Suppression of telomere-binding protein TPP1 resulted in telomere dysfunction and enhanced radiation sensitivity in telomerase-negative osteosarcoma cell line

Shaping DNA damage responses: Therapeutic potential of targeting telomeric proteins and DNA repair factors in cancer

Shelterin proteins regulate genomic stability by preventing inappropriate DNA damage responses (DDRs) at telomeres. Unprotected telomeres lead to persistent DDR causing cell cycle inhibition, growth arrest, and apoptosis. Cancer cells rely on DDR to protect themselves from DNA lesions and exogenous DNA-damaging agents such as chemotherapy and radiotherapy. Therefore, targeting DDR machinery is a promising strategy to increase the sensitivity of cancer cells to existing cancer therapies. However, the success of these DDR inhibitors depends on other mutations, and over time, patients develop resistance to these therapies. This suggests the need for alternative approaches. One promising strategy is co-inhibiting shelterin proteins with DDR molecules, which would offset cellular fitness in DNA repair in a mutation-independent manner. This review highlights the associations and dependencies of the shelterin complex with the DDR proteins and discusses potential co-inhibition strategies that might improve the therapeutic potential of current inhibitors.

The Altered Functions of Shelterin Components in ALT Cells

Telomeres are nucleoprotein complexes that cap the ends of eukaryotic linear chromosomes. Telomeric DNA is bound by shelterin protein complex to prevent telomeric chromosome ends from being recognized as damaged sites for abnormal repair. To overcome the end replication problem, cancer cells mostly preserve their telomeres by reactivating telomerase, but a minority (10-15%) of cancer cells use a homologous recombination-based pathway called alternative lengthening of telomeres (ALT). Recent studies have found that shelterin components play an important role in the ALT mechanism. The binding of TRF1, TRF2, and RAP1 to telomeres attenuates ALT activation, while the maintenance of ALT telomere requires TRF1 and TRF2. POT1 and TPP1 can also influence the occurrence of ALT. The elucidation of how shelterin regulates the initiation of ALT remains elusive. This review presents a comprehensive overview of the current findings on the regulation of ALT by shelterin components, aiming to enhance the insight into the altered functions of shelterin components in ALT cells and to identify potential targets for the treatment of ALT tumor cells.

POT1 involved in telomeric DNA damage repair and genomic stability of cervical cancer cells in response to radiation

Though telomeres play a crucial role in maintaining genomic stability in cancer cells and have emerged as attractive therapeutic targets in anticancer therapy, the relationship between telomere dysfunction and genomic instability induced by irradiation is still unclear. In this study, we identified that protection of telomeres 1 (POT1), a single-stranded DNA (ssDNA)-binding protein, was upregulated in γ-irradiated HeLa cells and in cancer patients who exhibit radiation tolerance. Knockdown of POT1 delayed the repair of radiation-induced telomeric DNA damage which was associated with enhanced H3K9 trimethylation and enhanced the radiosensitivity of HeLa cells. The depletion of POT1 also resulted in significant genomic instability, by showing a significant increase in end-to-end chromosomal fusions, and the formation of anaphase bridges and micronuclei. Furthermore, knockdown of POT1 disturbed telomerase recruitment to telomere, and POT1 could interact with phosphorylated ATM (p-ATM) and POT1 depletion decreased the levels of p-ATM induced by irradiation, suggesting that POT1 could regulate the telomerase recruitment to telomeres to repair irradiation-induced telomeric DNA damage of HeLa cells through interactions with p-ATM. The enhancement of radiosensitivity in cancer cells can be achieved through the combination of POT1 and telomerase inhibitors, presenting a potential approach for radiotherapy in cancer treatment.

A Role for Human DNA Polymerase λ in Alternative Lengthening of Telomeres

Telomerase negative cancer cell types use the Alternative Lengthening of Telomeres (ALT) pathway to elongate telomeres ends. Here, we show that silencing human DNA polymerase (Pol λ) in ALT cells represses ALT activity and induces telomeric stress. In addition, replication stress in the absence of Pol λ, strongly affects the survival of ALT cells. In vitro, Pol λ can promote annealing of even a single G-rich telomeric repeat to its complementary strand and use it to prime DNA synthesis. The noncoding telomeric repeat containing RNA TERRA and replication protein A negatively regulate this activity, while the Protection of Telomeres protein 1 (POT1)/TPP1 heterodimer stimulates Pol λ. Pol λ associates with telomeres and colocalizes with TPP1 in cells. In summary, our data suggest a role of Pol λ in the maintenance of telomeres by the ALT mechanism.

TPP1 Enhances the Therapeutic Effects of Transplanted Aged Mesenchymal Stem Cells in Infarcted Hearts via the MRE11/AKT Pathway

Background

Poor cell survival after transplantation restricts the therapeutic potential of mesenchymal stem cell (MSC) transplantation into infarcted hearts, particularly in older individuals. TPP1, a component of the shelterin complex that is involved in telomere protection, is highly expressed in young MSCs but declines in aged ones. Here, we explore whether TPP1 overexpression in aged mouse MSCs improves cell viability in vivo and in vitro.

Methods

Aged mouse MSCs overexpressing TPP1 were injected into the peri-infarct area of the mouse heart after left anterior descending coronary artery ligation. In parallel, to evaluate cellular-level effects, H₂O₂ was applied to MSCs in vitro to mimic the microenvironment of myocardial injury.

Results

In vivo, the transplantation of aged MSCs overexpressing TPP1 resulted in improved cell survival, enhanced cardiac function, and reduced fibrosis compared to unmodified aged MSCs. In vitro, TPP1 overexpression protected aged MSCs from H₂O₂-induced apoptosis and enhanced DNA double-strand break (DSB) repair. In addition, the phosphorylation of AKT and the key DSB repair protein MRE11 were both significantly upregulated in aged MSCs that overexpressed TPP1.

Conclusions

Our results reveal that TPP1 can enhance DNA repair through the AKT/MRE11 pathway, thereby improving the therapeutic effects of aged MSC transplantation and offering significant potential for the clinical application of autologous transplantation in aged patients.

Identification of three predictors of gastric cancer progression and prognosis

Abnormal gene expression is an established cause of gastric cancer (GC) initiation and progression. In this study, we aimed to identify several key genes that could be used to effectively predict progression and prognosis in patients with GC. The Cancer Genome Atlas and the Gene Expression Omnibus database were used to identify candidate genes. Fourteen genes were found to associate highly with progress, metastasis, and survival of GC. Five of these genes were overexpressed in tumor tissue compared to adjacent normal tissue. This was confirmed by reverse transcription‐polymerase chain reaction and western blotting for myosin‐Va (MYO5A), phospholipid transfer protein (PLTP), and tripeptidyl peptidase 1 (TPP1), while the CCK8 assay was used to show that these three genes promote GC cell proliferation. In summary, we demonstrate that MYO5A, PLTP, and TPP1 expression may be suitable markers for the progression and prognosis of GC.

BIBR1532, a Selective Telomerase Inhibitor, Enhances Radiosensitivity of Non-Small Cell Lung Cancer Through Increasing Telomere Dysfunction and ATM/CHK1 Inhibition

PURPOSE

Telomerase is reactivated in non-small cell lung cancer (NSCLC), and it increases cell resistance to irradiation through protecting damaged telomeres and enhancing DNA damage repair. We investigated the radiosensitizing effect of BIBR1532, a highly selective telomerase inhibitor, and its corresponding mechanism in NSCLC.

METHODS AND MATERIALS

Cell proliferation, telomerase activity, and telomere dysfunction-induced foci were measured with CCK-8 assay, real-time fluorescent quantitative polymerase chain reaction, and immunofluorescence. The effect of BIBR1532 on the response of NSCLC cells to radiation was analyzed using clonogenic survival and xenograft tumor assays. Cell death and cell senescence induced by BIBR1532 or ionizing radiation (IR), or both, were detected with western blotting, flow cytometry, and senescence-association β-galactosidase staining assay.

RESULTS

We observed dose-dependent direct cytotoxicity of BIBR1532 at relatively high concentrations in NSCLC cells. Low concentrations of BIBR1532 did not appear toxic to NSCLC cells; however, they substantially increased the therapeutic efficacy of IR in vitro by enhancing IR-induced apoptosis, senescence, and mitotic catastrophe. Moreover, in a mouse xenograft model, BIBR1532 treatment synergized with IR at nontoxic dose levels promoted the antitumor efficacy of IR without toxicity to hematologic and internal organs. Mechanistically, lower concentrations of BIBR1532 effectively inhibited telomerase activity and increased IR-induced telomere dysfunction, resulting in disruption of chromosomal stability and inhibition of the ATM/CHK1 (ataxia-telangiectasia-mutated/Checkpoint kinase 1) pathway, which impaired DNA damage repair.

CONCLUSIONS

Our findings demonstrate that disturbances in telomerase function by nontoxic dose levels of BIBR1532 effectively enhance the radiosensitivity of NSCLC cells. This finding provides a rationale for the clinical assessment of BIBR1532 as a radiosensitizer.

ALT cancer cells are specifically sensitive to lysine acetyl transferase inhibition

Some cancer cells elongate their telomeres through the ALT (alternative lengthening of telomeres) pathway, which is based on homologous recombination for the addition of telomere repeats without telomerase activity. General control non-derepressible 5 (GCN5) and P300/CBP-associated factor (PCAF), two homologous lysine acetyltransferases, exert opposite effects on the ALT pathway, inhibiting or favoring it respectively. Here we show that ALT cells are particularly sensitive to the inhibition of acetyltransferases activities using Anacardic Acid (AA). AA treatment recapitulates the effect of PCAF knockdown on several ALT features, suggesting that AA decreased the ALT mechanism through the inhibition of lysine transferase activity of PCAF, but not that of GCN5. Furthermore, AA specifically sensitizes human ALT cells to radiation as compared to telomerase-positive cells suggesting that the inhibition of lysine acetyltransferases activity may be used to increase the radiotherapy efficiency against ALT cancers.

HMGB1 and repair: focus on the heart

High-mobility group box 1 (HMGB1) is one of the most abundant proteins in eukaryotes and the best characterized damage-associated molecular pattern (DAMP). The biological activities of HMGB1 depend on its subcellular location, context and post-translational modifications. Inside the nucleus, HMGB1 is engaged in many DNA events such as DNA repair, transcription regulation and genome stability; in the cytoplasm, its main function is to regulate the autophagic flux while in the extracellular environment, it possesses more complicated functions and it is involved in a large variety of different processes such as inflammation, migration, invasion, proliferation, differentiation and tissue regeneration. Due to this pleiotropy, the role of HMGB1 has been vastly investigated in various pathological diseases and a large number of studies have explored its function in cardiovascular pathologies. However, in this contest, the precise mechanism of action of HMGB1 and its therapeutic potential are still very controversial since is debated whether HMGB1 is involved in tissue damage or plays a role in tissue repair and regeneration. The main focus of this review is to provide an overview of the effects of HMGB1 in different ischemic heart diseases and to discuss its functions in these pathological conditions.

Silencing of telomere-binding protein adrenocortical dysplasia (ACD) homolog enhances radiosensitivity in glioblastoma cells

Adrenocortical dysplasia (ACD) is a shelterin protein involved in the maintenance of telomere length and in cancer radioresistance. This study investigated the expression profile of ACD in human gliomas and its role in radioresistance of glioma cells. The expression of ACD was analyzed in 62 different grades of glioma tissues and correlated with prognosis. A radioresistant cell line was generated from U87MG cells. For mechanistic studies, ACD was inhibited by small interfering RNA-targeting ACD and the effect on cell radioresistance, telomerase activity, cyclinD1, caspase-3, hTERT, and BIRC1 was evaluated. Clonogenic assay was performed after irradiation, to investigate the effect of ACD silencing on radiation sensitivity. ACD expression appeared strongly upregulated in higher grade gliomas, and its expression was significantly correlated to grading and poor prognosis. In glioma cell lines, ACD expression pattern was similar to those observed in glioma tissues. In irradiated cells, ACD expression was increased in an ionizing radiation dose-dependent manner. A higher expression of ACD was observed in the radioresistant clones than parental cells. Silencing of ACD led to the enhanced radiation sensitivity, decreased telomerase activity and cyclin D1 expression, reduced expression of BIRC1, and finally to the upregulation of caspase-3. This study represents the first report, which demonstrated the expression pattern of ACD in gliomas and its prognostic value. Our results suggested that ACD is involved in glioblastoma radioresistance, likely through the modulation of telomerase activity, proliferation, and apoptosis. ACD might represent a potential molecular biomarker and a novel therapeutic target in glioblastoma.

Effect of therapies-mediated modulation of telomere and/or telomerase on cancer cells radiosensitivity

Cancer is one of the leading causes of death in the world. Many strategies of cancer treatment such as radiotherapy which plays a key role in cancer treatment are developed and used nowadays. However, the side effects post-cancer radiotherapy and cancer radioresistance are two major causes of the limitation of cancer radiotherapy effectiveness in the cancer patients. Moreover, reduction of the limitation of cancer radiotherapy effectiveness by reducing the side effects post-cancer radiotherapy and cancer radioresistance is the aim of several radiotherapy-oncologic teams. Otherwise, Telomere and telomerase are two cells components which play an important role in cancer initiation, cancer progression and cancer therapy resistance such as radiotherapy resistance. For resolving the problems of the limitation of cancer radiotherapy effectiveness especially the cancer radio-resistance problems, the radio-gene-therapy strategy which is the use of gene-therapy via modulation of gene expression combined with radiotherapy was developed and used as a new strategy to treat the patients with cancer. In this review, we summarized the information concerning the implication of telomere and telomerase modulation in cancer radiosensitivity.

Shelterin differentially respond to oxidative stress induced by TiO2-NPs and regulate telomere length in human hepatocytes and hepatocarcinoma cells in vitro

Titanium dioxide nanoparticles (TiO₂-NPs) have raised serious attention for their widely use and potential adverse effects on human mainly due to producing ROS. However, the influence of TiO₂-NPs on telomere maintaining has not been studied clearly. Shelterin plays core roles in telomere length (TL) regulation. Abnormal TL are associated with chromosome instability (CIN) and high risk of diseases. This study investigated whether TiO₂-NPs affect TL to induce CIN through ROS generation and the possible mechanisms. Human hepatocyte L-02 and hepatocarcinoma cells QGY were exposed to TiO₂-NPs (0, 40, 80 μg/mL) for 72 h. The intracellular hydrogen dioxide (H₂O₂) concentration were measured. The TL, Nrf-2, and three core shelterin components (TRF1, TRF2, and POT1) transcription level were determined by quantitative real-time PCR. CIN was measured by cytokinesis-block micronucleus assay. TiO₂-NPs exposure increased intracellular H₂O₂ in both L-02 and QGY cells, and induced Nrf-2, TRF1, TRF2, POT1 downregulated transcription compared with control (P < 0.001) in L-02 but all upregulated (P < 0.05) in QGY. Significant TL shortening (P < 0.001) and CIN increase (P < 0.01) in L-02 cells were observed but not in QGY cells. The differentially responses of the tested components of shelterin and Nrf-2 to oxidative stress induced by TiO₂-NPs led to the weakened telomere protection in normal cells and effective telomere maintenance in cancer cells, respectively. The upregulation of Nrf-2 and shelterin could protect TL and chromosome stability against TiO₂-NPs exposure.

UBE2D3 gene overexpression increases radiosensitivity of EC109 esophageal cancer cells in vitro and in vivo

Ubiquitin-conjugating enzyme E2D3 (UBE2D3), a key component in ubiquitin (Ub) proteasome system, plays a crucial role in tumorigenesis. We previously found that it is bound to hTERT, and UBE2D3 could attenuate radiosensitivity of human breast cancer cells. Here we investigated a contributing role of UBE2D3 in radiosensitivity of esophageal squamous carcinoma. We demonstrated that the overexpression of UBE2D3 in esophageal squamous carcinoma cells (EC109) resulted in prolonged G1 phase and shortened G2/M phase after irradiation. UBE2D3 overexpression also decreased length of telomere and activity of telomerase. In addition, the overexpression of UBE2D3 increased mRNA expression but decreased protein levels of hTERT in both vitro and vivo systems. Compared with untreated cells, the treatment of UBE2D3 overexpressing cells with the specific proteasome inhibitor (MG132) could up-regulate hTERT. MG132 treatment of UBE2D3 overexpressed cells caused a clear and dramatic increase in the amount of ubiquitinated hTERT species. These findings indicate that UBE2D3 enhances radiosensitivity of EC109 cells by degradating hTERT through the ubiquitin proteolysis pathway.

The dual role and therapeutic potential of high-mobility group box 1 in cancer

High-mobility group box 1 (HMGB1) is an abundant protein in most eukaryocytes. It can bind to several receptors such as advanced glycation end products (RAGE) and Toll-like receptors (TLRs), in direct or indirect way. The biological effects of HMGB1 depend on its expression and subcellular location. Inside the nucleus, HMGB1 is engaged in many DNA events such as DNA repair, transcription, telomere maintenance, and genome stability. While outside the nucleus, it possesses more complicated functions, including regulating cell proliferation, autophagy, inflammation and immunity. During tumor development, HMGB1 has been characterized as both a pro- and anti-tumoral protein by either promoting or suppressing tumor growth, proliferation, angiogenesis, invasion and metastasis. However, the current knowledge concerning the positive and negative effects of HMGB1 on tumor development is not explicit. Here, we evaluate the role of HMGB1 in tumor development and attempt to reconcile the dual effects of HMGB1 in carcinogenesis. Furthermore, we would like to present current strategies targeting against HMGB1, its receptor or release, which have shown potentially therapeutic value in cancer intervention.

Targeting telomerase and telomeres to enhance ionizing radiation effects in in vitro and in vivo cancer models

One of the hallmarks of cancer consists in the ability of tumor cells to divide indefinitely, and to maintain stable telomere lengths throughout the activation of specific telomere maintenance mechanisms (TMM). Therefore in the last fifteen years, researchers proposed to target telomerase or telomeric structure in order to block limitless replicative potential of cancer cells providing a fascinating strategy for a broad-spectrum cancer therapy. In the present review, we report in vitro and in vivo evidence regarding the use of chemical agents targeting both telomerase or telomere structure and showing promising antitumor effects when used in combination with ionizing radiation (IR). RNA interference, antisense oligonucleotides (e.g., GRN163L), non-nucleoside inhibitors (e.g., BIBR1532) and nucleoside analogs (e.g., AZT) represent some of the most potent strategies to inhibit telomerase activity used in combination with IR. Furthermore, radiosensitizing effects were demonstrated also for agents acting directly on the telomeric structure such as G4-ligands (e.g., RHPS4 and Telomestatin) or telomeric-oligos (T-oligos). To date, some of these compounds are under clinical evaluation (e.g., GRN163L and KML001). Advantages of Telomere/Telomerase Targeting Compounds (T/TTCs) coupled with radiotherapy may be relevant in the treatment of radioresistant tumors and in the development of new optimized treatment plans with reduced dose adsorbed by patients and consequent attenuation of short- end long-term side effects. Pros and cons of possible future applications in cancer therapy based on the combination of T/TCCs and radiation treatment are discussed.

The telomere proteins in tumorigenesis and clinical outcomes of oral squamous cell carcinoma

The "Hallmarks of Cancer" describe the ways by which cancer cells bypass homeostasis. Escape from replicative senescence is one of the earliest features of cancer cells. Maintenance of the telomeres through reactivation of telomerase was initially associated with replicative immortality in various cancers. The shelterin complex, a telomeric hexaprotein association, plays a key role in telomere maintenance and in the hallmarks of cancer. Some shelterin proteins are overexpressed in diverse cancers and can promote tumorigenesis in animal models. Shelterin can also have an impact on tumor size, tumor growth and resistance to treatment. Studies into the expression level of shelterin in oral squamous cell carcinoma (OSCC) report contradictory results. Moreover, the exact role of these proteins in OSCC tumorigenesis remains uncertain. In this review, we examined the data linking telomeres and hallmarks of OSCC. Furthermore, we examined the literature concerning telomeres and the clinical outcome of OSCC. Finally, we propose a model encompassing the role of shelterin proteins in oral tumorigenesis and treatment outcome.

The role of telomeres in predicting individual radiosensitivity of patients with cancer in the era of personalized radiotherapy

Radiotherapy plays a key role in cancer treatments, but tumor cell death differs from one tumor to another. The response of patients to radiotherapy varies considerably and adverse side effects are difficult to prevent. The mechanisms involved in the heterogeneity of this response are not well understood. In order to enhance the efficacy and safety of radiotherapy, it is important to identify subpopulations most at risk of developing a late adverse response to radiotherapy. Telomeres are composed of multiple repeats of a unique sequence of nucleotides forming a TTAGGG pattern. They protect chromosomes from end-to-end fusion and maintain genomic stability. Telomeres have been shown to be extremely sensitive to radiotherapy especially because of their atypical DNA damage repair response, which includes partial inhibition of the non-homologous end joining repair pathway. Ionizing Radiation (IR)-induced damage to telomere DNA could lead to chromosome instability and the initiation or progression of tumor processes. Telomeres could thus be a reliable marker of IR exposure and as such become a new parameter for predicting radiosensitivity. Furthermore, short telomeres are more sensitive to radiotherapy, which could partially explain differences in tumor cell death and in inter-individual sensitivity to radiotherapy. Telomere length could be used to identify subpopulations of patients who could benefit from higher or lower doses per fraction. Finally, pharmacological interference with tumor-cell telomere biology to reduce telomere length and/or telomere stability could also enhance the effectiveness and safety of radiotherapy. Telomeres could play a key role in radiotherapy in the era of personalized medicine.

Downregulation of high mobility group box 1 modulates telomere homeostasis and increases the radiosensitivity of human breast cancer cells

The functions of the high mobility group box 1 (HMGB1) in tumor cells include replenishing telomeric DNA and maintaining cell immortality. There is a negative correlation between human telomerase reverse transcriptase (hTERT) and radiosensitivity in tumor cells. Our aim was to elucidate the relationship among HMGB1, telomere homeostasis and radiosensitivity in MCF-7 cells. In this study, we established stably transfected control (MCF-7-NC) and HMGB1 knockdown (MCF-7-shHMGB1) cell lines. The expression of HMGB1 mRNA and the relative telomere length were examined by real-time PCR. Radiosensitivity was detected by clonogenic assay. The protein expressions were determined by western blot analysis. The telomerase activity was detected by PCR-ELISA. Proliferation ability was examined by CCK-8 assay. Cell cycle and apoptosis were examined by flow cytometry. DNA damage foci were detected by immunofluorescence. ShRNA-mediated downregulation of HMGB1 expression increased the radiosensitivity of MCF-7 cells, and reduced the accumulation of hTERT and cyclin D1. Moreover, knockdown of HMGB1 in MCF-7 cells inhibited telomerase activity and cell proliferation, while increasing the extent of apoptosis. Downregulation of HMGB1 modulated telomere homeostasis by changing the level of telomere-binding proteins, such as TPP1 (PTOP), TRF1 and TRF2. This downregulation also inhibited the ATM and ATR signaling pathways. The current data demonstrate that knockdown of HMGB1 breaks telomere homeostasis, enhances radiosensitivity, and suppresses the repair of DNA damage in human breast cancer cells. These results suggested that HMGB1 might be a potential radiotherapy target in human breast cancer.