of both position and intensity of cisplatin-DNA adducts than previously possible with manual and automated slab gel procedures. A problem with artefact bands was overcome by ethanol precipitation. It was found that cisplatin strongly formed adducts with telomeric DNA sequences.

Sodium butyrate enhances the cytotoxic effect of cisplatin by abrogating the cisplatin imposed cell cycle arrest

Background

Histone deacetylase inhibitors have been proposed as potential enhancers of the cytotoxic effect of cisplatin and other anticancer drugs. Their application would permit the use of lower therapeutic doses and reduction of the adverse side effects of the drugs. However, the molecular mechanisms by which they sensitize the cells towards anticancer drugs are not known in details, which is an obstacle in developing effective therapeutic protocols.

Results

In the present work, we studied the molecular mechanisms by which sodium butyrate sensitizes cancer cells towards cisplatin. HeLa cells were treated with 5 mM butyrate, with 8 μM cis-diaminedichloroplatinum II (cisplatin), or with both. Cells treated with both agents showed approximately two-fold increase of the mortality rate in comparison with cells treated with cisplatin only. Accordingly, the life span of albino mice transfected with Ehrlich ascites tumor was prolonged almost two-fold by treatment with cisplatin and butyrate in comparison with cisplatin alone. This showed that the observed synergism of cisplatin and butyrate was not limited to specific cell lines or in vitro protocols, but was also expressed in vivo during the process of tumor development. DNA labeling and fluorescence activated cell sorting experiments showed that cisplatin treatment inhibited DNA synthesis and arrested HeLa cells at the G1/S transition and early S phase of the cell cycle. Western blotting and chromatin immunoprecipitation revealed that this effect was accompanied with a decrease of histone H4 acetylation levels. Butyrate treatment initially reversed the effect of cisplatin by increasing the levels of histone H4 acetylation in euchromatin regions responsible for the G1/S phase transition and initiation of DNA synthesis. This abrogated the cisplatin imposed cell cycle arrest and the cells traversed S phase with damaged DNA. However, this effect was transient and continued only a few hours. The long-term effect of butyrate was a massive histone acetylation in both eu- and heterochromatin, inhibition of DNA replication and apoptosis.

Conclusion

The study presents evidence that cell sensitization towards cisplatin by sodium butyrate is due to hyperacetylation of histone H4 in specific chromatin regions, which temporarily abrogates the cisplatin imposed cell cycle arrest.

Biological activity of enantiomeric complexes [PtCl(2)L (2)] (L (2) is aromatic bisphosphanes and aromatic diamines)

Enantiomeric complexes of formula [PtCl(2)L(2)] [L(2) is (R)-(+)-BINAP and (S)-(-)-BINAP, where BINAP is 2,2'-bis(diphenylphosphane)-1,1'-binaphthyl, and (R)-(+)-DABN and (S)-(-)-DABN, where DABN is 1,1'-binaphthyl-2,2'-diamine], were tested for their cytotoxic activity against three cancer cell lines and for their ability to bind to the human telomeric sequence folded in the G-quadruplex structure. Similar experiments were carried out on prototypal complexes cisplatin and cis-[PtCl(2)(PPh(3))(2)] for comparison. Platinum complexes containing phosphanes proved less cytotoxic to cancer cell lines and less likely to interact with the nucleobases of the G-quadruplex than those containing amines; in both cases the S-(-) isomer was more active than the R-(+) counterpart. More specifically, whereas all the platinum complexes were able to platinate the G-quadruplex structure from the human telomeric repeat, the extent and sites of platination depended on the nature of the ligands. Complexes containing (bulky) phosphanes interacted only with the adenines of the loops, whereas those containing the less sterically demanding amines interacted with adenines and some guanines of the G-quartet.

Telomerase activity and telomere length in human tumor cells with acquired resistance to anticancer agents

Telomeres and telomerase are targets for anticancer drug development and specific inhibitors are currently under clinical investigation. However, it has been reported that standard cytotoxic agents can affect telomere length and telomerase activity suggesting that they also have of a role in drug resistance. in this study, telomere lengths and telomerase activity as well as drug efflux pump expression, glutathione (GSH) levels and polyadenosine-ribose polymerase (PARP) cleavage were assessed in a panel of human tumor cell lines made resistant to vindesine, gemcitabine and cisplatin. these included two lung cancer cell lines resistant to vindesine (LXFL 529L/Vind, LXFA 526L/Vind), a renal cancer cell line (RXF944L/Gem) and an ovarian cancer cell line (AG6000) resistant to gemcitabine, and one resistant to cisplatin (ADDP). The resistant clones were compared to their parental lines and evaluated for cross resistance to other cytotoxic agents. Several drug specific resistance patterns were found, and various complex patterns of cross resistance emerged from some cell lines, but these mechanisms of resistance could not be related to drug efflux pump expression, GSH levels or pARp cleavage. However, all displayed changes in telomerase activity and/or telomere length. Our studies present evidence that telomere maintenance should be taken into consideration in efforts not only to overcome drug resistance, but also to optimize the use of telomere-based therapeutics.

Telomere-mediated effects on melanogenesis and skin aging

UV-induced melanogenesis (tanning) and "premature aging" or photoaging result in large part from DNA damage. This article reviews data tying both phenomena to telomere-based DNA damage signaling and develops a conceptual framework in which both responses may be understood as cancer-avoidance protective mechanisms.Journal of Investigative Dermatology Symposium Proceedings (2009) 14, 25-31; doi:10.1038/jidsymp.2009.9.

Molecular disruption of RAD50 sensitizes human tumor cells to cisplatin-based chemotherapy

Platinum-based drugs that induce DNA damage are commonly used first-line chemotherapy agents for testicular, bladder, head and neck, lung, esophageal, stomach, and ovarian cancers. The inherent resistance of tumors to DNA damage often limits the therapeutic efficacy of these agents, such as cisplatin. An enhanced DNA repair and telomere maintenance response by the Mre11/Rad50/Nbs1 (MRN) complex is critical in driving this chemoresistance. We hypothesized therefore that the targeted impairment of native cellular MRN function could sensitize tumor cells to cisplatin. To test this, we designed what we believe to be a novel dominant-negative adenoviral vector containing a mutant RAD50 gene that significantly downregulated MRN expression and markedly disrupted MRN function in human squamous cell carcinoma cells. A combination of cisplatin and mutant RAD50 therapy produced significant tumor cytotoxicity in vitro, with a corresponding increase in DNA damage and telomere shortening. In cisplatin-resistant human squamous cell cancer xenografts in nude mice, this combination therapy caused dramatic tumor regression with increased apoptosis. Our findings suggest the use of targeted RAD50 disruption as what we believe to be a novel chemosensitizing approach for cancer therapy in the context of chemoresistance. This strategy is potentially applicable to several types of malignant tumors that demonstrate chemoresistance and may positively impact the treatment of these patients.

DNA repair proteins and telomerase reverse transcriptase in the cochlear lateral wall of cisplatin-treated rats

Cochlear lateral wall damage is a side effect of cisplatin chemotherapy. Recent studies have shown that cisplatin treatment precipitates platinated DNA adducts in the cochlear lateral wall which suggest that DNA damage may contribute to ototoxicity. Platinated adducts are high-affinity substrates for the global genomic nucleotide excision repair (GG-NeR) pathway which is facilitated by xeroderma pigmentosum (Xp) complementing proteins, such as XpC, XpD and XpA. tumor biology has shown that in addition to stimulating GG-NeR, cisplatin may deplete telomerase reverse transcriptase (teRt). in the current study Fischer344 rats were treated with cisplatin (2 mg/kg/4 days, i.p.) and their cochleae harvested for immunohistochemistry. XpC, XpD and XpA expression increased while teRt expression decreased among cisplatin treated animals compared to vehicle control. these findings suggest that in addition to forming platinated adducts, cisplatin chemotherapy may up-regulate DNA repair proteins and modify teRt expression in the cochlear lateral wall.

Platinum(II) complexes with dipyridophenazine ligands as human telomerase inhibitors and luminescent probes for G-quadruplex DNA

A series of platinum(II) complexes containing dipyridophenazine (dppz) and C-deprotonated 2-phenylpyridine (N-CH) ligands were prepared and assayed for G-quadruplex DNA binding activities. [PtII(dppz-COOH)(N-C)]CF3SO3 (1; dppz-COOH = 11-carboxydipyrido[3,2-a:2',3'-c]phenazine) binds G-quadruplex DNA through an external end-stacking mode with a binding affinity of approximately 10(7) dm3 mol-1. G-quadruplex DNA binding is accompanied by up to a 293-fold increase in the intensity of photoluminescence at lambdamax = 512 nm. Using a biotinylated-primer extension telomerase assay, 1 was shown to be an effective inhibitor of human telomerase in vitro, with a telIC50 value of 760 nM.

DNA adducts of antitumor cisplatin preclude telomeric sequences from forming G quadruplexes

We studied the effect of antitumor cisplatin and inefficient transplatin on the structure and stability of G quadruplexes formed by the model human telomere sequence 5'-GGG(TTAGGG)(3)-3' using circular dichroism, UV-monitored thermal denaturation, and gel electrophoresis. In addition, to investigate whether there is a connection between the ability of cisplatin or transplatin to affect telomerase activity and stability of G quadruplexes, we also used a modified telomere repeat amplification protocol assay that uses an oligonucleotide substrate for telomerase elongation susceptible to forming a G quadruplex. The results indicate that cisplatin is more efficient than transplatin in disturbing the quadruplex structure, thereby precluding telomeric sequences from forming quadruplexes. On the other hand, the results of this work also demonstrate that in absence of free platinum complex, DNA adducts of antitumor cisplatin inhibit telomerase catalysis, so the mechanism underlying this inhibition does not involve formation of the G quadruplexes which are not elongated by telomerase.

Platination of telomeric sequences and nuclease hypersensitive elements of human c-myc and PDGF-A promoters and their ability to form G-quadruplexes

Naturally occurring G-rich DNA sequences that are able to form G-quadruplex structures appear as potential targets for anti-cancer chemotherapy, and therefore play an important role in cellular processes, such as cell aging, death and carcinogenesis. The telomeric regions of DNA and nuclease hypersensitive elements of human c-myc and PDGF-A promoters represent a very appealing target for cisplatin and may interfere with normal DNA function. Platinum complexes bind covalently to nucleobases, and especially to the N7 atom of guanines, and the four guanines of a G-quartet have their N7 atoms involved in hydrogen bonding. Therefore, within a G-quadruplex structure, only the guanines out of the stack of G-quartets should react with electrophilic species such as platinum (II) complexes. Platinum complexes have significant influence on the formation of G-quadruplexes. Results obtained by CD spectroscopy and temperature gradient-gel electrophoresis clearly demonstrate that DNA platination significantly affects G-quadruplex folding for telomeric sequences; the abundance of un/misfolded DNAs compared to the G-quadruplex is proportional to the platinum concentration.

Reversal of the malignant phenotype of ovarian cancer A2780 cells through transfection with wild-type PTEN gene

OBJECTIVE

PTEN (phosphatase and tensin homologue deleted on chromosome 10) is a tumor suppressor gene identified on human chromosome 10q23. Substantial studies have demonstrated that PTEN can inhibit cell proliferation, migration and invasion of many cancer cells. The purpose of this study was to determine whether upregulation of PTEN gene by transfection wild-type PTEN gene to ovarian cancer cells can inhibit growth and migration and to explore the potential for PTEN gene therapy of ovarian cancers.

METHOD

Wild-type and phosphatase-inactive (C124A) PTEN plasmids were transfected into ovarian epithelial cancer A2780 cells, and their effects on cell apoptosis, cell proliferation, cell migration and cell invasion were analyzed by flow cytometry analysis, TUNEL assay, MTT assay, wound-healing assay and transwell assay.

RESULTS

Both wild-type and mutant PTEN can upregulate the expression of PTEN gene dramatically; however, it is wild-type PTEN not phosphatase-inactive PTEN that can induce apoptosis and decrease cell migration, invasion and proliferation in ovarian cancer cells.

CONCLUSION

These results demonstrated that PTEN had played an important role in the cell proliferation, cell migration and invasion dependent on its phosphatase activity. Enhanced expression of PTEN by gene transfer is sufficient to reverse the malignant phenotype of ovarian cancer cells and transfection of ovarian cancer cells with wild-type PTEN gene might be another novel approach for therapeutic intervention in ovarian cancer.

Consequences of telomerase inhibition by BIBR1532 on proliferation and chemosensitivity of chondrosarcoma cell lines

PURPOSE

Human chondrosarcomas are generally resistant to conventional treatments like chemotherapy and radiotherapy. We investigated the effects of BIBR1532, an inhibitor of telomerase on chondrosarcoma cells in vitro.

METHODS

Telomerase activity, telomere lengths, growth kinetics and chemosensitivity were analyzed in chondrosarcoma cell lines treated with BIBR1532.

RESULTS

BIBR1532 treatment resulted in telomerase inhibition, decrease of telomere length and reduction of growth capacity of telomerase positive chondrosarcoma cells. Although resistant to cisplatin, telomerase positive cells were sensitive to paclitaxel, which rapidly induced telomere erosion.

CONCLUSION

Targeting of telomeres might represent a valid strategy for the (re-)sensitization of chemoresistant chondrosarcomas.

Accelerated senescence: an emerging role in tumor cell response to chemotherapy and radiation

Treatment of malignancies with chemotherapeutic drugs and/or radiotherapy is designed to eliminate the disease by depriving the tumor cell of its reproductive potential. Frequently, the desired effect of cell killing is achieved through the promotion of apoptosis; however, accumulating evidence suggests that apoptosis may not be the exclusive or even primary mechanism whereby tumor cells lose their self-renewal capacity after radiation or drug treatment, particularly in the case of solid tumors. While failure to undergo apoptosis in response to chemotherapeutic drugs or radiation may represent a mechanism of drug and radiation resistance, particularly in the case of leukemias and lymphomas, it is gradually being recognized that in the case of solid tumors, loss of reproductive capacity can occur through alternative pathways including reproductive cell death or mitotic catastrophe, through autophagic cell death, and as described below, through a terminally arrested state similar to replicative senescence. Studies building upon the phenomenon of replicative senescence in normal cells approaching the limit of their reproductive potential have identified a comparable senescence-like arrest as a component of the tumor cell response to chemotherapeutic drugs and radiation. This response, which has been termed "premature senescence", "senescence-like growth arrest", "stress-induced premature senescence", and "accelerated senescence", can also result from supraphysiological mitogenic signaling, sub-optimal culture conditions, and ectopic expression of oncogenes. Here, we will use the term "accelerated senescence" in our consideration of the morphological, biochemical, and molecular aspects of treatment-induced senescence, its relationship to classical replicative senescence, its prevalence in clinical specimens and the implications of accelerated senescence for the outcome of cancer therapy.

A dyskerin motif reactivates telomerase activity in X-linked dyskeratosis congenita and in telomerase-deficient human cells

Dyskerin gene is mutated in patients with X-linked dyskeratosis congenita (X-DC), which results in greatly reduced levels of telomerase activity. A genetic suppressor element (GSE) termed GSE24-2 has been isolated in a screening for cisplatin resistance. GSE24-2-expressing cells presented impaired telomerase inhibition following in vitro exposure to chemotherapies, such as cisplatin, or telomerase inhibitors. The promoter of the telomerase component hTERT was constitutively activated in GSE24-2 cells in a c-myc expression-dependent manner. Deletion analyses and mutagenesis of the human c-myc promoter demonstrated that the target sequence for activation was the nuclease hypersensitive element-III (NHEIII) site located upstream to the P1 region of the promoter. Further, expression of GSE24-2 in cell lines derived from patients with X-DC and in VA13 cells induced increased hTERT RNA and hTR levels and recovery of telomerase activity. Finally, expression of GSE24-2 was able to rescue X-DC fibroblasts from premature senescence. These data demonstrate that this domain of dyskerin plays an important role in telomerase maintenance following cell insults such as cisplatin treatment, and in telomerase-defective cells in patients with X-DC. The expression of this dyskerin fragment has a dominant function in X-DC cells and could provide the basis for a therapeutic approach to this disease.