Emerging cancer therapeutic opportunities target DNA-repair systems

Natural products for combating multidrug resistance in cancer

Cancer cells frequently develop resistance to chemotherapeutic therapies and targeted drugs, which has been a significant challenge in cancer management. With the growing advances in technologies in isolation and identification of natural products, the potential of natural products in combating cancer multidrug resistance has received substantial attention. Importantly, natural products can impact multiple targets, which can be valuable in overcoming drug resistance from different perspectives. In the current review, we will describe the well-established mechanisms underlying multidrug resistance, and introduce natural products that could target these multidrug resistant mechanisms. Specifically, we will discuss natural compounds such as curcumin, resveratrol, baicalein, chrysin and more, and their potential roles in combating multidrug resistance. This review article aims to provide a systematic summary of recent advances of natural products in combating cancer drug resistance, and will provide rationales for novel drug discovery.

Recent advances in EZH2-based dual inhibitors in the treatment of cancers

The enhancer of zeste homolog 2 (EZH2) protein is the catalytic subunit of one of the histone methyltransferases. EZH2 catalyzes the trimethylation of lysine 27 of histone H3 (H3K27me3) and further alters downstream target levels. EZH2 is upregulated in cancer tissues, wherein its levels correlate strongly with cancer genesis, progression, metastasis, and invasion. Consequently, it has emerged as a novel anticancer therapeutic target. Nonetheless, developing EZH2 inhibitors (EZH2i) has encountered numerous difficulties, such as pre-clinical drug resistance and poor therapeutic effect. The EZH2i synergistically suppresses cancers when used in combination with additional antitumor drugs, such as PARP inhibitors, HDAC inhibitors, BRD4 inhibitors, EZH1 inhibitors, and EHMT2 inhibitors. Typically, the use of dual inhibitors of two different targets mediated by one individual molecule has been recognized as the preferred approach for overcoming the limitations of EZH2 monotherapy. The present review discusses the theoretical basis for designing EZH2-based dual-target inhibitors, and also describes some in vitro and in vivo analysis results.

Suppressing cancer by damaging cancer cell DNA using LED irradiation

BACKGROUND

High-energy irradiation eliminates cancer cells by destroying their genetic components. However, there are several side effects from doing this, such as fatigue, dermatitis, and hair loss, which remain obstacles to this treatment. Here, we propose a moderate method that uses low-energy white light from a light-emitting diode (LED) to selectively inhibit cancer cell proliferation without affecting normal cells.

METHODS

The association between LED irradiation and cancer cell growth arrest was evaluated based on cell proliferation, viability, and apoptotic activity. Immunofluorescence, polymerase chain reaction, and western blotting were performed in vitro and in vivo to identify the metabolism related to the inhibition of HeLa cell proliferation.

RESULTS

LED irradiation aggravated the defective p53 signaling pathway and induced cell growth arrest in cancer cells. Consequently, cancer cell apoptosis was induced by the increased DNA damage. Additionally, LED irradiation inhibited the proliferation of cancer cells by suppressing the MAPK pathway. Furthermore, the suppression of cancer growth by the regulation of p53 and MAPK was observed in cancer-bearing mice irradiated with LED.

CONCLUSIONS

Our findings suggest that LED irradiation can suppress cancer cell activity and may contribute to preventing the proliferation of cancer cells after medical surgery without causing side effects.

miRNAs are now starring in "No Time to Die: Overcoming the chemoresistance in cancer"

Cancer is a leading cause of death globally, with about 19.3 million new cases reported each year. Current therapies for cancer management include-chemotherapy, radiotherapy, and surgery. However, they are loaded with side effects and tend to cause toxicity in the patient's body posttreatment, ultimately hindering the response towards the treatment building up resistance. This is where noncoding RNAs such as miRNAs help provide us with a helping hand for taming the chemoresistance and providing potential holistic cancer management. MicroRNAs are promising targets for anticancer therapy as they perform critical regulatory roles in various signaling cascades related to cell proliferation, apoptosis, migration, and invasion. Combining miRNAs and anticancer drugs and devising a combination therapy has managed cancer well in various independent studies. This review aims to provide insights into how miRNAs play a mechanistic role in cancer development and progression and regulate drug resistance in various types of cancers. Furthermore, next-generation novel therapies using miRNAs in combination with anticancer treatments in multiple cancers have been put forth and how they improve the efficacy of the treatments. Exemplary studies currently in the preclinical and clinical models have been summarized. Ultimately, we briefly talk through the challenges that come forward with it and minimize them.

Cancer Chemoresistance; Recent Challenges and Future Considerations

Cancer remains one of the serious health hazards and major causes of human mortality across the world. Despite the development of many typical antineoplastic drugs and introduction of novel targeted agents, chemoresistance constitutes a major challenge in the effective therapeutic management of cancer. Drug inactivation, efflux of anticancer agents, modification of target sites, enhanced repair of DNA damage, apoptosis failure and induction of epithelial-mesenchymal transition are the principal mechanisms of cancer chemoresistance. Moreover, epigenetics, cell signaling, tumor heterogeneity, stem cells, microRNAs, endoplasmic reticulum, tumor microenvironment and exosomes have also been implicated in the multifaceted phenomenon of anticancer drug resistance. The tendency of resistance is either intrinsically possessed or subsequently acquired by cancerous cells. From clinical oncology standpoint, therapeutic failure and tumor progression are the most probable consequences of cancer chemoresistance. Combination therapy can help to overcome the issue of drug resistance, and therefore, the development of such treatment regimens is recommended for counteracting the emergence and dissemination of cancer chemoresistance. This chapter outlines the current knowledge on underlying mechanisms, contributory biological factors and likely consequences of cancer chemoresistance. Besides, prognostic biomarkers, diagnostic methods and potential approaches to overcome the emergence of antineoplastic drug resistance have also been described.

Repeated treatments of Capan-1 cells with PARP1 and Chk1 inhibitors promote drug resistance, migration and invasion

PARP1 and Chk1 inhibitors have been shown to be synergistic in different cancer models in relatively short time treatment modes. However, the consequences of long-term/repeated treatments with the combinations in cancer models remain unclear. In this study, the synergistic cytotoxicity of their combinations in 8 tumor cell lines was confirmed in a 7-day exposure mode. Then, pancreatic Capan-1 cells were repeatedly treated with the PARP1 inhibitor olaparib, the Chk1 inhibitor rabusertib or their combination for 211–214 days, during which the changes in drug sensitivity were monitored at a 35-day interval. Unexpectedly, among the 3 treatment modes, the combination treatments resulted in the highest-grade resistance to Chk1 (~14.6 fold) and PARP1 (~420.2 fold) inhibitors, respectively. Consistently, G2/M arrest and apoptosis decreased significantly in the resulting resistant variants exposed to olaparib. All 3 resistant variants also unexpectedly obtained enhanced migratory and invasive capabilities. Moreover, the combination treatments resulted in increased migration and invasion than olaparib alone. The expression of 124 genes changed significantly in all the resistant variants. We further demonstrate that activating CXCL3-ERK1/2 signaling might contribute to the enhanced migratory capabilities rather than the acquired drug resistance. Our findings indicate that repeated treatments with the rabusertib/olaparib combination result in increased drug resistance and a more aggressive cell phenotype than those with either single agent, providing new clues for future clinical anticancer tests of PARP1 and Chk1 inhibitor combinations.

Investigating the Anticancer Potential of Salvicine as a Modulator of Topoisomerase II and ROS Signaling Cascade

Salvicine is a new diterpenoid quinone substance from a natural source, specifically in a Chinese herb. It has powerful growth-controlling abilities against a broad range of human cancer cells in both in vitro and in vivo environments. A significant inhibitory effect of salvicine on multidrug-resistant (MDR) cells has also been discovered. Several research studies have examined the activities of salvicine on topoisomerase II (Topo II) by inducing reactive oxygen species (ROS) signaling. As opposed to the well-known Topo II toxin etoposide, salvicine mostly decreases the catalytic activity with a negligible DNA breakage effect, as revealed by several enzymatic experiments. Interestingly, salvicine dramatically reduces lung metastatic formation in the MDA-MB-435 orthotopic lung cancer cell line. Recent investigations have established that salvicine is a new non-intercalative Topo II toxin by interacting with the ATPase domains, increasing DNA-Topo II interaction, and suppressing DNA relegation and ATP hydrolysis. In addition, investigations have revealed that salvicine-induced ROS play a critical role in the anticancer-mediated signaling pathway, involving Topo II suppression, DNA damage, overcoming multidrug resistance, and tumor cell adhesion suppression, among other things. In the current study, we demonstrate the role of salvicine in regulating the ROS signaling pathway and the DNA damage response (DDR) in suppressing the progression of cancer cells. We depict the mechanism of action of salvicine in suppressing the DNA-Topo II complex through ROS induction along with a brief discussion of the anticancer perspective of salvicine.

Discovery of a Dual Tubulin and Poly(ADP-Ribose) Polymerase-1 Inhibitor by Structure-Based Pharmacophore Modeling, Virtual Screening, Molecular Docking, and Biological Evaluation

Dual inhibition of tubulin and poly(ADP-ribose) polymerase-1 (PARP-1) may become an attractive approach for cancer therapy. Here, we discover a dual tubulin/PARP-1 inhibitor (termed as TP-3) using structure-based virtual screening. TP-3 shows strong dual inhibitory effects on both tubulin and PARP-1. Cellular assays reveal that TP-3 shows superior antiproliferative activities against human cancer cells, including breast, liver, ovarian, and cervical cancers. Further studies indicate that TP-3 plays an antitumor role through multiple mechanisms, including the disturbance of the microtubule network and the PARP-1 DNA repairing function, accumulation of DNA double-strand breaks, inhibition of the tube formation, and induction of G2/M cell cycle arrest and apoptosis. In vivo assessment indicates that TP-3 inhibits the growth of MDA-MB-231 xenograft tumors in nude mouse with no notable side effects. These data demonstrate that TP-3 is a dual-targeting, high-efficacy, and low-toxic antitumor agent.

The splicing factor XAB2 interacts with ERCC1-XPF and XPG for R-loop processing

RNA splicing, transcription and the DNA damage response are intriguingly linked in mammals but the underlying mechanisms remain poorly understood. Using an in vivo biotinylation tagging approach in mice, we show that the splicing factor XAB2 interacts with the core spliceosome and that it binds to spliceosomal U4 and U6 snRNAs and pre-mRNAs in developing livers. XAB2 depletion leads to aberrant intron retention, R-loop formation and DNA damage in cells. Studies in illudin S-treated cells and Csb^m/m developing livers reveal that transcription-blocking DNA lesions trigger the release of XAB2 from all RNA targets tested. Immunoprecipitation studies reveal that XAB2 interacts with ERCC1-XPF and XPG endonucleases outside nucleotide excision repair and that the trimeric protein complex binds RNA:DNA hybrids under conditions that favor the formation of R-loops. Thus, XAB2 functionally links the spliceosomal response to DNA damage with R-loop processing with important ramifications for transcription-coupled DNA repair disorders.

XPA-binding protein (XAB)-2 is the human homologue of the yeast pre-mRNA splicing factor Syf1. Here the authors use an in vivo biotinylation tagging approach to show XAB2’s role in DNA repair, RNA splicing and transcription during mammalian development.

Identification of probe-quality degraders for Poly(ADP-ribose) polymerase-1 (PARP-1)

Poly(ADP-ribose) polymerase-1 (PARP-1), a critical DNA repair enzyme in the base excision repair pathway, has been pursued as an attractive cancer therapeutic target. Intervention with PARP-1 has been proved to be more sensitive to cancer cells carrying BRCA1/2 mutations. Several PARP-1 inhibitors have been available on market for the treatment of breast, ovarian and prostatic cancer. Promisingly, the newly developed proteolysis targeting chimaeras (PROTACs) may provide a more potential strategy based on the degradation of PARP-1. Here we report the design, synthesis, and evaluation of a proteolysis targeting chimaera (PROTAC) based on the combination of PARP-1 inhibitor olaparib and the CRBN (cereblon) ligand lenalidomide. In SW620 cells, our probe-quality degrader compound 2 effectively induced PARP-1 degradation which results in anti-proliferation, cells apoptosis, cell cycle arresting, and cancer cells migratory inhibition. Thus, our findings qualify a new chemical probe for PARP-1 knockdown.

Apurinic/apyrimidinic endonuclease 1/reduction-oxidation effector factor-1 (APE1) regulates the expression of NLR family pyrin domain containing 3 (NLRP3) inflammasome through modulating transcription factor NF-κB and promoting the secretion of inflammatory mediators in macrophages

Background

Inflammatory mediators play an important role in the occurrence, development, and metastasis of tumors. The aim of the present study was to elucidate the effect of apurinic/apyrimidinic endonuclease 1/reduction-oxidation effector factor-1 (APE1) on inflammatory mediator secretion, which is dependent on the APE1-mediated NLR family pyrin domain containing 3 (NLRP3) regulatory mechanism.

Methods

The human myeloid leukemia mononuclear cell line (THP-1) cells were cultured and polarized to M2 subset macrophages. Enzyme-linked immunosorbent assay was used for determining tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-18, IL-10, and IL-33 levels. Reverse transcription–polymerase chain reaction and western blot were used for evaluating TNF-α, NLR family pyrin domain containing 1 (NLRP1), NLRP3, caspase-1, and apoptosis-associated speck-like protein containing a card expression. Plasmid silencing APE1 gene (APE1^shRNA) was synthesized and packaged into lentiviral. For activating inflammasomes, M2-type THP-1 cells were transfected with lentiviral vector APE1^shRNA incubated with lipopolysaccharide (LPS) (100 ng/mL)/APE1 inhibitor (E3330, 20 µM) and ATP. Electrophoretic mobility shift assay and dual-luciferase reporter assay were used for determining the interaction between NLRP3 and nuclear factor-κB (NF-κB) molecule.

Results

APE1 significantly induced LPS-induced pro-inflammatory cytokine production, including TNF-α, IL-1β, and IL18, compared with THP-1 cells without APE1 treatment (P<0.05). APE1 promoted LPS-induced NLRP3 inflammasome activation by modulating the gene transcription of NLRP3-associated molecules. APE1 enhanced LPS-induced NLRP3 inflammasome activation by regulating NLRP3 and caspase-1 protein expression. APE1 improved NLRP3 activity by modulating the interaction between NLRP3 and NF-κB, and the modulation of NF-κB. APE1 promoted LPS-induced NLRP3 inflammasome activation through an NF-κB-dependent pathway.

Conclusions

APE1 regulates the expression of NLRP3 by modulating transcription factor NF-κB and further promoting the secretion of inflammatory mediators IL-1β and IL-18 in macrophages. The findings of the present study provide theoretical and experimental bases for the design of tumor-associated macrophage (TAM)-targeted therapy, with APE1 as the target molecule.

3D culture technologies of cancer stem cells: promising ex vivo tumor models

Cancer stem cells have been shown to be important in tumorigenesis processes, such as tumor growth, metastasis, and recurrence. As such, many three-dimensional models have been developed to establish an ex vivo microenvironment that cancer stem cells experience under in vivo conditions. Cancer stem cells propagating in three-dimensional culture systems show physiologically related signaling pathway profiles, gene expression, cell-matrix and cell-cell interactions, and drug resistance that reflect at least some of the tumor properties seen in vivo. Herein, we discussed the presently available Cancer stem cell three-dimensional culture models that use biomaterials and engineering tools and the biological implications of these models compared to the conventional ones.

PARP inhibitors in pancreatic cancer: molecular mechanisms and clinical applications

Pancreatic cancer is a highly lethal disease with a poor prognosis, and existing therapies offer only limited effectiveness. Mutation gene sequencing has shown several gene associations that may account for its carcinogenesis, revealing a promising research direction. Poly (ADP-ribose) polymerase (PARP) inhibitors target tumor cells with a homologous recombination repair (HRR) deficiency based on the concept of synthetic lethality. The most prominent target gene is BRCA, in which mutations were first identified in breast cancer and ovarian cancer. PARP inhibitors can trap the PARP-1 protein at a single-stranded break/DNA lesion and disrupt its catalytic cycle, ultimately leading to replication fork progression and consequent double-strand breaks. For tumor cells with BRCA mutations, HRR loss would result in cell death. Pancreatic cancer has also been reported to have a strong relationship with BRCA gene mutations, which indicates that pancreatic cancer patients may benefit from PARP inhibitors. Several clinical trials are being conducted and have begun to yield results. For example, the POLO (Pancreatic Cancer Olaparib Ongoing) trial has demonstrated that the median progression-free survival was observably longer in the olaparib group than in the placebo group. However, PARP inhibitor resistance has partially precluded their use in clinical applications, and the major mechanism underlying this resistance is the restoration of HRR. Therefore, determining how to use PARP inhibitors in more clinical applications and how to avoid adverse effects, as well as prognosis and treatment response biomarkers, require additional research. This review elaborates on future prospects for the application of PARP inhibitors in pancreatic cancer.

Polymerase independent repression of FoxO1 transcription by sequence-specific PARP1 binding to FoxO1 promoter

Poly(ADP-ribose) polymerase 1 (PARP1) regulates gene transcription in addition to functioning as a DNA repair factor. Forkhead box O1 (FoxO1) is a transcription factor involved in extensive biological processes. Here, we report that PARP1 binds to two separate motifs on the FoxO1 promoter and represses its transcription in a polymerase-independent manner. Using PARP1-knock out (KO) cells, wild-type-PARP1-complemented cells and catalytic mutant PARP1^E988K-reconstituted cells, we investigated transcriptional regulation by PARP1. PARP1 loss led to reduced DNA damage response and ~362-fold resistance to five PARP inhibitors (PARPis) in Ewing sarcoma cells. RNA sequencing showed 492 differentially expressed genes in a PARP1-KO subline, in which the FoxO1 mRNA levels increased up to more than five times. The change in the FoxO1 expression was confirmed at both mRNA and protein levels in different PARP1-KO and complemented cells. Moreover, exogenous PARP1 overexpression reduced the endogenous FoxO1 protein in RD-ES cells. Competitive EMSA and ChIP assays revealed that PARP1 specifically bound to the FoxO1 promoter. DNase I footprinting, mutation analyses, and DNA pulldown FREP assays showed that PARP1 bound to two particular nucleotide sequences separately located at −813 to −826 bp and −1805 to −1828 bp regions on the FoxO1 promoter. Either the PARPi olaparib or the PARP1 catalytic mutation (E988K) did not impair the repression of PARP1 on the FoxO1 expression. Exogenous FoxO1 overexpression did not impair cellular PARPi sensitivity. These findings demonstrate a new PARP1-gene promoter binding mode and a new transcriptional FoxO1 gene repressor.

Tissue-infiltrating macrophages mediate an exosome-based metabolic reprogramming upon DNA damage

DNA damage and metabolic disorders are intimately linked with premature disease onset but the underlying mechanisms remain poorly understood. Here, we show that persistent DNA damage accumulation in tissue-infiltrating macrophages carrying an ERCC1-XPF DNA repair defect (Er1^F/−) triggers Golgi dispersal, dilation of endoplasmic reticulum, autophagy and exosome biogenesis leading to the secretion of extracellular vesicles (EVs) in vivo and ex vivo. Macrophage-derived EVs accumulate in Er1^F/− animal sera and are secreted in macrophage media after DNA damage. The Er1^F/− EV cargo is taken up by recipient cells leading to an increase in insulin-independent glucose transporter levels, enhanced cellular glucose uptake, higher cellular oxygen consumption rate and greater tolerance to glucose challenge in mice. We find that high glucose in EV-targeted cells triggers pro-inflammatory stimuli via mTOR activation. This, in turn, establishes chronic inflammation and tissue pathology in mice with important ramifications for DNA repair-deficient, progeroid syndromes and aging.

DNA damage is associated with metabolic disorders, but the mechanism in unclear. Here, the authors show that persistent DNA damage induced by lack of the endonuclease XPF-ERCC1 triggers extracellular vesicle biogenesis in tissue infiltrating macrophages, and that vesicle uptake stimulates glucose uptake in recipient cells, leading to increased inflammation.

DNA Repair Proteins as Therapeutic Targets in Ovarian Cancer

Epithelial ovarian cancer is a serious public health problem worldwide with the highest mortality rate of all gynecologic cancers. The current standard-of-care for the treatment of ovarian cancer is based on chemotherapy based on adjuvant cisplatin/carboplatin and taxane regimens that represent the first-line agents for patients with advanced disease. The DNA repair activity of cancer cells determines the efficacy of anticancer drugs. These features make DNA repair mechanisms a promising target for novel cancer treatments. In this context a better understanding of the DNA damage response caused by antitumor agents has provided the basis for the use of DNA repair inhibitors to improve the therapeutic use of DNA-damaging drugs. In this review, we will discuss the functions of DNA repair proteins and the advances in targeting DNA repair pathways with special emphasis in the inhibition of HRR and BER in ovarian cancer. We focused in the actual efforts in the development and clinical use of poly (ADPribose) polymerase (PARP) inhibitors for the intervention of BRCA1/BRCA2-deficient ovarian tumors. The clinical development of PARP inhibitors in ovarian cancer patients with germline BRCA1/2 mutations and sporadic high-grade serous ovarian cancer is ongoing. Some phase II and phase III trials have been completed with promising results for ovarian cancer patients.

Targeting DNA Replication Stress and DNA Double-Strand Break Repair for Optimizing SCLC Treatment

Small cell lung cancer (SCLC), accounting for about 15% of all cases of lung cancer worldwide, is the most lethal form of lung cancer. Despite an initially high response rate of SCLC to standard treatment, almost all patients are invariably relapsed within one year. Effective therapeutic strategies are urgently needed to improve clinical outcomes. Replication stress is a hallmark of SCLC due to several intrinsic factors. As a consequence, constitutive activation of the replication stress response (RSR) pathway and DNA damage repair system is involved in counteracting this genotoxic stress. Therefore, therapeutic targeting of such RSR and DNA damage repair pathways will be likely to kill SCLC cells preferentially and may be exploited in improving chemotherapeutic efficiency through interfering with DNA replication to exert their functions. Here, we summarize potentially valuable targets involved in the RSR and DNA damage repair pathways, rationales for targeting them in SCLC treatment and ongoing clinical trials, as well as possible predictive biomarkers for patient selection in the management of SCLC.

Effect of transcription inhibition and generation of suppressive viral non-coding RNAs

Background

HIV-1 patients receiving combination antiretroviral therapy (cART) survive infection but require life-long adherence at high expense. In chronic cART-treated patients with undetectable viral titers, cell-associated viral RNA is still detectable, pointing to low-level viral transcriptional leakiness. To date, there are no FDA-approved drugs against HIV-1 transcription. We have previously shown that F07#13, a third generation Tat peptide mimetic with competitive activity against Cdk9/T1-Tat binding sites, inhibits HIV-1 transcription in vitro and in vivo.

Results

Here, we demonstrate that increasing concentrations of F07#13 (0.01, 0.1, 1 µM) cause a decrease in Tat levels in a dose-dependent manner by inhibiting the Cdk9/T1-Tat complex formation and subsequent ubiquitin-mediated Tat sequestration and degradation. Our data indicate that complexes I and IV contain distinct patterns of ubiquitinated Tat and that transcriptional inhibition induced by F07#13 causes an overall reduction in Tat levels. This reduction may be triggered by F07#13 but ultimately is mediated by TAR-gag viral RNAs that bind suppressive transcription factors (similar to 7SK, NRON, HOTAIR, and Xist lncRNAs) to enhance transcriptional gene silencing and latency. These RNAs complex with PRC2, Sin3A, and Cul4B, resulting in epigenetic modifications. Finally, we observed an F07#13-mediated decrease of viral burden by targeting the R region of the long terminal repeat (HIV-1 promoter region, LTR), promoting both paused polymerases and increased efficiency of CRISPR/Cas9 editing in infected cells. This implies that gene editing may be best performed under a repressed transcriptional state.

Conclusions

Collectively, our results indicate that F07#13, which can terminate RNA Polymerase II at distinct sites, can generate scaffold RNAs, which may assemble into specific sets of “RNA Machines” that contribute to gene regulation. It remains to be seen whether these effects can also be seen in various clades that have varying promoter strength, mutant LTRs, and in patient samples.

Electronic supplementary material

The online version of this article (10.1186/s12977-019-0475-0) contains supplementary material, which is available to authorized users.

Increased PARP1-DNA binding due to autoPARylation inhibition of PARP1 on DNA rather than PARP1-DNA trapping is correlated with PARP1 inhibitor's cytotoxicity

PARP1 inhibitors (PARPis) are used clinically during cancer therapy and are thought to exert their cytotoxicity through PARP1 polymerase inhibition and PARP1-DNA trapping. Here, we showed no significant correlation between PARP1-DNA trapping and cytotoxicity induced by PARPis. We complemented PARP1-knockout sublines with wild-type PARP1 and 11 mutants with different point mutations that affect the polymerase activity. When examining the PARPi talazoparib, the induced cytotoxicity was highly significantly correlated with cellular PARP1 polymerase activity, but not with its PARP1-DNA trapping or polymerase inhibition. Similarly, talazoparib's PARP1-DNA trapping revealed significant correlation with the polymerase activity rather than its inhibition. Differently, however, when evaluating purified wild-type and mutated PARP1, we identified an almost linear relationship between PARPis' inhibiting PARP1 dissociation from DNA and their cytotoxicity in 17 cancer cell lines. In contrast, no significant correlation existed between PARP1 polymerase inhibition in the histone-based systems and the cytotoxicity. After careful comparisons on different methods and detection targets, we conclude that the PARPi-mediated increase in PARP1-DNA binding by inhibiting autoPARylation of PARP1 on DNA rather than in PARP1-DNA trapping is correlated with PARPi's cytotoxicity. Accordingly, we established a new PARPi screening model that more closely predicts cytotoxicity.

Silencing Artemis Enhances Colorectal Cancer Cell Sensitivity to DNA-Damaging Agents

Artemis is a key protein of NHEJ (nonhomologous end joining), which is the major pathway for the repair of IR-induced DSBs in mammalian cells. However, the expression of Artemis in tumors and the influence of silencing Artemis on tumor sensitivity to radiation have not been investigated fully. In this study, we investigated how the expression levels of Artemis may affect the treatment outcome of radiotherapy and chemotherapy in colorectal cancer cells. First, we found that the expression of Artemis is strong in some human rectal cancer samples, being higher than in adjacent normal tissues using immunohistochemical staining. We then knocked down Artemis gene in a human colorectal cancer cell line (RKO) using lentivirus-mediated siRNAs. Compared to the control RKO cells, the Artemis knockdown cells showed significantly increased sensitivity to bleomycin, etoposide, camptothecin, and IR. Induced by DNA-damaging agents, delayed DNA repair kinetics was found by the γ-H2AX foci assay, and a significantly increased cell apoptosis occurred in the Artemis knockdown RKO cells through apoptosis detection methods and Western blot. We also found that the p53/p21 signaling pathway may be involved in the apoptosis process. Taken together, our study indicates that manipulating Artemis can enhance colorectal cancer cell sensitivity to DNA-damaging agents. Therefore, Artemis can serve as a therapeutic target in rectal cancer therapy.

Anti-cancer effects of Rhizoma Curcumae against doxorubicin-resistant breast cancer cells

Background

Chemotherapy is a primary approach in cancer treatment after routine surgery. However, chemo-resistance tends to occur with chemotherapy in clinic, resulting in poor prognosis and recurrence. Nowadays, Chinese medicine may shed light on design of new therapeutic modes to overcome chemo-resistance. Although Rhizoma Curcumae possesses anti-cancer activities in various types of cancers, the effects and underlying mechanisms of its bioactive components against chemo-resistance are not clear. Therefore, the present study aims to explore the potential effects of Rhizoma Curcumae on doxorubicin-resistant breast cancer cells.

Methods

The expression and function of ABC transporters in doxorubicin-resistant MCF-7 breast cancer cells were measured by western blotting and flow cytometry. Cell viability was detected using MTT assay. The combination index was analyzed using the CalcuSyn program (Biosoft, Ferguson, MO), based on the Chou–Talalay method.

Results

In our present study, P-gp was overexpressed at protein level in doxorubicin-resistant MCF-7 cell line, but short of MRP1 and BCRP1. Essential oil of Rhizoma Curcumae and the main bioactive components were assessed on doxorubicin-resistant MCF-7 cell line. We found that the essential oil and furanodiene both display powerful inhibitory effects on cell viability, but neither of these is the specific inhibitor of ABC transporters. Moreover, furanodiene fails to enhance the efficacy of doxorubicin to improve multidrug resistance.

Conclusion

Overall, our findings fill the gaps of the researches on chemo-resistance improvement of Rhizoma Curcumae and are also beneficial for Rhizoma Curcumae being developed as a promising natural product for cancer adjuvant therapy in the future.

Electronic supplementary material

The online version of this article (10.1186/s13020-018-0203-z) contains supplementary material, which is available to authorized users.

Novel PARP1/2 inhibitor mefuparib hydrochloride elicits potent in vitro and in vivo anticancer activity, characteristic of high tissue distribution

The approval of poly(ADP-ribose) polymerase (PARP) inhibitor AZD2281 in 2014 marked the successful establishment of the therapeutic strategy targeting homologous recombination repair defects of cancers in the clinic. However, AZD2281 has poor water solubility, low tissue distribution and relatively weak in vivo anticancer activity, which appears to become limiting factors for its clinical use. In this study, we found that mefuparib hydrochloride (MPH) was a potent PARP inhibitor, possessing prominent in vitro and in vivo anticancer activity. Notably, MPH displayed high water solubility (> 35 mg/ml) and potent PARP1/2 inhibition in a substrate-competitive manner. It reduced poly(ADP-ribose) (PAR) formation, enhanced γH2AX levels, induced G2/M arrest and subsequent apoptosis in homologous recombination repair (HR)-deficient cells. Proof-of-concept studies confirmed the MPH-caused synthetic lethality. MPH showed potent in vitro and in vivo proliferation and growth inhibition against HR-deficient cancer cells and synergistic sensitization of HR-proficient xenografts to the anticancer drug temozolomide. A good relationship between the anticancer activity and the PARP inhibition of MPH suggested that PAR formation and γH2AX accumulation could serve as its pharmacodynamic biomarkers. Its high bioavailability (40%~100%) and high tissue distribution in both monkeys and rats were its most important pharmacokinetic features. Its average concentrations were 33-fold higher in the tissues than in the plasma in rats. Our work supports the further clinical development of MPH as a novel PARP1/2 inhibitor for cancer therapy.

Acquired resistance of phosphatase and tensin homolog-deficient cells to poly(ADP-ribose) polymerase inhibitor and Ara-C mediated by 53BP1 loss and SAMHD1 overexpression

With increasing uses of poly(ADP‐ribose) polymerase (PARP) inhibitors (PARPi) for cancer therapy, understanding their resistance is becoming urgent. However, acquired PARPi resistance in the phosphatase and tensin homolog (PTEN)‐deficient background is poorly understood. We generated 3 PARPi‐resistant PTEN‐deficient glioblastoma U251 variants separately with olaparib (U251/OP), talazoparib (U251/TP) and simmiparib (U251/SP). These variants displayed consistent resistance (2.46‐71.78‐fold) to all 5 PARPi, including niraparib and rucaparib, and showed higher degrees of resistance to the PARPi to which the parental cells were more sensitive. The resistance was characteristic of fast emergence and high stability. However, the resistance acquirement did not cause an increasingly aggressive phenotype. The resistance was not correlated to various factors, including PTEN mutations. The PARPi‐treated variants produced less γH2AX and G2/M arrest. Consistently, loss of 53BP1 occurred in all variants and its compensation enhanced their sensitivity to PARPi by approximately 76%. The variants revealed slightly different cross‐resistance profiles to 13 non‐PARPi anticancer drugs. All were resistant to Ara‐C (6‐8‐fold) but showed differential resistance to 5‐fluorouracil, gemcitabine and paclitaxel. Almost no resistance was observed to the rest drugs, including cisplatin. SAMHD1 was overexpressed in all the variants and its knockout completely restored their sensitivity to Ara‐C but did not affect their PARPi sensitivity. The present study demonstrates a consistent resistance profile to PARPi and a unique cross‐resistance profile to non‐PARPi drugs in different PARPi‐resistant U251 cells and reveals 53BP1 loss and SAMHD1 overexpression as the primary mechanisms responsible for their resistance to PARPi and Ara‐C, respectively. These effects probably result from heritable gene change(s) caused by persistent PARPi exposure.

ERCC1-XPF cooperates with CTCF and cohesin to facilitate the developmental silencing of imprinted genes

Inborn defects in DNA repair are associated with complex developmental disorders whose causal mechanisms are poorly understood. Using an in vivo biotinylation tagging approach in mice, we show that the nucleotide excision repair (NER) structure-specific endonuclease ERCC1-XPF complex interacts with the insulator binding protein CTCF, the cohesin subunits SMC1A and SMC3 and with MBD2; the factors co-localize with ATRX at the promoters and control regions (ICRs) of imprinted genes during postnatal hepatic development. Loss of Ercc1 or exposure to MMC triggers the localization of CTCF to heterochromatin, the dissociation of the CTCF-cohesin complex and ATRX from promoters and ICRs, altered histone marks and the aberrant developmental expression of imprinted genes without altering DNA methylation. We propose that ERCC1-XPF cooperates with CTCF and cohesin to facilitate the developmental silencing of imprinted genes and that persistent DNA damage triggers chromatin changes that affect gene expression programs associated with NER disorders.

DDRI-9: a novel DNA damage response inhibitor that blocks mitotic progression

The DNA damage response (DDR) is an emerging target for cancer therapy. By modulating the DDR, including DNA repair and cell cycle arrest, the efficacy of anticancer drugs can be enhanced and side effects reduced. We previously screened a chemical library and identified novel DDR inhibitors including DNA damage response inhibitor-9 (DDRI-9; 1H-Purine-2,6-dione,7-[(4-fluorophenyl)methyl]-3,7-dihydro-3-methyl-8-nitro). In this study, we characterized DDRI-9 activity and found that it inhibited phosphorylated histone variant H2AX foci formation upon DNA damage, delayed DNA repair, and enhanced the cytotoxicity of etoposide and ionizing radiation. It also reduced the foci formation of DNA repair-related proteins, including the protein kinase ataxia-telangiectasia mutated, DNA-dependent protein kinase, breast cancer type 1 susceptibility protein, and p53-binding protein 1, but excluding mediator of DNA damage checkpoint protein 1. Cell cycle analysis revealed that DDRI-9 blocked mitotic progression. Like other mitotic inhibitors, DDRI-9 treatment resulted in the accumulation of mitotic protein and induced cell death. Thus, DDRI-9 may affect both DDR signal amplification and mitotic progression. This study suggests that DDRI-9 is a good lead molecule for the development of anticancer drugs.

DNA Polymerases λ and β: The Double-Edged Swords of DNA Repair

DNA is constantly exposed to both endogenous and exogenous damages. More than 10,000 DNA modifications are induced every day in each cell's genome. Maintenance of the integrity of the genome is accomplished by several DNA repair systems. The core enzymes for these pathways are the DNA polymerases. Out of 17 DNA polymerases present in a mammalian cell, at least 13 are specifically devoted to DNA repair and are often acting in different pathways. DNA polymerases β and λ are involved in base excision repair of modified DNA bases and translesion synthesis past DNA lesions. Polymerase λ also participates in non-homologous end joining of DNA double-strand breaks. However, recent data have revealed that, depending on their relative levels, the cell cycle phase, the ratio between deoxy- and ribo-nucleotide pools and the interaction with particular auxiliary proteins, the repair reactions carried out by these enzymes can be an important source of genetic instability, owing to repair mistakes. This review summarizes the most recent results on the ambivalent properties of these enzymes in limiting or promoting genetic instability in mammalian cells, as well as their potential use as targets for anticancer chemotherapy.

An Update on Poly(ADP-ribose)polymerase-1 (PARP-1) Inhibitors: Opportunities and Challenges in Cancer Therapy

Poly(ADP-ribose)polymerase-1 (PARP-1) is a critical DNA repair enzyme in the base excision repair pathway. Inhibitors of this enzyme comprise a new type of anticancer drug that selectively kills cancer cells by targeting homologous recombination repair defects. Since 2010, important advances have been achieved in PARP-1 inhibitors. Specifically, the approval of olaparib in 2014 for the treatment of ovarian cancer with BRCA mutations validated PARP-1 as an anticancer target and established its clinical importance in cancer therapy. Here, we provide an update on PARP-1 inhibitors, focusing on breakthroughs in their clinical applications and investigations into relevant mechanisms of action, biomarkers, and drug resistance. We also provide an update on the design strategies and the structural types of PARP-1 inhibitors. Opportunities and challenges in PARP-1 inhibitors for cancer therapy will be discussed based on the above advances.

Caffeic Acid Phenethyl Ester Increases Radiosensitivity of Estrogen Receptor-Positive and -Negative Breast Cancer Cells by Prolonging Radiation-Induced DNA Damage

Purpose

Breast cancer is an important cause of death among women. The development of radioresistance in breast cancer leads to recurrence after radiotherapy. Caffeic acid phenethyl ester (CAPE), a polyphenolic compound of honeybee propolis, is known to have anticancer properties. In this study, we examined whether CAPE enhanced the radiation sensitivity of MDA-MB-231 (estrogen receptor-negative) and T47D (estrogen receptor-positive) cell lines.

Methods

The cytotoxic effect of CAPE on MDA-MB-231 and T47D breast cancer cells was evaluated by performing an 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) assay. To assess clonogenic ability, MDA-MB-231 and T47D cells were treated with CAPE (1 µM) for 72 hours before irradiation, and then, a colony assay was performed. A comet assay was used to determine the number of DNA strand breaks at four different times.

Results

CAPE decreased the viability of both cell lines in a dose- and time-dependent manner. In the clonogenic assay, pretreatment of cells with CAPE before irradiation significantly reduced the surviving fraction of MDA-MB-231 cells at doses of 6 and 8 Gy. A reduction in the surviving fraction of T47D cells was observed relative to MDA-MB-231 at lower doses of radiation. Additionally, CAPE maintained radiation-induced DNA damage in T47D cells for a longer period than in MDA-MB-231 cells.

Conclusion

Our results indicate that CAPE impairs DNA damage repair immediately after irradiation. The induction of radiosensitivity by CAPE in radioresistant breast cancer cells may be caused by prolonged DNA damage.

Nuclear-Targeting MSNs-Based Drug Delivery System: Global Gene Expression Analysis on the MDR-Overcoming Mechanisms

The biological mechanisms of nuclear-targeting mesoporous silica nanoparticles (MSNs)-based DDSs (DOX@NT-MSNs) in overcoming multidrug resistance of cancer cells are studied. It is interesting to find for the first time that DOX@NT-MSNs down-regulate the expression of apoptosis suppressor genes and inhibit DNA repair process by disturbing the p53 pathway.

A novelly synthesized phenanthroline derivative is a promising DNA-damaging anticancer agent inhibiting G1/S checkpoint transition and inducing cell apoptosis in cancer cells

PURPOSE

The study mainly aimed to determine the biological function of a novelly synthesized phenanthroimidazole derivative, named L233, and to explore its potential mechanisms.

METHODS

Cell survival was examined using the MTT assays, and the DNA-damaging role of L233 was explored using the comet assay. Moreover, the western blotting assays and immunofluorescence assays were used to detect DNA damage biomarkers. Afterward, the flow cytometry was used to assess the effects of L233 on cell cycle distribution. As for the detection of cell apoptosis upon L233 treatment, the Hoechst 33342 staining, flow cytometry, and western blotting assays were all put into practice.

RESULTS

We find that L233 inhibits tumor cell growth more efficiently and safely than cisplatin. Moreover, it is a DNA-damaging agent, interrupting the cell cycle G1/S checkpoint transition and inducing cell apoptosis by not only activating ATM/CHK1 signaling pathway, but also targeting CHK1 to reduce the expression of RAP80 and PARP-1 to compromise the DNA damage repair in tumor cells.

CONCLUSIONS

In summary, L233 is a promising anticancer drug for the development of novel chemotherapies in the future.

Lactate dehydrogenase inhibitors sensitize lymphoma cells to cisplatin without enhancing the drug effects on immortalized normal lymphocytes

Up-regulation of glycolysis, a well recognized hallmark of cancer cells, was also found to be predictive of poor chemotherapy response. This observation suggested the attempt of sensitizing cancer cells to conventional chemotherapeutic agents by inhibiting glucose metabolism. Lactate dehydrogenase (LDH) inhibition can be a way to hinder glycolysis of cancer cells without affecting the metabolism of normal tissues, which usually does not require this enzymatic activity. In this paper, we showed that two LDH inhibitors (oxamate and galloflavin) can increase the efficacy of cisplatin in cultured Burkitt's lymphoma (BL) cells and that this potentiating effect is not exerted in proliferating normal lymphocytes. This result was explained by the finding that in BL cells LDH inhibition induced reactive oxygen species (ROS) generation, which was not evidenced in proliferating normal lymphocytes. In BL cells treated with the association of cisplatin and LDH inhibitors, these ROS can be a further cause of DNA damage, to be added to that produced by cisplatin, leading to the failure of the response repair. At present LDH inhibitors suitable for clinical use are actively searched; our results can allow a better understanding of the potentiality of LDH as a possible target to develop innovative anticancer treatments.

hMSH2 expression is associated with paclitaxel resistance in ovarian carcinoma, and inhibition of hMSH2 expression in vitro restores paclitaxel sensitivity

The objective of the present study was to investigate the association between paclitaxel resistance, gene copy number, and gene expression in ovarian carcinoma, and to restore paclitaxel sensitivity in a paclitaxel-resistant ovarian carcinoma cell line by using hMSH2-targeting siRNA. Paclitaxel-resistant ovarian carcinoma cell lines OC3/TAX300 and OC3/TAX50 and their parental cell lines were analyzed by comparative genomic hybridization, and the expression levels of hMSH2 in ovarian carcinoma cell lines and tissues were determined. An siRNA targeted to hMSH2 mRNA was used to transfect a paclitaxel-resistant cell line. We assessed the morphological features, proliferation, and susceptibility to apoptosis of the transfected cells after paclitaxel treatment. Chromosome 2p21 (gene locus of hMSH2) was amplified in OC3/TAX300 cells. hMSH2 was overexpressed in 93.9 and 47.6% of paclitaxel-treated and untreated ovarian carcinoma tissue samples (P=0.0001), respectively. hMSH2 was overexpressed in 93.3 and 54.2% of low-differentiated and moderate-to-highly differentiated ovarian carcinoma tissue samples (P=0.0008), respectively. hMSH2 expression was inhibited in the OC3/TAX300 cells transfected with hMSH2 siRNA. hMSH2 siRNA increased paclitaxel sensitivity, inhibited OC3/TAX300 cell proliferation (G2/M arrest), and increased susceptibility to apoptosis. hMSH2 expression was upregulated in ovarian carcinoma cell lines and tissues after paclitaxel treatment. hMSH2 overexpression is related to paclitaxel resistance and poor prognosis. Inhibition of hMSH2 expression in vitro restores paclitaxel sensitivity in paclitaxel‑resistant ovarian carcinoma cell lines and indicates a new direction in adjuvant therapy for ovarian carcinoma.

TRIP13 promotes error-prone nonhomologous end joining and induces chemoresistance in head and neck cancer

Head and neck cancer (SCCHN) is a common, aggressive, treatment-resistant cancer with a high recurrence rate and mortality, but the mechanism of treatment-resistance remains unclear. Here we describe a mechanism where the AAA-ATPase TRIP13 promotes treatment-resistance. Overexpression of TRIP13 in non-malignant cells results in malignant transformation. High expression of TRIP13 in SCCHN leads to aggressive, treatment-resistant tumors and enhanced repair of DNA damage. Using mass spectrometry, we identify DNA-PKcs complex proteins that mediate non homologous end joining (NHEJ), as TRIP13 binding partners. Using repair-deficient reporter systems, we show that TRIP13 promotes NHEJ, even when homologous recombination is intact. Importantly, overexpression of TRIP13 sensitizes SCCHN to an inhibitor of DNA-PKcs. Thus, this study defines a new mechanism of treatment resistance in SCCHN and underscores the importance of targeting NHEJ to overcome treatment failure in SCCHN and potentially in other cancers that overexpress TRIP13.

Up-regulation of microRNA-138 induce radiosensitization in lung cancer cells

Deregulation of microRNAs (miRNAs) is implicated in tumor progression. We attempt to identify the association between miR-138 and Sentrin/SUMO-specific protease 1 (SENP1) as a radiosensitization-related gene and characterize the biological function by which SENP1 was regulated by miR-138 to influence radiosensitization in lung cancer cells. In this study, we showed that miRNA-138 is reduced in both lung cancer clinical specimens and cell lines and is effective to inhibit SENP1 expression. Moreover, high levels of miR-138 are associated with lower levels of lung cancer cell proliferation and colony formation. Then, we investigated the underlying mechanisms responsible for the increase in the radiosensitivity of lung cancer cells when SENP1 is inhibited by miR-138. We further show that the increased radiosensitivity may be the result of an increased γ-H2AX expression, an increased rate of apoptosis, and changes in the cell cycle. In conclusion, our data demonstrate that the miR-138/SENP1 cascade is relative to radiosensitization in lung cancer cells and is a potential radiotherapy target.

Chemo-sensitisation of HeLa cells to etoposide by a benzoxazine in the absence of DNA-PK inhibition

The benzoaxines have been developed from structurally similar chromones as specific inhibitors of the PI3K family to sensitize cancer cells to the effects of chemotherapeutic agents; most have been shown to do this through specific inhibition of DNA-PK and DNA repair mechanisms. In this study we examined the benzoxazine, 2-((3-methoxybut-3en-2-yl)amino)-8methyl-4H-benzo[1,3]oxazin-4one (LTUSI54). This compound had no DNA-PK or PI3K inhibitory activity but still sensitized HeLa cells to the effects of Etoposide. LTUSI54 works synergistically with Etoposide to inhibit growth of HeLa cells and sub G1 analysis indicates that this is not due to an increase in apoptosis. LTUSI54 neither enhances DSB formation due to Etoposide nor does it delay the repair of such damage. Cell cycle analysis shows a clear G2 block with Etoposide alone while, in combination with LTUSI54 there is an additional S phase arrest. Phospho-kinase analysis indicated that LTUSI54 engages key regulators of cell cycle progression, specifically p38α, p53 and ERK 1/2. From our results we hypothesize that LTUSI54 is promoting the cell cycle arrest through activation of p38α pathways, independent of p53 mechanisms. This results in a decrease in p53 phosphorylation and hence, restricted apoptosis. Changes in cell number appear to be the result of p38α pathways disrupting cell cycle progression, at the S and G2 checkpoints. Further investigation into the finer mechanisms by which LTUSI54 effects cell cycle progression would be of great interest in assessing this compound as a chemosensitising agent.

DNA damage triggers a chronic autoinflammatory response, leading to fat depletion in NER progeria

Lipodystrophies represent a group of heterogeneous disorders characterized by loss of fat tissue. However, the underlying mechanisms remain poorly understood. Using mice carrying an ERCC1-XPF DNA repair defect systematically or in adipocytes, we show that DNA damage signaling triggers a chronic autoinflammatory response leading to fat depletion. Ercc1-/- and aP2-Ercc1F/- fat depots show extensive gene expression similarities to lipodystrophic Pparγ(ldi/+) animals, focal areas of ruptured basement membrane, the reappearance of primary cilia, necrosis, fibrosis, and a marked decrease in adiposity. We find that persistent DNA damage in aP2-Ercc1F/- fat depots and in adipocytes ex vivo triggers the induction of proinflammatory factors by promoting transcriptionally active histone marks and the dissociation of nuclear receptor corepressor complexes from promoters; the response is cell autonomous and requires ataxia telangiectasia mutated (ATM). Thus, persistent DNA damage-driven autoinflammation plays a causative role in adipose tissue degeneration, with important ramifications for progressive lipodystrophies and natural aging.

ATP-dependent chromatin remodeling and histone acetyltransferases in 5-FU cytotoxicity in Saccharomyces cerevisiae

Chromatin is thought to modulate access of repair proteins to DNA lesions, and may be altered by chromatin remodelers to facilitate repair. We investigated the participation of chromatin remodelers and DNA repair in 5-fluorouracil (5-FU) cytotoxicity in Saccharomyces cerevisiae. 5-FU is an antineoplastic drug commonly used in clinical settings. Among the several strains tested, only those with deficiencies in ATP-dependent chromatin remodeling (CR) and some histone acetyltransferases (HAT) exhibited sensitivity to 5-FU. CR and HAT double-mutants exhibited increased resistance to 5-FU in comparison to the wild-type mutant, but were still arrested in G2/M, as were the sensitive strains. The participation of Htz1p in 5-FU toxicity was also evaluated in single- and double-mutants of CR and HAT; the most significant effect was on cell cycle distribution. 5-FU lesions are repaired by different DNA repair machineries, including homologous recombination (HR) and post-replication repair (PRR). We investigated the role of CR and HAT in these DNA repair pathways. Deficiencies in Nhp10 and CR combined with deficiencies in HR or PRR increased 5-FU sensitivity; however, combined deficiencies of HAT, HR, and PRR did not. CRs are directly recruited to DNA damage and lead to chromatin relaxation, which facilitates access of HR and PRR proteins to 5-FU lesions. Combined deficiencies in HAT with defects in HR and PRR did not potentiate 5-FU cytotoxicity, possibly because they function in a common pathway.

Single-cell microarray enables high-throughput evaluation of DNA double-strand breaks and DNA repair inhibitors

A key modality of non-surgical cancer management is DNA damaging therapy that causes DNA double-strand breaks that are preferentially toxic to rapidly dividing cancer cells. Double-strand break repair capacity is recognized as an important mechanism in drug resistance and is therefore a potential target for adjuvant chemotherapy. Additionally, spontaneous and environmentally induced DSBs are known to promote cancer, making DSB evaluation important as a tool in epidemiology, clinical evaluation and in the development of novel pharmaceuticals. Currently available assays to detect double-strand breaks are limited in throughput and specificity and offer minimal information concerning the kinetics of repair. Here, we present the CometChip, a 96-well platform that enables assessment of double-strand break levels and repair capacity of multiple cell types and conditions in parallel and integrates with standard high-throughput screening and analysis technologies. We demonstrate the ability to detect multiple genetic deficiencies in double-strand break repair and evaluate a set of clinically relevant chemical inhibitors of one of the major double-strand break repair pathways, non-homologous end-joining. While other high-throughput repair assays measure residual damage or indirect markers of damage, the CometChip detects physical double-strand breaks, providing direct measurement of damage induction and repair capacity, which may be useful in developing and implementing treatment strategies with reduced side effects.

Silencing of APE1 enhances sensitivity of human hepatocellular carcinoma cells to radiotherapy in vitro and in a xenograft model

Resistance to radiotherapy is a key limitation for the treatment of human hepatocellular carcinoma (HCC). To overcome this problem, we investigated the correlation between radioresistance and the human apurinic/apyrimidinic endonuclease (APE1), a bifunctional protein, which plays an important role in DNA repair and redox regulation activity of transcription factors. In the present study, we examined the radiosensitivity profiles of three human HCC cell lines, HepG2, Hep3B, and MHCC97L, using the adenoviral vector Ad5/F35-mediated APE1 siRNA (Ad5/F35-siAPE1). The p53 mutant cell lines MHCC97L showed radioresistance, compared with HepG2 and Hep3B cells. APE1 was strongly expressed in MHCC97L cells and was induced by irradiation in a dose-dependent manner, and Ad5/F35-siAPE1 effectively inhibited irradiation-induced APE1 and p53 expression. Moreover, silencing of APE1 significantly potentiated the growth inhibition and apoptosis induction by irradiation in all tested human HCC cell lines. In addition, Ad5/F35-siAPE1 significantly enhanced inhibition of tumor growth and potentiated cell apoptosis by irradiation both in HepG2 and MHCC97L xenografts. In conclusion, down regulation of APE1 could enhance sensitivity of human HCC cells to radiotherapy in vitro and in vivo.

Characterization of DDRI-18 (3,3'-(1H,3'H-5,5'-bibenzo[d]imidazole-2,2'-diyl)dianiline), a novel small molecule inhibitor modulating the DNA damage response

BACKGROUND AND PURPOSE

Recently, the DNA damage response (DDR) has emerged as a promising target for anticancer drug development. In our previous study, we identified several DDR-inhibiting compounds via high-content screening of a small molecule library using γH2AX foci as a biomarker. Here, we studied the effects of the DNA damage response inhibitor DDRI-18 (3,3'-(1H,3'H-5,5'-bibenzo[d]imidazole-2,2'-diyl)dianiline) on DDR.

EXPERIMENTAL APPROACH

Osteosarcoma U2OS cells were treated with etoposide to induce DDR. The nuclear foci of γH2AX and other signalling molecules in DDR were visualized by immunofluorescence and quantified using an IN Cell Analyzer. The DNA repair capacity of cells was analysed using the comet assay and in vivo DNA end-joining assay. Cell survival after drug treatment was quantified using the MTT assay, and apoptotic cell death was analysed by Annexin V staining and flow cytometry.

KEY RESULTS

DDRI-18 inhibited the non-homologous end-joining (NHEJ) DNA repair process and delayed the resolution of DNA damage-related proteins (γH2AX, ATM and BRCA1) from DNA lesions at a later phase of DDR. Furthermore, DDRI-18 enhanced the cytotoxic effects of anticancer DNA-damaging drugs, including etoposide, camptothecin, doxorubicin and bleomycin. This synergistic effect on cell death was shown to be due to caspase-dependent apoptosis.

CONCLUSIONS AND IMPLICATIONS

We identified a chemical compound, DDRI-18, that has chemosensitization activity. Although the target molecule and mechanism of action of DDRI-18 remain unknown, DDRI-18 is an effective chemosensitizing agent and may improve the therapy with classical anticancer drugs.

Complex DNA repair pathways as possible therapeutic targets to overcome temozolomide resistance in glioblastoma

Many conventional chemotherapeutic drugs exert their cytotoxic function by inducing DNA damage in the tumor cell. Therefore, a cell-inherent DNA repair pathway, which reverses the DNA-damaging effect of the cytotoxic drugs, can mediate therapeutic resistance to chemotherapy. The monofunctional DNA-alkylating agent temozolomide (TMZ) is a commonly used chemotherapeutic drug and the gold standard treatment for glioblastoma (GBM). Although the activity of DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) has been described as the main modulator to determine the sensitivity of GBM to TMZ, a subset of GBM does not respond despite MGMT inactivation, suggesting that another DNA repair mechanism may also modulate the tolerance to TMZ. Considerable interest has focused on MGMT, mismatch repair (MMR), and the base excision repair (BER) pathway in the mechanism of mediating TMZ resistance, but emerging roles for the DNA strand-break repair pathway have been demonstrated. In the first part of this review article, we briefly review the significant role of MGMT, MMR, and the BER pathway in the tolerance to TMZ; in the last part, we review the recent publications that demonstrate possible roles of DNA strand-break repair pathways, such as single-strand break repair and double-strand break repair, as well as the Fanconi anemia pathway in the repair process after alkylating agent-based therapy. It is possible that all of these repair pathways have a potential to modulate the sensitivity to TMZ and aid in overcoming the therapeutic resistance in the clinic.

Targeting DNA repair mechanisms in cancer

Preservation of genomic integrity is an essential process for cell homeostasis. DNA-damage response (DDR) promotes faithful transmission of genomes in dividing cells by reversing the extrinsic and intrinsic DNA damage, and is required for cell survival during replication. Radiation and genotoxic drugs have been widely used in the clinic for years to treat cancer but DNA repair mechanisms are often associated with chemo- and radio-resistance. To increase the efficacy of these treatments, inhibitors of the major components of the DDR such as ATM (ataxia telangiectasia mutated), ATR (ATM and Rad3-related), DNA-PK (DNA-dependent protein kinase, catalytic subunit), Chk1 (checkpoint protein 1) and Chk2 (checkpoint protein 2) have been used to confer radio- and/or chemosensitivity upon cancer cells. The elucidation of the molecular mechanisms of DNA repair and the discovery that tumors are frequently repair-deficient provide a therapeutic opportunity to selectively target this deficiency. Genetic mutations in the DNA repair genes constitute not only the initiating event of the cancer cell but also its weakness since the mutated gene is often needed by the cancer cell to maintain its own survival. This weakness has been exploited to specifically kill the tumor cells while sparing the normal ones, a concept known as 'synthetic lethality'. Recent efforts in the design of cancer therapies are directed towards exploiting synthetic lethal interactions with cancer-associated mutations in the DDR. In this review, we will discuss the latest concepts in targeting DNA repair mechanisms in cancer and the novel and promising compounds currently in clinical trials.