DNA repair inhibition: a selective tumour targeting strategy

Function and inhibition of the DNA repair enzyme SNM1A

SNM1A is an enzyme involved in several important biological pathways. To date, most investigations have focused on its role in repairing interstrand crosslinks, a highly cytotoxic form of DNA damage. SNM1A acts as a 5'-3' exonuclease, displaying an unusual capability to digest DNA past the site of a crosslink lesion. Recently, additional functions of this enzyme in the repair of DNA double-strand breaks and critically shortened telomeres have been uncovered. Furthermore, SNM1A is involved in two cell cycle checkpoints that arrest cell division in response to DNA damage. Inhibition of both DNA repair enzymes and cell cycle checkpoint proteins are effective strategies for cancer treatment, and SNM1A is therefore of significant interest as a potential therapeutic target. As a member of the metallo-β-lactamase superfamily, SNM1A is postulated to contain two metal ions in the active site that catalyse hydrolysis of the phosphodiester backbone of DNA. Substrate-mimic probes based on a nucleoside or oligonucleotide scaffold appended with a metal-binding group have proven effective in vitro. High-throughput screening campaigns have identified potent inhibitors, some of which were successful in sensitising cells to DNA-damaging cancer drugs. This review discusses the biological role, structure, and mechanism of action of SNM1A, and the development of SNM1A inhibitors for cancer therapy.

Beyond inhibition against the PD-1/PD-L1 pathway: development of PD-L1 inhibitors targeting internalization and degradation of PD-L1

Tumor cells hijack the programmed cell death protein-1 (PD-1)/programmed cell death ligand-1 (PD-L1) pathway to suppress the immune response through overexpressing PD-L1 to interact with PD-1 of T cells. With in-depth ongoing research, tumor-intrinsic PD-L1 is found to play important roles in tumor progression without interaction with PD-1 expressed on T cells, which provides an additional important target and therapeutic approach for development of PD-L1 inhibitors. Existing monoclonal antibody (mAb) drugs against the PD-1/PD-L1 pathway generally behave by conformationally blocking the interactions of PD-1 with PD-L1 on the cell surface. Beyond general inhibition of the protein-protein interaction (PPI), inhibitors targeting PD-L1 currently focus on the functional inhibition of the interaction between PD-1/PD-L1 and degradation of tumor-intrinsic PD-L1. This perspective will clarify the evolution of PD-L1 inhibitors and provide insights into the current development of PD-L1 inhibitors, especially targeting internalization and degradation of PD-L1.

A New Drug Discovery Platform: Application to DNA Polymerase Eta and Apurinic/Apyrimidinic Endonuclease 1

The ability to quickly discover reliable hits from screening and rapidly convert them into lead compounds, which can be verified in functional assays, is central to drug discovery. The expedited validation of novel targets and the identification of modulators to advance to preclinical studies can significantly increase drug development success. Our SaXPyTM ("SAR by X-ray Poses Quickly") platform, which is applicable to any X-ray crystallography-enabled drug target, couples the established methods of protein X-ray crystallography and fragment-based drug discovery (FBDD) with advanced computational and medicinal chemistry to deliver small molecule modulators or targeted protein degradation ligands in a short timeframe. Our approach, especially for elusive or "undruggable" targets, allows for (i) hit generation; (ii) the mapping of protein-ligand interactions; (iii) the assessment of target ligandability; (iv) the discovery of novel and potential allosteric binding sites; and (v) hit-to-lead execution. These advances inform chemical tractability and downstream biology and generate novel intellectual property. We describe here the application of SaXPy in the discovery and development of DNA damage response inhibitors against DNA polymerase eta (Pol η or POLH) and apurinic/apyrimidinic endonuclease 1 (APE1 or APEX1). Notably, our SaXPy platform allowed us to solve the first crystal structures of these proteins bound to small molecules and to discover novel binding sites for each target.

The Current Status of DNA-Repair-Directed Precision Oncology Strategies in Epithelial Ovarian Cancers

Survival outcomes for patients with advanced ovarian cancer remain poor despite advances in chemotherapy and surgery. Platinum-based systemic chemotherapy can result in a response rate of up to 80%, but most patients will have recurrence and die from the disease. Recently, the DNA-repair-directed precision oncology strategy has generated hope for patients. The clinical use of poly(ADP-ribose) polymerase (PARP) inhibitors in BRCA germ-line-deficient and/or platinum-sensitive epithelial ovarian cancers has improved survival. However, the emergence of resistance is an ongoing clinical challenge. Here, we review the current clinical state of PARP inhibitors and other clinically viable targeted approaches in epithelial ovarian cancers.

Evolving DNA repair synthetic lethality targets in cancer

DNA damage signaling response and repair (DDR) is a critical defense mechanism against genomic instability. Impaired DNA repair capacity is an important risk factor for cancer development. On the other hand, up-regulation of DDR mechanisms is a feature of cancer chemotherapy and radiotherapy resistance. Advances in our understanding of DDR and its complex role in cancer has led to several translational DNA repair-targeted investigations culminating in clinically viable precision oncology strategy using poly(ADP-ribose) polymerase (PARP) inhibitors in breast, ovarian, pancreatic, and prostate cancers. While PARP directed synthetic lethality has improved outcomes for many patients, the lack of sustained clinical response and the development of resistance pose significant clinical challenges. Therefore, the search for additional DDR-directed drug targets and novel synthetic lethality approaches is highly desirable and is an area of intense preclinical and clinical investigation. Here, we provide an overview of the mammalian DNA repair pathways and then focus on current state of PARP inhibitors (PARPi) and other emerging DNA repair inhibitors for synthetic lethality in cancer.

Unboxing the molecular modalities of mutagens in cancer

The etiology of the majority of human cancers is associated with a myriad of environmental causes, including physical, chemical, and biological factors. DNA damage induced by such mutagens is the initial step in the process of carcinogenesis resulting in the accumulation of mutations. Mutational events are considered the major triggers for introducing genetic and epigenetic insults such as DNA crosslinks, single- and double-strand DNA breaks, formation of DNA adducts, mismatched bases, modification in histones, DNA methylation, and microRNA alterations. However, DNA repair mechanisms are devoted to protect the DNA to ensure genetic stability, any aberrations in these calibrated mechanisms provoke cancer occurrence. Comprehensive knowledge of the type of mutagens and carcinogens and the influence of these agents in DNA damage and cancer induction is crucial to develop rational anticancer strategies. This review delineated the molecular mechanism of DNA damage and the repair pathways to provide a deep understanding of the molecular basis of mutagenicity and carcinogenicity. A relationship between DNA adduct formation and cancer incidence has also been summarized. The mechanistic basis of inflammatory response and oxidative damage triggered by mutagens in tumorigenesis has also been highlighted. We elucidated the interesting interplay between DNA damage response and immune system mechanisms. We addressed the current understanding of DNA repair targeted therapies and DNA damaging chemotherapeutic agents for cancer treatment and discussed how antiviral agents, anti-inflammatory drugs, and immunotherapeutic agents combined with traditional approaches lay the foundations for future cancer therapies.

Alterations in Molecular Profiles Affecting Glioblastoma Resistance to Radiochemotherapy: Where Does the Good Go?

Simple Summary

Glioblastoma is a type of brain cancer that remains incurable. Despite multiple past and ongoing preclinical studies and clinical trials, involving adjuvants to the conventional therapy and based on molecular targeting, no relevant benefit for patients’ survival has been achieved so far. The current first-line treatment regimen is based on ionizing radiation and the monoalkylating compound, temozolomide, and has been administered for more than 15 years. Glioblastoma is extremely resistant to most agents due to a mutational background that elicits quick response to insults and adapts to microenvironmental and metabolic changes. Here, we present the most recent evidence concerning the molecular features and their alterations governing pathways involved in GBM response to the standard radio-chemotherapy and discuss how they collaborate with acquired GBM’s resistance.

Abstract

Glioblastoma multiforme (GBM) is a brain tumor characterized by high heterogeneity, diffuse infiltration, aggressiveness, and formation of recurrences. Patients with this kind of tumor suffer from cognitive, emotional, and behavioral problems, beyond exhibiting dismal survival rates. Current treatment comprises surgery, radiotherapy, and chemotherapy with the methylating agent, temozolomide (TMZ). GBMs harbor intrinsic mutations involving major pathways that elicit the cells to evade cell death, adapt to the genotoxic stress, and regrow. Ionizing radiation and TMZ induce, for the most part, DNA damage repair, autophagy, stemness, and senescence, whereas only a small fraction of GBM cells undergoes treatment-induced apoptosis. Particularly upon TMZ exposure, most of the GBM cells undergo cellular senescence. Increased DNA repair attenuates the agent-induced cytotoxicity; autophagy functions as a pro-survival mechanism, protecting the cells from damage and facilitating the cells to have energy to grow. Stemness grants the cells capacity to repopulate the tumor, and senescence triggers an inflammatory microenvironment favorable to transformation. Here, we highlight this mutational background and its interference with the response to the standard radiochemotherapy. We discuss the most relevant and recent evidence obtained from the studies revealing the molecular mechanisms that lead these cells to be resistant and indicate some future perspectives on combating this incurable tumor.

Inula graveolens induces selective cytotoxicity in glioblastoma and chronic leukemia cells

OBJECTIVE

Crude oil extracts, components of extracts, and ethanolic extracts of Inula graveolens possess various pharmacological activities on various cancer cells including antioxidative and antiproliferative effects. Aqueous extract of this species has not been investigated on the liquid malignancies and solid tumors with a high incidence of treatment refractoriness and poor survival outcomes such as glioblastoma and leukemia. Hence, the present study aimed to evaluate the cytotoxic efficiency of I. graveolens aqueous extracts on human glioblastoma multiforme and chronic myelogenous leukemia cell lines in comparison to non-cancerous primary rat cerebral cortex and human peripheral blood mononuclear cells.

METHODS

The cells were treated with the extracts of I. graveolens (125-1000 μg/mL) for 48 h, the cellular viability was identified using 3'-(4,5dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay, and lactate dehydrogenase release was measured to determine the cytotoxic potential. Total oxidant status and apurinic/apyrimidinic endodeoxyribonuclease 1 assays were used to determine the oxidative status of cells and DNA damage, respectively.

RESULTS

I. graveolens showed selective cytotoxicity toward human glioblastoma multiforme and chronic myelogenous leukemia cell lines and exhibited a higher antiproliferative effect against cancer cells in comparison to non-cancerous cells. Moreover, it significantly reduced the apurinic/apyrimidinic endodeoxyribonuclease 1 levels on both cancer cell lines as compared with their control cells without changing the levels of an oxidative stress marker.

CONCLUSION

The extracts of I. graveolens have anti-cancer potential on human glioblastoma multiforme and chronic myelogenous leukemia cell lines without causing oxidative stress.

Early Drug Discovery and Development of Novel Cancer Therapeutics Targeting DNA Polymerase Eta (POLH)

Polymerase eta (or Pol η or POLH) is a specialized DNA polymerase that is able to bypass certain blocking lesions, such as those generated by ultraviolet radiation (UVR) or cisplatin, and is deployed to replication foci for translesion synthesis as part of the DNA damage response (DDR). Inherited defects in the gene encoding POLH (a.k.a., XPV) are associated with the rare, sun-sensitive, cancer-prone disorder, xeroderma pigmentosum, owing to the enzyme's ability to accurately bypass UVR-induced thymine dimers. In standard-of-care cancer therapies involving platinum-based clinical agents, e.g., cisplatin or oxaliplatin, POLH can bypass platinum-DNA adducts, negating benefits of the treatment and enabling drug resistance. POLH inhibition can sensitize cells to platinum-based chemotherapies, and the polymerase has also been implicated in resistance to nucleoside analogs, such as gemcitabine. POLH overexpression has been linked to the development of chemoresistance in several cancers, including lung, ovarian, and bladder. Co-inhibition of POLH and the ATR serine/threonine kinase, another DDR protein, causes synthetic lethality in a range of cancers, reinforcing that POLH is an emerging target for the development of novel oncology therapeutics. Using a fragment-based drug discovery approach in combination with an optimized crystallization screen, we have solved the first X-ray crystal structures of small novel drug-like compounds, i.e., fragments, bound to POLH, as starting points for the design of POLH inhibitors. The intrinsic molecular resolution afforded by the method can be quickly exploited in fragment growth and elaboration as well as analog scoping and scaffold hopping using medicinal and computational chemistry to advance hits to lead. An initial small round of medicinal chemistry has resulted in inhibitors with a range of functional activity in an in vitro biochemical assay, leading to the rapid identification of an inhibitor to advance to subsequent rounds of chemistry to generate a lead compound. Importantly, our chemical matter is different from the traditional nucleoside analog-based approaches for targeting DNA polymerases.

Synthesis and evaluation of squaramide and thiosquaramide inhibitors of the DNA repair enzyme SNM1A

SNM1A is a zinc-dependent nuclease involved in the removal of interstrand crosslink lesions from DNA. Inhibition of interstrand crosslink repair enzymes such as SNM1A is a promising strategy for improving the efficacy of crosslinking chemotherapy drugs. Initial studies have demonstrated the feasibility of developing SNM1A inhibitors, but the full potential of this enzyme as a drug target has yet to be explored. Herein, the synthesis of a family of squaramide- and thiosquaramide-bearing nucleoside derivatives and their evaluation as SNM1A inhibitors is reported. A gel electrophoresis assay was used to identify nucleoside derivatives bearing an N-hydroxysquaramide or squaric acid moiety at the 3′-position, and a thymidine derivative bearing a 5′-thiosquaramide, as candidate SNM1A inhibitors. Quantitative IC₅₀ determination showed that a thymidine derivative bearing a 5′-thiosquaramide was the most potent inhibitor, followed by a thymidine derivative bearing a 3′-squaric acid. UV–Vis titrations were carried out to evaluate the binding of the (thio)squaramides to zinc ions, allowing the order of inhibitory potency to be rationalised. The membrane permeability of the active inhibitors was investigated, with several compounds showing promise for future in vivo applications.

Impact of Hypoxia on Relative Biological Effectiveness and Oxygen Enhancement Ratio for a 62-MeV Therapeutic Proton Beam

This study uses the yields of double-strand breaks (DSBs) to determine the relative biological effectiveness (RBE) of proton beams, using cell survival as a biological endpoint. DSB induction is determined when cells locate at different depths (6 positions) along the track of 62 MeV proton beams. The DNA damage yields are estimated using Monte Carlo Damage Simulation (MCDS) software. The repair outcomes are estimated using Monte Carlo excision repair (MCER) simulations. The RBE for cell survival at different oxygen concentrations is calculated using the repair-misrepair-fixation (RMF) model. Using 60Co γ-rays (linear energy transfer (LET) = 2.4 keV/μm) as the reference radiation, the RBE for DSB induction and enzymatic DSB under aerobic condition (21% O2) are in the range 1.0-1.5 and 1.0-1.6 along the track depth, respectively. In accord with RBE obtained from experimental data, RMF model-derived RBE values for cell survival are in the range of 1.0-3.0. The oxygen enhancement ratio (OER) for cell survival (10%) decreases from 3.0 to 2.5 as LET increases from 1.1 to 22.6 keV/μm. The RBE values for severe hypoxia (0.1% O2) are in the range of 1.1-4.4 as LET increases, indicating greater contributions of direct effects for protons. Compared with photon therapy, the overall effect of 62 MeV proton beams results in greater cell death and is further intensified under hypoxic conditions.

The effect of eIF3a on anthracycline-based chemotherapy resistance by regulating DSB DNA repair

BACKGROUND

Anthracycline are inhibitors of topoisomerase II leading to DNA double strand breaks, and it is widely used for treatment of breast cancer. eIF3a is the largest subunit of eukaryotic translation initiation factor 3 (eIF3) and highly expressed in breast cancer. In this study, we investigated the role of eIF3a in DSB DNA repair and the response of breast cancer patients to anthracycline-based chemotherapy.

METHODS

MTT assay was used to detect anthracycline sensitivity in cell lines. Real-time reverse transcriptase PCR, western blotting and immunofluorescence were performed to assess changes in gene expression levels. Cometassay and end-joining activity assay were conducted to explore the effect of eIF3a in NHEJ repair. Luciferase reporter assay was performed to detect LIG4 5'UTR activity. Immunohistochemistry was used to detect eIF3a, LIG4 and DNA-PKcs expression levels in breast cancer tissues.

RESULTS

The results showed that eIF3a increased cellular response to anthracyclines by regulating DSB repair activity via influencing the expression of LIG4 and DNA-PKcs at translational level. Breast cancer patients with high level of eIF3a or low level of LIG4 or low level of DNA-PKcs had better anthracycline-based chemotherapy prognosis compared. Moreover, Combined expressions of eIF3a, LIG4 and DNA-PKcs could be better to predict PFS in breast cancer patients with anthracycline-based chemotherapy.

CONCLUSION

Our findings suggest that eIF3a effects anthracycline-based chemotherapy response by regulating DSB DNA repair.

The Chromatin Response to Double-Strand DNA Breaks and Their Repair

Cellular DNA is constantly being damaged by numerous internal and external mutagenic factors. Probably the most severe type of insults DNA could suffer are the double-strand DNA breaks (DSBs). They sever both DNA strands and compromise genomic stability, causing deleterious chromosomal aberrations that are implicated in numerous maladies, including cancer. Not surprisingly, cells have evolved several DSB repair pathways encompassing hundreds of different DNA repair proteins to cope with this challenge. In eukaryotic cells, DSB repair is fulfilled in the immensely complex environment of the chromatin. The chromatin is not just a passive background that accommodates the multitude of DNA repair proteins, but it is a highly dynamic and active participant in the repair process. Chromatin alterations, such as changing patterns of histone modifications shaped by numerous histone-modifying enzymes and chromatin remodeling, are pivotal for proficient DSB repair. Dynamic chromatin changes ensure accessibility to the damaged region, recruit DNA repair proteins, and regulate their association and activity, contributing to DSB repair pathway choice and coordination. Given the paramount importance of DSB repair in tumorigenesis and cancer progression, DSB repair has turned into an attractive target for the development of novel anticancer therapies, some of which have already entered the clinic.

Effects of indirect actions and oxygen on relative biological effectiveness: estimate of DSB inductions and conversions induced by therapeutic proton beams

Purpose: This study evaluated the DNA double strand breaks (DSBs) induced by indirect actions and its misrepairs to estimate the relative biological effectiveness (RBE) of proton beams.Materials and methods: From experimental data, DSB induction was evaluated in cells irradiated by 62 MeV proton beams in the presence of dimethylsulphoxide (DMSO) and under hypoxic conditions. The DNA damage yields for calculating the RBE were estimated using Monte Carlo Damage Simulation (MCDS) software. The repair outcomes (correct repairs, mutations and DSB conversions) were estimated using Monte Carlo Excision Repair (MCER) simulations.Results: The values for RBE of 62 MeV protons (LET = 1.051 keV/μm) for DSB induction and enzymatic DSB under aerobic condition (21% O₂) was 1.02 and 0.94, respectively, as comparing to ⁶⁰Co γ-rays (LET = 2.4 keV/μm). DMSO mitigated the inference of indirect action and reduced DSB induction to a greater extent when damaged by protons rather than γ-rays, resulting in a decreased RBE of 0.86. DMSO also efficiently prevented enzymatic DSB yields triggered by proton irradiation and reduced the RBE to 0.83. However, hypoxia (2% O₂) produced a similar level of DSB induction with respect to the protons and γ-rays, with a comparable RBE of 1.02.Conclusions: The RBE values of proton beams estimated from DSB induction and enzymatic DSB decreased by 16% and 12%, respectively, in the presence of DMSO. Our findings indicate that the overall effects of DSB induction and enzymatic DSB could intensify the tumor killing, while alleviate normal tissue damage when indirect actions are effectively interrupted.

Effects of electromagnetic field, cisplatin and morphine on cytotoxicity and expression levels of DNA repair genes

Morphine (Mor) is widely used as an analgesic drug in cancers and in combination with chemotherapy is known to have DNA damaging effects on non-targeted cell. This study surveyed the effect of Mor in combination with 50-Hz electromagnetic field (EMF) and co-treatment of cisplatin in combination with Mor and EMF on the expression of genes involved in DNA repair pathways. MCF-7 and SH-SY5Y cells were treated with 5.0 µM Mor and then exposed to 50-Hz 0.50 mT EMF in the intermittent pattern of 15 min field-on/15 min field-off. Gene expression, cisplatin and bleomycin cytotoxicity were measured using real-time PCR and MTT assay. Mor treated cells showed significant down-regulation of the examined genes, while in "Mor + EMF" treatments the genes were not significantly changed. IC₅₀ of cisplatin was significantly elevated in both cell lines when co-treated with "Mor + EMF" compared with Mor treated cells. Non-homologous end joining (NHEJ) related genes were significantly decreased in co-treatment of cisplatin and "Mor + EMF" which led to bleomycin higher cytotoxicity in SH-SY5Y not in MCF-7. Our data is promising for providing a cell line-specific sensitization by combination of cisplatin and "Mor + EMF" treatment with local administration of double strand breaking agents.

Establishment and genomic characterizations of patient-derived esophageal squamous cell carcinoma xenograft models using biopsies for treatment optimization

Background

Squamous cell carcinoma is the dominant type of esophageal cancer in China with many patients initially diagnosed at advanced stage. Patient-derived xenografts (PDX) models have been developed to be an important platform for preclinical research. This study aims to establish and characterize PDX models using biopsy tissue from advanced esophageal cancer patients to lay the foundation of preclinical application.

Methods

Fresh endoscopic biopsy tissues were harvested from patients with advanced esophageal cancer and implanted subcutaneously into NOD/SCID mice. Then, the PDXs were serially passaged for up to four generations. Transplantation was analyzed and genomic characteristics of xenografts were profiled using next-generation sequencing.

Results

Twenty-five PDX models were established (13.3%, 25/188). The latency period was 75.12 ± 19.87 days (50–120 days) for the first passage and it decreased with increasing passaging. Other than tumor stages, no differences were found between transplantations of xenografts and patient characteristics, irrespective of chemotherapy. Histopathological features and chemosensitivity of PDXs were in great accordance with primary patient tumors. Each PDX was assessed for molecular characteristics including copy number variations, somatic mutations, and signaling pathway abnormalities and these were similar to patient results.

Conclusions

Our PDX models were established from real time biopsies and molecularly profiled. They might be promising for drug development and individualized therapy.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1379-9) contains supplementary material, which is available to authorized users.

Different profiles of the mRNA levels of DNA repair genes in MCF-7 and SH-SY5Y cells after treatment with combination of cisplatin, 50-Hz electromagnetic field and bleomycin

Neurotoxicity is known to be a major dose-limiting adverse effect of cisplatin (CDDP), alone or in combination with other chemicals. DNA repair capacity serve as a neuroprotective factor against CDDP. The purpose of this study was to evaluate the effect of 50-Hz electromagnetic field (EMF) in combination with CDDP and bleomycin (Bleo) on expression of some of DNA repair genes (GADD45A, XRCC1, XRCC4, Ku70, Ku80, DNA-PKcs and LIG4) in MCF-7 (breast cancer) and SH-SY5Y (neuroblastoma) cell lines. MCF-7 and SH-SY5Y cells were pre-treated with CDDP in the presence or absence of EMF and then exposed to different concentration of Bleo. EMF (0.50mT intensity) was used in the intermittenet pattern of "15min field on/15min field off" with 30min total exposure. Cell viability assay was done and then the transcript levels of the examined genes were measured using quantitative real-time PCR in "CDDP+Bleo" and "CDDP+EMF+Bleo" treatments. Our results indicated that MCF-7 cells treated with "CDDP+EMF+Bleo" showed more susceptibility compared with "CDDP+Bleo" treated ones, while SH-SY5Y susceptibility was not changed between the two treatments. The represented data indicated that MCF-7 and SH-SY5Y cells showed non-random disagreement in DNA repair gene expression in 11 conditions (out of 14 conditions) with each other (χ2=4.52, df=1, P=0.033). This finding can be promising for sensitizing breast cancer cells while protecting against CDDP induced neuropathy in cancer patients.

Investigating the Frequency of the ERCC1 Gene C8092A Polymorphism in Iranian Patients with Advanced Gastric Cancer Receiving Platinum-based Chemotherapy

BACKGROUND

Platinum compounds are the main drugs for treatment of advanced gastric cancer. Previous studies have shown that clinical outcome with platinum-based compounds depends on ERCC1 polymorphisms. The aim of this study was to investigate the frequency of a common polymorphism of ERCC1 gene (C8092A) in Iranian patients with advanced gastric cancer receiving platinum chemotherapy.

MATERIALS AND METHODS

Genetic analysis of the ERCC1 C8092A polymorphism was performed by the PCR - RFLP method using 50 paraffin-embedded tissue specimens.

RESULTS

Of the 50 cases, 32% of individuals showed CC genotype, 24% of them had CA genotype and 44% of patients had AA genotype.

CONCLUSIONS

Based on the results, using of platinum-based chemotherapy would be expected to be specifically beneficial in only 32% of patients.

Innocent But Deadly: Nontoxic Organoiridium Catalysts Promote Selective Cancer Cell Death

We demonstrate that nontoxic organoiridum complexes can selectively chemosensitize cancer cells toward platinum antiproliferative agents. Treatment of human cancer cells (breast, colon, eye/retina, head/neck, lung, ovary, and blood) with the iridium chemosensitizers led to lowering of the 50 % growth inhibition concentration (IC₅₀ ) of the Pt drug carboplatin by up to ∼30-50 %. Interestingly, non-cancer cells were mostly resistant to the chemosensitizing effects of the iridium complexes. Cell culture studies indicate that cancer cells that were administered with Ir show significantly higher reactive oxygen species concentrations as well as NAD⁺ /NADH ratios (oxidized vs. reduced nicotinamide adenine dinucleotide) than Ir-treated non-cancer cells. These biochemical changes are consistent with a catalytic transfer hydrogenation cycle involving the formation of iridium-hydride species from the reaction of the iridium catalysts with NADH and subsequent oxidation in air to generate hydrogen peroxide.

The ascidian natural product eusynstyelamide B is a novel topoisomerase II poison that induces DNA damage and growth arrest in prostate and breast cancer cells

As part of an anti-cancer natural product drug discovery program, we recently identified eusynstyelamide B (EB), which displayed cytotoxicity against MDA-MB-231 breast cancer cells (IC₅₀ = 5 μM) and induced apoptosis. Here, we investigated the mechanism of action of EB in cancer cell lines of the prostate (LNCaP) and breast (MDA-MB-231). EB inhibited cell growth (IC₅₀ = 5 μM) and induced a G2 cell cycle arrest, as shown by a significant increase in the G2/M cell population in the absence of elevated levels of the mitotic marker phospho-histone H3. In contrast to MDA-MB-231 cells, EB did not induce cell death in LNCaP cells when treated for up to 10 days. Transcript profiling and Ingenuity Pathway Analysis suggested that EB activated DNA damage pathways in LNCaP cells. Consistent with this, CHK2 phosphorylation was increased, p21^CIP1/WAF1 was up-regulated and CDC2 expression strongly reduced by EB. Importantly, EB caused DNA double-strand breaks, yet did not directly interact with DNA. Analysis of topoisomerase II-mediated decatenation discovered that EB is a novel topoisomerase II poison.

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.

Nanoparticle formulation of small DNA molecules, Dbait, improves the sensitivity of hormone-independent prostate cancer to radiotherapy

Intensification of radiotherapy has been shown to improve prostate cancer (PCa) outcomes. We hypothesized that we could further improve radiotherapy efficacy through the use DNA repair inhibitors. In this study, we evaluated the use of a new class of DNA damage repair inhibitor, nanoparticle (NP) Dbait, in radiosensitization of PCa. NP Dbait was formulated using H1 nanopolymer (folate-polyethylenimine600-cyclodextrin). We demonstrated that NP Dbait was a potent radiosensitizer in vitro by colony forming assay using PCa cell lines. The result was validated in vivo using mouse xenograft models of PCa and we showed that NP Dbait significantly suppressed tumor growth and prolonged survival. Western blot, immunofluorescence and immunohistochemistry showed that NP Dbait inhibited DNA damage repair signaling pathways by mimicking DNA double-strand breaks. Our study supports further investigations of NP Dbait in improving the therapeutic efficacy of cancer radiotherapy.

Inhibition of AKT sensitizes chemoresistant ovarian cancer cells to cisplatin by abrogating S and G2/M arrest

The phosphatidylinositol 3-kinase (PI3K)/Akt pathway is frequently altered in human malignancies and Akt over-expression and/or activation induces malignant transformation and chemoresistance. However, the role of Akt in the mechanisms of chemoresistance remains elusive. Here we reported that cisplatin treatment of chemosensitive, but not resistant, ovarian cancer cells (OVCAs) markedly increased the cell proportion in sub-G1 phase. Cisplatin however caused a significant accumulation of the resistant cells in S and G2/M phases, which was associated with a rapid and sustained checkpoint kinase 1 (Chk1) activation. In contrast, while cisplatin also elicited a rapid activation of Chk1 in sensitive cells, it markedly decreased total ChK1 and phospho-Chk1 contents over 12 h. Over-expression of dominant negative (DN)-AKT alone increased phospho-Chk1 content, and induced G2/M arrest and apoptosis. However, it inhibited Chk1 activation and G2/M arrest with combination of cisplatin treatment, resulting in p53-independent apoptosis. Furthermore, the responses of the chemoresistant cells to cisplatin were attenuated with forced expression of constitutive active AKT2. Chk1 knock-down also facilitated cisplatin-induced apoptosis in chemoresistant cells. Our studies implicate that, in addition to its cell survival and anti-apoptotic actions, Akt might also play an important role in the regulation of G2-M transition, possibly via up-regulation of Chk1 activity and stability. These data provide strong support for the concept that Akt is important in cell cycle regulation in the control of chemosensitivity in OVCAs and offers an alternate regulatory pathway for the development of rationale therapy for cisplatin-resistant ovarian cancer.

DNA-Dependent Protein Kinase As Molecular Target for Radiosensitization of Neuroblastoma Cells

Tumor cells might resist therapy with ionizing radiation (IR) by non-homologous end-joining (NHEJ) of IR-induced double-strand breaks. One of the key players in NHEJ is DNA-dependent protein kinase (DNA-PK). The catalytic subunit of DNA-PK, i.e. DNA-PKcs, can be inhibited with the small-molecule inhibitor NU7026. In the current study, the in vitro potential of NU7026 to radiosensitize neuroblastoma cells was investigated. DNA-PKcs is encoded by the PRKDC (protein kinase, DNA-activated, catalytic polypeptide) gene. We showed that PRKDC levels were enhanced in neuroblastoma patients and correlated with a more advanced tumor stage and poor prognosis, making DNA-PKcs an interesting target for radiosensitization of neuroblastoma tumors. Optimal dose finding for combination treatment with NU7026 and IR was performed using NGP cells. One hour pre-treatment with 10 μM NU7026 synergistically sensitized NGP cells to 0.63 Gy IR. Radiosensitizing effects of NU7026 increased in time, with maximum effects observed from 96 h after IR-exposure on. Combined treatment of NGP cells with 10 μM NU7026 and 0.63 Gy IR resulted in apoptosis, while no apoptotic response was observed for either of the therapies alone. Inhibition of IR-induced DNA-PK activation by NU7026 confirmed the capability of NGP cells to, at least partially, resist IR by NHEJ. NU7026 also synergistically radiosensitized other neuroblastoma cell lines, while no synergistic effect was observed for low DNA-PKcs-expressing non-cancerous fibroblasts. Results obtained for NU7026 were confirmed by PRKDC knockdown in NGP cells. Taken together, the current study shows that DNA-PKcs is a promising target for neuroblastoma radiosensitization.

3-(Dipropylamino)-5-hydroxybenzofuro[2,3-f]quinazolin-1(2H)-one (DPA-HBFQ-1) plays an inhibitory role on breast cancer cell growth and progression

A series of unknown 3-(alkyl(dialkyl)amino)benzofuro[2,3-f]quinazolin-1(2H)-ones 4-17 has been synthesized as new ellipticine analogs, in which the carbazole moiety and the pyridine ring were replaced by a dibenzofuran residue and a pyrimidine ring, respectively. The synthesis of these benzofuroquinazolinones 4-17 was performed in a simple one-pot reaction using 3-aminodibenzofuran or its 2-methoxy derivative, as starting materials. From 3-(dipropylamino)-5-methoxybenzofuro[2,3-f] quinazolin-1(2H)-one (13), we prepared 3-(dipropylamino)-5-hydroxybenzofuro[2,3-f]quinazolin-1(2H)-one (18), referred to as DPA-HBFQ-1. The cytotoxic activities of all the synthesized compounds, tested in different human breast cancer cell lines, revealed that DPA-HBFQ-1 was the most active compound. In particular, the latter was able to inhibit anchorage-dependent and -independent cell growth and to induce apoptosis in estrogen receptor alpha (ERα)-positive and -negative breast cancer cells. It did not affect proliferation and apoptotic responses in MCF-10A normal breast epithelial cells. The observed effects have been ascribed to an enhanced p21(Cip1/WAF1) expression in a p53-dependent manner of tumor suppressor and to a selective inhibition of human topoisomerase II. In addition, DPA-HBFQ-1 exerted growth inhibitory effects also in other cancer cell lines, even though with a lower cytotoxic activity. Our results indicate DPA-HBFQ-1 as a good candidate to be useful as cancer therapeutic agent, particularly for breast cancer.

Recurrent Glioblastomas Reveal Molecular Subtypes Associated with Mechanistic Implications of Drug-Resistance

Previously, transcriptomic profiling studies have shown distinct molecular subtypes of glioblastomas. It has also been suggested that the recurrence of glioblastomas could be achieved by transcriptomic reprograming of tumors, however, their characteristics are not yet fully understood. Here, to gain the mechanistic insights on the molecular phenotypes of recurrent glioblastomas, gene expression profiling was performed on the 43 cases of glioblastomas including 15 paired primary and recurrent cases. Unsupervised clustering analyses revealed two subtypes of G1 and G2, which were characterized by proliferation and neuron-like gene expression traits, respectively. While the primary tumors were classified as G1 subtype, the recurrent glioblastomas showed two distinct expression types. Compared to paired primary tumors, the recurrent tumors in G1 subtype did not show expression alteration. By contrast, the recurrent tumors in G2 subtype showed expression changes from proliferation type to neuron-like one. We also observed the expression of stemness-related genes in G1 recurrent tumors and the altered expression of DNA-repair genes (i.e., AURK, HOX, MGMT, and MSH6) in the G2 recurrent tumors, which might be responsible for the acquisition of drug resistance mechanism during tumor recurrence in a subtype-specific manner. We suggest that recurrent glioblastomas may choose two different strategies for transcriptomic reprograming to escape the chemotherapeutic treatment during tumor recurrence. Our results might be helpful to determine personalized therapeutic strategy against heterogeneous glioma recurrence.

DNA Repair Endonucleases: Physiological Roles and Potential as Drug Targets

Genomic DNA is constantly exposed to endogenous and exogenous damaging agents. To overcome these damaging effects and maintain genomic stability, cells have robust coping mechanisms in place, including repair of the damaged DNA. There are a number of DNA repair pathways available to cells dependent on the type of damage induced. The removal of damaged DNA is essential to allow successful repair. Removal of DNA strands is achieved by nucleases. Exonucleases are those that progressively cut from DNA ends, and endonucleases make single incisions within strands of DNA. This review focuses on the group of endonucleases involved in DNA repair pathways, their mechanistic functions, roles in cancer development, and how targeting these enzymes is proving to be an exciting new strategy for personalized therapy in cancer.

Phosphatidylinositol 3-Kinase (PI3K) and phosphatidylinositol 3-kinase-related kinase (PIKK) inhibitors: importance of the morpholine ring

Phosphatidylinositol 3-kinases (PI3Ks) and phosphatidylinositol 3-kinase-related protein kinases (PIKKs) are two related families of kinases that play key roles in regulation of cell proliferation, metabolism, migration, survival, and responses to diverse stresses including DNA damage. To design novel efficient strategies for treatment of cancer and other diseases, these kinases have been extensively studied. Despite their different nature, these two kinase families have related origin and share very similar kinase domains. Therefore, chemical inhibitors of these kinases usually carry analogous structural motifs. The most common feature of these inhibitors is a critical hydrogen bond to morpholine oxygen, initially present in the early nonspecific PI3K and PIKK inhibitor 3 (LY294002), which served as a valuable chemical tool for development of many additional PI3K and PIKK inhibitors. While several PI3K pathway inhibitors have recently shown promising clinical responses, inhibitors of the DNA damage-related PIKKs remain thus far largely in preclinical development.

Oxidatively induced DNA damage and its repair in cancer

Oxidatively induced DNA damage is caused in living organisms by endogenous and exogenous reactive species. DNA lesions resulting from this type of damage are mutagenic and cytotoxic and, if not repaired, can cause genetic instability that may lead to disease processes including carcinogenesis. Living organisms possess DNA repair mechanisms that include a variety of pathways to repair multiple DNA lesions. Mutations and polymorphisms also occur in DNA repair genes adversely affecting DNA repair systems. Cancer tissues overexpress DNA repair proteins and thus develop greater DNA repair capacity than normal tissues. Increased DNA repair in tumors that removes DNA lesions before they become toxic is a major mechanism for development of resistance to therapy, affecting patient survival. Accumulated evidence suggests that DNA repair capacity may be a predictive biomarker for patient response to therapy. Thus, knowledge of DNA protein expressions in normal and cancerous tissues may help predict and guide development of treatments and yield the best therapeutic response. DNA repair proteins constitute targets for inhibitors to overcome the resistance of tumors to therapy. Inhibitors of DNA repair for combination therapy or as single agents for monotherapy may help selectively kill tumors, potentially leading to personalized therapy. Numerous inhibitors have been developed and are being tested in clinical trials. The efficacy of some inhibitors in therapy has been demonstrated in patients. Further development of inhibitors of DNA repair proteins is globally underway to help eradicate cancer.

The prognostic value of ERCC1 expression in gastric cancer patients treated with platinum-based chemotherapy: a meta-analysis

Numerous studies examined the association between excision repair complementation group 1 (ERCC1) expression and the prognosis of gastric cancer patients receiving platinum-based chemotherapy but yielded controversial results. We thus conducted a meta-analysis to quantitatively evaluate the prognostic value of ERCC1 expression in gastric cancer patients receiving platinum-based chemotherapy. A systematic literature search was performed to identify relevant studies in PubMed, Web of Science, Embase, Cochrane Library, China National Knowledge Infrastructure, Chinese Biomedical Literature Database, and WanFang Database up to December 17, 2013. Pooled hazard ratios (HRs) or odds ratios (ORs) with 95 % confidence intervals (CIs) were estimated. Moreover, meta-regression analysis and subgroup analysis were conducted according to ethnicity, HR extraction, detection methods, survival analysis, and quality score. A total of 1,409 patients from 21 studies were subjected to final analysis. Positive/high ERCC1 expression was significantly associated with poorer overall survival (HR, 1.58; 95 % CI, 1.09-2.28), especially in Asians (HR, 1.81; 95 % CI, 1.20-2.73), and lower response rate (OR, 0.26; 95 % CI, 0.18-0.36), but not with clinicopathological features, such as gender (OR, 1.01; 95 % CI, 0.68-1.51), grade (OR, 0.66; 95 % CI, 0.43-1.01), and stage (OR, 1.05; 95 % CI, 0.58-1.90). This meta-analysis suggested that ERCC1 expression might be a useful biomarker to predict response and survival for gastric cancer patients receiving platinum-based chemotherapy, particularly in Asians.

A multiplex assay based on encoded microbeads conjugated to DNA NanoBeacons to monitor base excision repair activities by flow cytometry

We reported here a new assay to detect base excision repair activities from purified enzymes, as well as in cell-free extracts. The multiplex format rests upon the encoding of magnetic beads with the fluorophore Alexa 488, thanks to a fast and original procedure. Fluorescently encoded microbeads are subsequently functionalized by lesion-containing DNA NanoBeacons labeled with the fluorophore/quencher pair Cyanine 5/BHQ2. Probes cleavage, induced by targeted enzymes leads to Cyanine 5 signal enhancement, which is finally quantified by flow cytometry. The multiplex assay was applied to the detection of restriction enzymes activities as well as base excision repair processes. Each test requires only 500fmol of DNA substrate, which constitutes great sensitivity compared to other BER functional assays. The present biosensor is able to detect both uracil DNA N-glycosylase (UNG) and AP-endonuclease 1 (APE1) within few nanograms of nuclear extract. Additionally, we demonstrated that the corresponding assay has potential application in DNA repair inhibitor search. Finally, the current multiplexed tool shows several advantages in comparison to other functional BER assays with no need of electrophoretic separation, straightforward, easy and reproducible functionalization of encoded microbeads and a high stability of DNA probes in cell-free extracts.

DNA repair in cancer: emerging targets for personalized therapy

Genomic deoxyribonucleic acid (DNA) is under constant threat from endogenous and exogenous DNA damaging agents. Mammalian cells have evolved highly conserved DNA repair machinery to process DNA damage and maintain genomic integrity. Impaired DNA repair is a major driver for carcinogenesis and could promote aggressive cancer biology. Interestingly, in established tumors, DNA repair activity is required to counteract oxidative DNA damage that is prevalent in the tumor microenvironment. Emerging clinical data provide compelling evidence that overexpression of DNA repair factors may have prognostic and predictive significance in patients. More recently, DNA repair inhibition has emerged as a promising target for anticancer therapy. Synthetic lethality exploits intergene relationships where the loss of function of either of two related genes is nonlethal, but loss of both causes cell death. Exploiting this approach by targeting DNA repair has emerged as a promising strategy for personalized cancer therapy. In the current review, we focus on recent advances with a particular focus on synthetic lethality targeting in cancer.

Ouabain, a cardiac glycoside, inhibits the Fanconi anemia/BRCA pathway activated by DNA interstrand cross-linking agents

Modulation of the DNA repair pathway is an emerging target for the development of anticancer drugs. DNA interstrand cross-links (ICLs), one of the most severe forms of DNA damage caused by anticancer drugs such as cisplatin and mitomycin C (MMC), activates the Fanconi anemia (FA)/BRCA DNA repair pathway. Inhibition of the FA/BRCA pathway can enhance the cytotoxic effects of ICL-inducing anticancer drugs and can reduce anticancer drug resistance. To find FA/BRCA pathway inhibitory small molecules, we established a cell-based high-content screening method for quantitating the activation of the FA/BRCA pathway by measuring FANCD2 foci on DNA lesions and then applied our method to chemical screening. Using commercial LOPAC1280 chemical library screening, ouabain was identified as a competent FA/BRCA pathway inhibitory compound. Ouabain, a member of the cardiac glycoside family, binds to and inhibits Na(+)/K(+)-ATPase and has been used to treat heart disease for many years. We observed that ouabain, as well as other cardiac glycoside family members--digitoxin and digoxin--down-regulated FANCD2 and FANCI mRNA levels, reduced monoubiquitination of FANCD2, inhibited FANCD2 foci formation on DNA lesions, and abrogated cell cycle arrest induced by MMC treatment. These inhibitory activities of ouabain required p38 MAPK and were independent of cellular Ca(2+) ion increase or the drug uptake-inhibition effect of ouabain. Furthermore, we found that ouabain potentiated the cytotoxic effects of MMC in tumor cells. Taken together, we identified an additional effect of ouabain as a FA/BRCA pathway-inhibiting chemosensitization compound. The results of this study suggest that ouabain may serve as a chemosensitizer to ICL-inducing anticancer drugs.

Delineation of key XRCC4/Ligase IV interfaces for targeted disruption of non-homologous end joining DNA repair

Efficient DNA repair mechanisms frequently limit the effectiveness of chemotherapeutic agents that act through DNA damaging mechanisms. Consequently, proteins involved in DNA repair have increasingly become attractive targets of high-throughput screening initiatives to identify modulators of these pathways. Disruption of the XRCC4-Ligase IV interaction provides a novel means to efficiently halt repair of mammalian DNA double strand break repair; however; the extreme affinity of these proteins presents a major obstacle for drug discovery. A better understanding of the interaction surfaces is needed to provide a more specific target for inhibitor studies. To clearly define key interface(s) of Ligase IV necessary for interaction with XRCC4, we developed a competitive displacement assay using ESI-MS/MS and determined the minimal inhibitory fragment of the XRCC4-interacting region (XIR) capable of disrupting a complex of XRCC4/XIR. Disruption of a single helix (helix 2) within the helix-loop-helix clamp of Ligase IV was sufficient to displace XIR from a preformed complex. Dose-dependent response curves for the disruption of the complex by either helix 2 or helix-loop-helix fragments revealed that potency of inhibition was greater for the larger helix-loop-helix peptide. Our results suggest a susceptibility to inhibition at the interface of helix 2 and future studies would benefit from targeting this surface of Ligase IV to identify modulators that disrupt its interaction with XRCC4. Furthermore, helix 1 and loop regions of the helix-loop-helix clamp provide secondary target surfaces to identify adjuvant compounds that could be used in combination to more efficiently inhibit XRCC4/Ligase IV complex formation and DNA repair.

Inhibition of apurinic/apyrimidinic endonuclease I's redox activity revisited

The essential base excision repair protein, apurinic/apyrimidinic endonuclease 1 (APE1), plays an important role in redox regulation in cells and is currently targeted for the development of cancer therapeutics. One compound that binds APE1 directly is (E)-3-[2-(5,6-dimethoxy-3-methyl-1,4-benzoquinonyl)]-2-nonylpropenoic acid (E3330). Here, we revisit the mechanism by which this negatively charged compound interacts with APE1 and inhibits its redox activity. At high concentrations (millimolar), E3330 interacts with two regions in the endonuclease active site of APE1, as mapped by hydrogen-deuterium exchange mass spectrometry. However, this interaction lowers the melting temperature of APE1, which is consistent with a loss of structure in APE1, as measured by both differential scanning fluorimetry and circular dichroism. These results are consistent with other findings that E3330 concentrations of >100 μM are required to inhibit APE1's endonuclease activity. To determine the role of E3330's negatively charged carboxylate in redox inhibition, we converted the carboxylate to an amide by synthesizing (E)-2-[(4,5-dimethoxy-2-methyl-3,6-dioxocyclohexa-1,4-dien-1-yl)methylene]-N-methoxy-undecanamide (E3330-amide), a novel uncharged derivative. E3330-amide has no effect on the melting temperature of APE1, suggesting that it does not interact with the fully folded protein. However, E3330-amide inhibits APE1's redox activity in in vitro electrophoretic mobility shift redox and cell-based transactivation assays, producing IC(50) values (8.5 and 7 μM) lower than those produced with E3330 (20 and 55 μM, respectively). Thus, E3330's negatively charged carboxylate is not required for redox inhibition. Collectively, our results provide additional support for a mechanism of redox inhibition involving interaction of E3330 or E3330-amide with partially unfolded APE1.

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.

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.

APE1/Ref-1 role in redox signaling: translational applications of targeting the redox function of the DNA repair/redox protein APE1/Ref-1

The heterogeneity of most cancers diminishes the treatment effectiveness of many cancer-killing regimens. Thus, treatments that hold the most promise are ones that block multiple signaling pathways essential to cancer survival. One of the most promising proteins in that regard is APE1, whose reduction-oxidation activity influences multiple cancer survival mechanisms, including growth, proliferation, metastasis, angiogenesis, and stress responses. With the continued research using APE1 redox specific inhibitors alone or coupled with developing APE1 DNA repair inhibitors it will now be possible to further delineate the role of APE1 redox, repair and protein-protein interactions. Previously, use of siRNA or over expression approaches, while valuable, do not give a clear picture of the two major functions of APE1 since both techniques severely alter the cellular milieu. Additionally, use of the redox-specific APE1 inhibitor, APX3330, now makes it possible to study how inhibition of APE1's redox signaling can affect multiple tumor pathways and can potentiate the effectiveness of existing cancer regimens. Because APE1 is an upstream effector of VEGF, as well as other molecules that relate to angiogenesis and the tumor microenvironment, it is also being studied as a possible treatment for agerelated macular degeneration and diabetic retinopathy. This paper reviews all of APE1's functions, while heavily focusing on its redox activities. It also discusses APE1's altered expression in many cancers and the therapeutic potential of selective inhibition of redox regulation, which is the subject of intense preclinical studies.

Methoxyamine sensitizes the resistant glioblastoma T98G cell line to the alkylating agent temozolomide

Chemoresistance represents a major obstacle to successful treatment for malignant glioma with temozolomide. N (7)-methyl-G and N (3)-methyl-A adducts comprise more than 80 % of DNA lesions induced by temozolomide and are processed by the base excision repair, suggesting that the cellular resistance could be caused, in part, by this efficient repair pathway, although few studies have focused on this subject. The aim of this study was to evaluate the cellular responses to temozolomide treatment associated with methoxyamine (blocker of base excision repair) in glioblastoma cell lines, in order to test the hypothesis that the blockage of base excision repair pathway might sensitize glioblastoma cells to temozolomide. For all the tested cell lines, only T98G showed significant differences between temozolomide and temozolomide plus methoxyamine treatment, observed by reduced survival rates, enhanced the levels of DNA damage, and induced an arrest at G2-phase. In addition, ~10 % of apoptotic cells (sub-G1 fraction) were observed at 48 h. Western blot analysis demonstrated that APE1 and FEN1 presented a slightly reduced expression levels under the combined treatment, probably due to AP sites blockade by methoxyamine, thus causing a minor requirement of base excision repair pathway downstream to the AP removal by APE1. On the other hand, PCNA expression in temozolomide plus methoxyamine-treated cells does not rule out the possibility that such alteration might be related to the blockage of cell cycle (G2-phase), as observed at 24 h of recovery time. The results obtained in the present study demonstrated the efficiency of methoxyamine to overcome glioblastoma resistance to temozolomide treatment.

Oxidatively induced DNA damage: mechanisms, repair and disease

Endogenous and exogenous sources cause oxidatively induced DNA damage in living organisms by a variety of mechanisms. The resulting DNA lesions are mutagenic and, unless repaired, lead to a variety of mutations and consequently to genetic instability, which is a hallmark of cancer. Oxidatively induced DNA damage is repaired in living cells by different pathways that involve a large number of proteins. Unrepaired and accumulated DNA lesions may lead to disease processes including carcinogenesis. Mutations also occur in DNA repair genes, destabilizing the DNA repair system. A majority of cancer cell lines have somatic mutations in their DNA repair genes. In addition, polymorphisms in these genes constitute a risk factor for cancer. In general, defects in DNA repair are associated with cancer. Numerous DNA repair enzymes exist that possess different, but sometimes overlapping substrate specificities for removal of oxidatively induced DNA lesions. In addition to the role of DNA repair in carcinogenesis, recent evidence suggests that some types of tumors possess increased DNA repair capacity that may lead to therapy resistance. DNA repair pathways are drug targets to develop DNA repair inhibitors to increase the efficacy of cancer therapy. Oxidatively induced DNA lesions and DNA repair proteins may serve as potential biomarkers for early detection, cancer risk assessment, prognosis and for monitoring therapy. Taken together, a large body of accumulated evidence suggests that oxidatively induced DNA damage and its repair are important factors in the development of human cancers. Thus this field deserves more research to contribute to the development of cancer biomarkers, DNA repair inhibitors and treatment approaches to better understand and fight cancer.

A comparative study of recombinant mouse and human apurinic/apyrimidinic endonuclease

Mammalian apurinic/apyrimidinic endonuclease (APE1) initiates the repair of abasic sites (AP-sites), which are highly toxic, mutagenic, and implicated in carcinogenesis. Also, reducing the activity of APE1 protein in cancer cells and tumors sensitizes mammalian tumor cells to a variety of laboratory and clinical chemotherapeutic agents. In general, mouse models are used in studies of basic mechanisms of carcinogenesis, as well as pre-clinical studies before transitioning into humans. Human APE1 (hAPE1) has previously been cloned, expressed, and extensively characterized. However, the knowledge regarding the characterization of mouse APE1 (mAPE1) is very limited. Here we have expressed and purified full-length hAPE1 and mAPE1 in and from E. coli to near homogeneity. mAPE1 showed comparable fast reaction kinetics to its human counterpart. Steady-state enzyme kinetics showed an apparent K(m) of 91 nM and k(cat) of 4.2 s(-1) of mAPE1 for the THF cleavage reaction. For hAPE1 apparent K(m) and k(cat) were 82 nM and 3.2 s(-1), respectively, under similar reaction conditions. However, k(cat)/K(m) were in similar range for both APE1s. The optimum pH was in the range of 7.5-8 for both APE1s and had an optimal activity at 50-100 mM KCl, and they showed Mg(2+) dependence and abrogation of activity at high salt. Circular dichroism spectroscopy revealed that increasing the Mg(2+) concentration altered the ratio of "turns" to "β-strands" for both proteins, and this change may be associated with the conformational changes required to achieve an active state. Overall, compared to hAPE1, mAPE1 has higher K(m) and k(cat) values. However, overall results from this study suggest that human and mouse APE1s have mostly similar biochemical and biophysical properties. Thus, the conclusions of mouse studies to elucidate APE1 biology and its role in carcinogenesis may be extrapolated to apply to human biology. This includes the development and validation of effective APE1 inhibitors as chemosensitizers in clinical studies.

Soy isoflavones sensitize cancer cells to radiotherapy

Soy isoflavones are dietary compounds isolated from soybeans, which are safe for human use and have mild anti-cancer properties. Soy isoflavones inhibit the activity of transcription factors and genes essential for tumor cell proliferation, invasion, and neovascularization, and it appears that soy isoflavones may enhance the effectiveness of conventional therapies against cancer. Soy isoflavones could be an effective complementary treatment given that they inhibit the survival signaling pathways of various cancer cells through altered activation of APE1/Ref-1, NF-κB, and HIF-1α, which are genes essential for tumor cell survival, tumor growth, and angiogenesis, thus making such cells more sensitive to radiotherapy. Studies in which soy isoflavones were given in conjunction with radiotherapy to prostate cancer patients suggest that soy isoflavones might also mitigate the adverse effects of radiation on normal tissues, probably by acting as antioxidants. These observations open new avenues for exploiting soy isoflavones as supplements to conventional therapies.

Development and evaluation of human AP endonuclease inhibitors in melanoma and glioma cell lines

Aims:

Modulation of DNA base excision repair (BER) has the potential to enhance response to chemotherapy and improve outcomes in tumours such as melanoma and glioma. APE1, a critical protein in BER that processes potentially cytotoxic abasic sites (AP sites), is a promising new target in cancer. In the current study, we aimed to develop small molecule inhibitors of APE1 for cancer therapy.

Methods:

An industry-standard high throughput virtual screening strategy was adopted. The Sybyl8.0 (Tripos, St Louis, MO, USA) molecular modelling software suite was used to build inhibitor templates. Similarity searching strategies were then applied using ROCS 2.3 (Open Eye Scientific, Santa Fe, NM, USA) to extract pharmacophorically related subsets of compounds from a chemically diverse database of 2.6 million compounds. The compounds in these subsets were subjected to docking against the active site of the APE1 model, using the genetic algorithm-based programme GOLD2.7 (CCDC, Cambridge, UK). Predicted ligand poses were ranked on the basis of several scoring functions. The top virtual hits with promising pharmaceutical properties underwent detailed in vitro analyses using fluorescence-based APE1 cleavage assays and counter screened using endonuclease IV cleavage assays, fluorescence quenching assays and radiolabelled oligonucleotide assays. Biochemical APE1 inhibitors were then subjected to detailed cytotoxicity analyses.

Results:

Several specific APE1 inhibitors were isolated by this approach. The IC₅₀ for APE1 inhibition ranged between 30 nM and 50 μM. We demonstrated that APE1 inhibitors lead to accumulation of AP sites in genomic DNA and potentiated the cytotoxicity of alkylating agents in melanoma and glioma cell lines.

Conclusions:

Our study provides evidence that APE1 is an emerging drug target and could have therapeutic application in patients with melanoma and glioma.

Distinct roles of Ape1 protein in the repair of DNA damage induced by ionizing radiation or bleomycin

Ionizing radiation (IR) and bleomycin (BLM) are used to treat various types of cancers. Both agents generate cytotoxic double strand breaks (DSB) and abasic (apurinic/apyrimidinic (AP)) sites in DNA. The human AP endonuclease Ape1 acts on abasic or 3'-blocking DNA lesions such as those generated by IR or BLM. We examined the effect of siRNA-mediated Ape1 suppression on DNA repair and cellular resistance to IR or BLM in human B-lymphoblastoid TK6 cells and HCT116 colon tumor cells. Partial Ape1 deficiency (∼30% of normal levels) sensitized cells more dramatically to BLM than to IR cytotoxicity. In both cases, expression of the unrelated yeast AP endonuclease, Apn1, largely restored resistance. Ape1 deficiency increased DNA AP site accumulation due to IR treatment but reduced the number of DSB. In contrast, for BLM, there were more DSB under Ape1 deficiency, with little change in the accumulation of AP sites. Although the role of Ape1 in generating DSB was greater for IR, the enzyme facilitated removal of AP sites, which may mitigate the cytotoxic effects of IR. In contrast, BLM generates scattered AP sites, and the DSB have 3'-phosphoglycolate termini that require Ape1 processing. These DSB persist under Ape1 deficiency. Apoptosis induced by BLM (but not by IR) under Ape1 deficiency was partially p53-dependent, more dramatically in TK6 than HCT116 cells. Thus, Ape1 suppression or inhibition may be a more efficacious adjuvant for BLM than for IR cancer therapy, particularly for tumors with a functional p53 pathway.

Cisplatin sensitivity of testis tumour cells is due to deficiency in interstrand-crosslink repair and low ERCC1-XPF expression

Background

Cisplatin based chemotherapy cures over 80% of metastatic testicular germ cell tumours (TGCT). In contrast, almost all other solid cancers in adults are incurable once they have spread beyond the primary site. Cell lines derived from TGCTs are hypersensitive to cisplatin reflecting the clinical response. Earlier findings suggested that a reduced repair capacity might contribute to the cisplatin hypersensitivity of testis tumour cells (TTC), but the critical DNA damage has not been defined. This study was aimed at investigating the formation and repair of intrastrand and interstrand crosslinks (ICLs) induced by cisplatin in TTC and their contribution to TTC hypersensitivity.

Results

We observed that repair of intrastrand crosslinks is similar in cisplatin sensitive TTC and resistant bladder cancer cells, whereas repair of ICLs was significantly reduced in TTC. γH2AX formation, which serves as a marker of DNA breaks formed in response to ICLs, persisted in cisplatin-treated TTC and correlated with sustained phosphorylation of Chk2 and enhanced PARP-1 cleavage. Expression of the nucleotide excision repair factor ERCC1-XPF, which is implicated in the processing of ICLs, is reduced in TTC. To analyse the causal role of ERCC1-XPF for ICL repair and cisplatin sensitivity, we over-expressed ERCC1-XPF in TTC by transient transfection. Over-expression increased ICL repair and rendered TTC more resistant to cisplatin, which suggests that ERCC1-XPF is rate-limiting for repair of ICLs resulting in the observed cisplatin hypersensitivity of TTC.

Conclusion

Our data indicate for the first time that the exceptional sensitivity of TTC and, therefore, very likely the curability of TGCT rests on their limited ICL repair due to low level of expression of ERCC1-XPF.

Redox regulation of DNA repair: implications for human health and cancer therapeutic development

Redox reactions are known to regulate many important cellular processes. In this review, we focus on the role of redox regulation in DNA repair both in direct regulation of specific DNA repair proteins as well as indirect transcriptional regulation. A key player in the redox regulation of DNA repair is the base excision repair enzyme apurinic/apyrimidinic endonuclease 1 (APE1) in its role as a redox factor. APE1 is reduced by the general redox factor thioredoxin, and in turn reduces several important transcription factors that regulate expression of DNA repair proteins. Finally, we consider the potential for chemotherapeutic development through the modulation of APE1's redox activity and its impact on DNA repair.