A novel DNA-dependent protein kinase inhibitor, NU7026, potentiates the cytotoxicity of topoisomerase II poisons used in the treatment of leukemia

MRE11 facilitates the removal of human topoisomerase II complexes from genomic DNA

Topoisomerase II creates a double-strand break intermediate with topoisomerase covalently coupled to the DNA via a 5′-phosphotyrosyl bond. These intermediate complexes can become cytotoxic protein-DNA adducts and DSB repair at these lesions requires removal of topoisomerase II. To analyse removal of topoisomerase II from genomic DNA we adapted the trapped in agarose DNA immunostaining assay. Recombinant MRE11 from 2 sources removed topoisomerase IIα from genomic DNA in vitro, as did MRE11 immunoprecipitates isolated from A-TLD or K562 cells. Basal topoisomerase II complex levels were very high in A-TLD cells lacking full-length wild type MRE11, suggesting that MRE11 facilitates the processing of topoisomerase complexes that arise as part of normal cellular metabolism. In K562 cells inhibition of MRE11, PARP or replication increased topoisomerase IIα and β complex levels formed in the absence of an anti-topoisomerase II drug.

Dual effect of heat shock on DNA replication and genome integrity

The data presented here suggest that in an asynchronous cell culture, heat shock might affect DNA integrity both directly and via arrest of replication fork progression and that the phosphorylation of histone H2AX has a protective effect on the arrested replication forks in addition to its known DNA damage signaling function.

Heat shock (HS) is one of the better-studied exogenous stress factors. However, little is known about its effects on DNA integrity and the DNA replication process. In this study, we show that in G1 and G2 cells, HS induces a countable number of double-stranded breaks (DSBs) in the DNA that are marked by γH2AX. In contrast, in S-phase cells, HS does not induce DSBs but instead causes an arrest or deceleration of the progression of the replication forks in a temperature-dependent manner. This response also provoked phosphorylation of H2AX, which appeared at the sites of replication. Moreover, the phosphorylation of H2AX at or close to the replication fork rescued the fork from total collapse. Collectively our data suggest that in an asynchronous cell culture, HS might affect DNA integrity both directly and via arrest of replication fork progression and that the phosphorylation of H2AX has a protective effect on the arrested replication forks in addition to its known DNA damage signaling function.

Secondary leukemia associated with the anti-cancer agent, etoposide, a topoisomerase II inhibitor

Etoposide is an anticancer agent, which is successfully and extensively used in treatments for various types of cancers in children and adults. However, due to the increases in survival and overall cure rate of cancer patients, interest has arisen on the potential risk of this agent for therapy-related secondary leukemia. Topoisomerase II inhibitors, including etoposide and teniposide, frequently cause rearrangements involving the mixed lineage leukemia (MLL) gene on chromosome 11q23, which is associated with secondary leukemia. The prognosis is extremely poor for leukemias associated with rearrangements in the MLL gene, including etoposide-related secondary leukemias. It is of great importance to gain precise knowledge of the clinical aspects of these diseases and the mechanism underlying the leukemogenesis induced by this agent to ensure correct assessments of current and future therapy strategies. Here, I will review current knowledge regarding the clinical aspects of etoposide-related secondary leukemia, some probable mechanisms, and strategies for treating etoposide-induced leukemia.

Inhibition of non-homologous end joining repair impairs pancreatic cancer growth and enhances radiation response

Pancreatic ductal adenocarcinoma (PDAC) is amongst the deadliest of human cancers, due to its late diagnosis as well as its intense resistance to currently available therapeutics. To identify mechanisms as to why PDAC are refractory to DNA damaging cytoxic chemotherapy and radiation, we performed a global interrogation of the DNA damage response of PDAC. We find that PDAC cells generally harbor high levels of spontaneous DNA damage. Inhibition of Non-Homologous End Joining (NHEJ) repair either pharmacologically or by RNAi resulted in a further accumulation of DNA damage, inhibition of growth, and ultimately apoptosis even in the absence of exogenous DNA damaging agents. In response to radiation, PDAC cells rely on the NHEJ pathway to rapidly repair DNA double strand breaks. Mechanistically, when NHEJ is inhibited there is a compensatory increase in Homologous Recombination (HR). Despite this upregulation of HR, DNA damage persists and cells are significantly more sensitive to radiation. Together, these findings support the incorporation of NHEJ inhibition into PDAC therapeutic approaches, either alone, or in combination with DNA damaging therapies such as radiation.

Role of DNA-dependent protein kinase catalytic subunit in cancer development and treatment

DNA-dependent protein kinase catalytic subunit (DNA-PKcs), a key component of the non-homologous end-joining (NHEJ) pathway, is involved in DNA double-strand break repair, immunocompetence, genomic integrity, and epidermal growth factor receptor signaling. Clinical studies indicate that expression and activity of DNA-PKcs is correlated with cancer progression and response to treatment. Various anti-DNA-PKcs strategies have been developed and tested in preclinical studies to exploit the benefit of DNA-PKcs inhibition in sensitization of radiotherapy and in combined modality therapy with other antitumor agents. In this article, we review the association between DNA-PKcs and cancer development and discuss current approaches and mechanisms for inhibition of DNA-PKcs. The future challenges are to understand how DNA-PKcs activity is correlated with cancer susceptibility and to identify those patients who would most benefit from DNA-PKcs inhibition.

Model for MLL translocations in therapy-related leukemia involving topoisomerase IIβ-mediated DNA strand breaks and gene proximity

Topoisomerase poisons such as the epipodophyllotoxin etoposide are widely used effective cytotoxic anticancer agents. However, they are associated with the development of therapy-related acute myeloid leukemias (t-AMLs), which display characteristic balanced chromosome translocations, most often involving the mixed lineage leukemia (MLL) locus at 11q23. MLL translocation breakpoints in t-AMLs cluster in a DNase I hypersensitive region, which possesses cryptic promoter activity, implicating transcription as well as topoisomerase II activity in the translocation mechanism. We find that 2-3% of MLL alleles undergoing transcription do so in close proximity to one of its recurrent translocation partner genes, AF9 or AF4, consistent with their sharing transcription factories. We show that most etoposide-induced chromosome breaks in the MLL locus and the overall genotoxicity of etoposide are dependent on topoisomerase IIβ, but that topoisomerase IIα and -β occupancy and etoposide-induced DNA cleavage data suggest factors other than local topoisomerase II concentration determine specific clustering of MLL translocation breakpoints in t-AML. We propose a model where DNA double-strand breaks (DSBs) introduced by topoisomerase IIβ into pairs of genes undergoing transcription within a common transcription factory become stabilized by antitopoisomerase II drugs such as etoposide, providing the opportunity for illegitimate end joining and translocation.

Chronic myelogenous leukemia stem and progenitor cells demonstrate chromosomal instability related to repeated breakage-fusion-bridge cycles mediated by increased nonhomologous end joining

Chromosomal aberrations are an important consequence of genotoxic exposure and contribute to pathogenesis and progression of several malignancies. We investigated the susceptibility to chromosomal aberrations in chronic myelogenous leukemia (CML) progenitors after exposure to ionizing radiation. In normal progenitors, ionizing radiation induced both stable and unstable chromosomal lesions, but only stable aberrations persisted after multiple divisions. In contrast, radiation of chronic phase CML progenitors resulted in enhanced generation of unstable lesions that persisted after multiple divisions. CML progenitors demonstrated active cell cycle checkpoints and increased nonhomologous end joining DNA repair, suggesting that persistence of unstable aberrations was the result of continued generation of these lesions. CML progenitors demonstrated enhanced susceptibility to repeated cycles of chromosome damage, repair, and damage through a breakage-fusion-bridge mechanism. Perpetuation of breakage-fusion-bridge cycles in CML progenitors was mediated by classic nonhomologous end joining repair. These studies reveal a previously unrecognized mechanism of chromosomal instability in leukemia progenitors because of continued generation of unstable chromosomal lesions through repeated cycles of breakage and repair of such lesions.

Further characterisation of the cellular activity of the DNA-PK inhibitor, NU7441, reveals potential cross-talk with homologous recombination

PURPOSE

Inhibition of DNA repair is emerging as a new therapeutic strategy for cancer treatment. One promising target is DNA-PK, a pivotal kinase in double-strand break repair. The purpose of this study was to further characterise the activity of the DNA-PK inhibitor NU7441, giving some new insights into the biology of DNA-PK.

METHODS

We used NU7441, a potent DNA-PK inhibitor, to evaluate potential pharmacodynamic markers of DNA-PK inhibition, inhibition of DNA repair and chemo- and radio-potentiation in isogenic human cancer cells proficient (M059-Fus1) and deficient (M059 J) in DNA-PK.

RESULTS

NU7441 strongly inhibited DNA-PK in cell lines (IC(50) = 0.3 μM) but only weakly inhibited PI3 K (IC(50) = 7 μM). The only available anti-phospho-DNA-PK antibody also recognised some phosphoprotein targets of ATM. NU7441 caused doxorubicin- and IR-induced DNA DSBs (measured by γ-H2AX foci) to persist and also slightly decreased homologous recombination activity, as assessed by Rad51 foci. Chemo- and radio-potentiation were induced by NU7441 in M059-Fus-1, but not in DNA-PK-deficient M059 J cells. DNA-PK was highly expressed in a chronic lymphocytic leukaemia sample but undetectable in resting normal human lymphocytes, although it could be induced by PHA-P treatment. In K652 cells, DNA-PK expression was not related to cell cycle phase.

CONCLUSION

These data confirm NU7441 not only as a potent chemo- and radio-sensitiser clinical candidate but also as a powerful tool to study the biology of DNA-PK.

Targeting DNA damage and repair: embracing the pharmacological era for successful cancer therapy

DNA is under constant assault from genotoxic agents which creates different kinds of DNA damage. The precise replication of the genome and the continuous surveillance of its integrity are critical for survival and the avoidance of carcinogenesis. Cells have evolved an arsenal of repair pathways and cell cycle checkpoints to detect and repair DNA damage. When repair fails, typically cell cycle progression is halted and apoptosis is initiated. Here, we review the different sources and types of DNA damage including DNA replication stress and oxidative stress, the repair pathways that cells utilize to repair damaged DNA, and discuss their biological significance, especially with reference to cancer induction and cancer therapy. We also describe the main methodologies currently used for the detection of DNA damage with their strengths and limitations. We conclude with an outline as to how this information can be used to identify novel pharmacological targets for DNA repair pathways or enhancers of DNA damage to develop improved treatment strategies that will benefit cancer patients.

Sublethal doses of β-amyloid peptide abrogate DNA-dependent protein kinase activity

Accumulation of DNA damage and deficiency in DNA repair potentially contribute to the progressive neuronal loss in neurodegenerative disorders, including Alzheimer disease (AD). In multicellular eukaryotes, double strand breaks (DSBs), the most lethal form of DNA damage, are mainly repaired by the nonhomologous end joining pathway, which relies on DNA-PK complex activity. Both the presence of DSBs and a decreased end joining activity have been reported in AD brains, but the molecular player causing DNA repair dysfunction is still undetermined. β-Amyloid (Aβ), a potential proximate effector of neurotoxicity in AD, might exert cytotoxic effects by reactive oxygen species generation and oxidative stress induction, which may then cause DNA damage. Here, we show that in PC12 cells sublethal concentrations of aggregated Aβ(25-35) inhibit DNA-PK kinase activity, compromising DSB repair and sensitizing cells to nonlethal oxidative injury. The inhibition of DNA-PK activity is associated with down-regulation of the catalytic subunit DNA-PK (DNA-PKcs) protein levels, caused by oxidative stress and reversed by antioxidant treatment. Moreover, we show that sublethal doses of Aβ(1-42) oligomers enter the nucleus of PC12 cells, accumulate as insoluble oligomeric species, and reduce DNA-PK kinase activity, although in the absence of oxidative stress. Overall, these findings suggest that Aβ mediates inhibition of the DNA-PK-dependent nonhomologous end joining pathway contributing to the accumulation of DSBs that, if not efficiently repaired, may lead to the neuronal loss observed in AD.

Coronavirus infection induces DNA replication stress partly through interaction of its nonstructural protein 13 with the p125 subunit of DNA polymerase δ

Perturbation of cell cycle regulation is a characteristic feature of infection by many DNA and RNA viruses, including Coronavirus infectious bronchitis virus (IBV). IBV infection was shown to induce cell cycle arrest at both S and G(2)/M phases for the enhancement of viral replication and progeny production. However, the underlying mechanisms are not well explored. In this study we show that activation of cellular DNA damage response is one of the mechanisms exploited by Coronavirus to induce cell cycle arrest. An ATR-dependent cellular DNA damage response was shown to be activated by IBV infection. Suppression of the ATR kinase activity by chemical inhibitors and siRNA-mediated knockdown of ATR reduced the IBV-induced ATR signaling and inhibited the replication of IBV. Furthermore, yeast two-hybrid screens and subsequent biochemical and functional studies demonstrated that interaction between Coronavirus nsp13 and DNA polymerase δ induced DNA replication stress in IBV-infected cells. These findings indicate that the ATR signaling activated by IBV replication contributes to the IBV-induced S-phase arrest and is required for efficient IBV replication and progeny production.

Rad51 and BRCA2--New molecular targets for sensitizing glioma cells to alkylating anticancer drugs

First line chemotherapeutics for brain tumors (malignant gliomas) are alkylating agents such as temozolomide and nimustine. Despite growing knowledge of how these agents work, patients suffering from this malignancy still face a dismal prognosis. Alkylating agents target DNA, forming the killing lesion O⁶-alkylguanine, which is converted into DNA double-strand breaks (DSBs) that trigger apoptosis. Here we assessed whether inhibiting repair of DSBs by homologous recombination (HR) or non-homologous end joining (NHEJ) is a reasonable strategy for sensitizing glioma cells to alkylating agents. For down-regulation of HR in glioma cells, we used an interference RNA (iRNA) approach targeting Rad51 and BRCA2, and for NHEJ we employed the DNA-PK inhibitor NU7026. We also assessed whether inhibition of poly(ADP)ribosyltransferase (PARP) by olaparib would enhance the killing effect. The data show that knockdown of Rad51 or BRCA2 greatly sensitizes cells to DSBs and the induction of cell death following temozolomide and nimustine (ACNU). It did not sensitize to ionizing radiation (IR). The expression of O⁶-methylguanine-DNA methyltransferase (MGMT) abolished all these effects, indicating that O⁶-alkylguanine induced by these drugs is the primary lesion responsible for the formation of DSBs and increased sensitivity of glioma cells following knockdown of Rad51 and BRCA2. Inhibition of DNA-PK only slightly sensitized to temozolomide whereas a significant effect was observed with IR. A triple strategy including siRNA and the PARP inhibitor olaparib further improved the killing effect of temozolomide. The data provides evidence that down-regulation of Rad51 or BRCA2 is a reasonable strategy for sensitizing glioma cells to killing by O⁶-alkylating anti-cancer drugs. The data also provide proof of principle that a triple strategy involving down-regulation of HR, PARP inhibition and MGMT depletion may greatly enhance the therapeutic effect of temozolomide.

Salinomycin sensitizes cancer cells to the effects of doxorubicin and etoposide treatment by increasing DNA damage and reducing p21 protein

BACKGROUND AND PURPOSE

Salinomycin (Sal) has recently been shown to inhibit various cancer stem cells. Here, we investigated whether Sal could sensitize cancer cells to the effects of doxorubicin (DOX) or etoposide (ETO).

EXPERIMENTAL APPROACH

Using the Comet assay, immunocytochemistry and Western blot analysis, we assessed the ability of Sal to increase DNA breakage. We performed a cell proliferation assay to determine cell viability, cellular detachment, increased pre-G1 region, Annexin V staining and TUNEL assay to measure the ability of Sal to increase apoptosis.

KEY RESULTS

Sal increased DNA breakage and phosphorylated levels of p53 and H2AX. Sal also induced the formation of DNA foci with pH2AX and 53BP1. Furthermore, Sal increased the sensitivity of cancer cells to the apoptotic effects of DOX or ETO. We found that pH2AX, pBRCA1, p53BP1 and pChk1 levels were greatly increased after co-treatment of Sal with DOX or ETO. The level of anti-apoptotic p21 protein was increased by DOX or ETO but decreased by Sal, which increased proteasome activity.

CONCLUSIONS AND IMPLICATIONS

This is the first study to report that Sal increases DNA damage, and this effect plays an important role in the increased apoptosis caused by Sal. Overall, we demonstrated that the ability of Sal to sensitize cancer cells to the effects of DOX or ETO is associated with an increase in DNA damage and a decrease in anti-apoptotic protein p21 levels. These results may contribute to the development of Sal-based chemotherapy for cancer patients receiving DOX or ETO treatment.

Repair at single targeted DNA double-strand breaks in pluripotent and differentiated human cells

Differences in ex vivo cell culture conditions can drastically affect stem cell physiology. We sought to establish an assay for measuring the effects of chemical, environmental, and genetic manipulations on the precision of repair at a single DNA double-strand break (DSB) in pluripotent and somatic human cells. DSBs in mammalian cells are primarily repaired by either homologous recombination (HR) or nonhomologous end-joining (NHEJ). For the most part, previous studies of DSB repair in human cells have utilized nonspecific clastogens like ionizing radiation, which are highly nonphysiologic, or assayed repair at randomly integrated reporters. Measuring repair after random integration is potentially confounded by locus-specific effects on the efficiency and precision of repair. We show that the frequency of HR at a single DSB differs up to 20-fold between otherwise isogenic human embryonic stem cells (hESCs) based on the site of the DSB within the genome. To overcome locus-specific effects on DSB repair, we used zinc finger nucleases to efficiently target a DSB repair reporter to a safe-harbor locus in hESCs and a panel of somatic human cell lines. We demonstrate that repair at a targeted DSB is highly precise in hESCs, compared to either the somatic human cells or murine embryonic stem cells. Differentiation of hESCs harboring the targeted reporter into astrocytes reduces both the efficiency and precision of repair. Thus, the phenotype of repair at a single DSB can differ based on either the site of damage within the genome or the stage of cellular differentiation. Our approach to single DSB analysis has broad utility for defining the effects of genetic and environmental modifications on repair precision in pluripotent cells and their differentiated progeny.

Effects of the novel DNA dependent protein kinase inhibitor, IC486241, on the DNA damage response to doxorubicin and cisplatin in breast cancer cells

The purpose of this study was to determine the degree to which the novel DNA-PKcs inhibitor, IC486241 (ICC), synergizes the cytotoxicity of DNA damaging agents in 3 genetically diverse breast cancer cell lines. The sulforhodamine B (SRB) assay was employed as a primary screening method to determine the in-vitro cytotoxicity and the degree of synergy of ICC in combination with the topoisomerase II inhibitor, doxorubicin, or the DNA cross linking agent, cisplatin. Molecular mechanisms underlying drug toxicity were probed using immunostaining and flow cytometry, as well as, the alkaline comet assay to detect DNA damage. In this study, improved cytotoxicity and significant synergy were observed with both anticancer agents in the presence of nontoxic concentrations of ICC. Moreover, ICC decreased doxorubicin-induced DNA-PKcs autophosphorylation on Ser2056 and increased doxorubicin-induced DNA fragmentation. In conclusion, the novel DNA-PKcs inhibitor, ICC, synergistically sensitized 3 breast cancer cell lines to doxorubicin and cisplatin. Enhanced efficacy of doxorubicin was achieved by inhibiting non-homologous end joining resulting in increased accumulation of DNA damage.

A cell-based screen identifies ATR inhibitors with synthetic lethal properties for cancer-associated mutations

Oncogene activation has been shown to generate replication-born DNA damage, also known as replicative stress. The primary responder to replicative stress is not Ataxia-Telangiectasia Mutated (ATM) but rather the kinase ATM and Rad3-related (ATR). One limitation for the study of ATR is the lack of potent inhibitors. We here describe a cell-based screening strategy that has allowed us to identify compounds with ATR inhibitory activity in the nanomolar range. Pharmacological inhibition of ATR generates replicative stress, leading to chromosomal breakage in the presence of conditions that stall replication forks. Moreover, ATR inhibition is particularly toxic for p53-deficient cells, this toxicity being exacerbated by replicative stress-generating conditions such as the overexpression of cyclin E. Notably, one of the compounds we identified is NVP-BEZ235, a dual phosphatidylinositol-3-OH kinase (PI3K) and mTOR inhibitor that is being tested for cancer chemotherapy but that we now show is also very potent against ATM, ATR and the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs).

Genetic instability in inherited and sporadic leukemias

Genetic instability due to increased DNA damage and altered DNA repair is of central significance in the initiation and progression of inherited and sporadic human leukemias. Although very rare, some inherited DNA repair insufficiency syndromes (e.g., Fanconi anemia, Bloom's syndrome) have added substantially to our understanding of crucial mechanisms of leukemogenesis in recent years. Conversely, sporadic leukemias account for the main proportion of leukemias and here DNA damaging reactive oxygen species (ROS) play a central role. Although the exact mechanisms of increased ROS production remain largely unknown and no single pathway has been detected thus far, some oncogenic proteins (e.g., the activated tyrosine kinases BCR-ABL1 and FLT3-ITD) seem to play a key role in driving genetic instability by increased ROS generation which influences the disease course (e.g., blast crisis in chronic myeloid leukemia or relapse in FLT3-ITD positive acute myeloid leukemia). Of course other mechanisms, which promote genetic instability in leukemia also exist. A newly emerging mechanism is the genome-wide alteration of epigenetic marks (e.g., hypomethylation of histone H3K79), which promotes chromosomal instability. Taken together genetic instability plays a critical role both in inherited and sporadic leukemias and emerges as a common theme in both inherited and sporadic leukemias. Beyond its theoretical impact, the analysis of genetic instability may lead the way to the development of innovative therapy strategies.

Targeting nonhomologous end-joining through epidermal growth factor receptor inhibition: rationale and strategies for radiosensitization

DNA double-strand breaks (DSBs) are the most lethal type of DNA damage induced by ionizing radiation or chemotherapeutic drugs used to eradicate cancer cells. The ability of cancer cells to effectively repair DSBs significantly influences the outcome of therapeutic regimens. Therefore, a new and important area of clinical cancer research is the development of DNA repair inhibitors that can be used as radio- or chemosensitizers. Nonhomologous end joining (NHEJ) is the predominant pathway for the repair of radiation-induced DSBs. A series of recent reports indicates that the epidermal growth factor receptor (EGFR) or its downstream components may modulate NHEJ through direct interaction with the DNA repair enzyme, DNA-dependent protein kinase. Because EGFR is overexpressed or activated in many cancers, these findings provide a compelling rationale for combining radiotherapy with therapies that block EGFR or its downstream signaling components. In this review, we delineate how these novel connections between a cell-surface receptor (EGFR) and a predominantly nuclear event (NHEJ) provide vulnerable nodes that can be selectively targeted to improve cancer therapy.

Irinotecan and DNA-PKcs inhibitors synergize in killing of colon cancer cells

This study sought to measure the degree of synergy induced by specific small molecule inhibitors of DNA-PK [NU7026 and IC486241 (ICC)], a major component of the non-homologous end-joining (NHEJ) pathway, with SN38 or oxaliplatin. Synergy between the DNA damaging drugs and the DNA-PK inhibitors was assessed using the sulforhodamine-B assay (SRB). Effects of drug combinations on cell cycle and DNA-PK activity were determined using flow cytometry and western blot analysis. DNA damage was assessed via comet assay and quantification of γH2AX. The role of homologous recombination repair (HRR) was determined by nuclear Rad51 protein levels and a GFP reporter recombination assay. Significant reductions in the IC(50) values of SN38 were observed at 5 and 10 μM of DNA-PK inhibitors. Moreover, at 1-2 μM (attainable concentrations with ICC in mice) these DNA-PKcs inhibitors demonstrated synergistic reductions in the IC(50) of SN38. Flow cytometric data indicated that SN38 and SN38 in combination with DNA-PKcs inhibitors showed dramatic G2/M arrest at 24 h. Furthermore, reduced phosphorylation of DNA-PKcs and increased DNA damage were observed at this time point with SN38 in combination with DNA-PKcs inhibitors as compared to cells treated with SN38 alone. SN38 alone and in the presence of ICC increased nuclear Rad51 protein levels. Furthermore, inhibition of DNA-PKcs increased HRR suggesting that NHEJ is a negative regulator of HRR. These data indicate that small molecule inhibitors of DNA-PKcs dramatically enhance the efficacy of SN38 in colon cancer cell lines.

Nuclear phospho-Akt increase predicts synergy of PI3K inhibition and doxorubicin in breast and ovarian cancer

The phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway is frequently disrupted in cancer and implicated in multiple aspects of tumor growth and survival. In addition, increased activity of this pathway in cancer is associated with resistance to chemotherapeutic agents. Therefore, it has been hypothesized that PI3K inhibitors could help to overcome resistance to chemotherapies. We used preclinical cancer models to determine the effects of combining the DNA-damaging drug doxorubicin with GDC-0941, a class I PI3K inhibitor that is currently being tested in early-stage clinical trials. We found that PI3K inhibition significantly increased apoptosis and enhanced the antitumor effects of doxorubicin in a defined set of breast and ovarian cancer models. Doxorubicin treatment caused an increase in the amount of nuclear phospho-Akt(Ser473) in cancer cells that rely on the PI3K pathway for survival. This increased phospho-Akt(Ser473) response to doxorubicin correlates with the strength of GDC-0941's effect to augment doxorubicin action. These studies predict that clinical use of combination therapies with GDC-0941 in addition to DNA-damaging agents will be effective in tumors that rely on the PI3K pathway for survival.

Mitoxantrone in combination with an inhibitor of DNA-dependent protein kinase: a potential therapy for high risk B-cell chronic lymphocytic leukaemia

Defects in the DNA damage response pathway [e.g. del(17p)] are associated with drug-resistant B-cell chronic lymphocytic leukaemia (CLL). We previously demonstrated that over-expression of DNA-dependent protein kinase (DNA-PK) correlates with chemo-resistance and that inhibition of DNA-PK sensitizes CLL cells to chemotherapeutics. Here, we investigated expression of DNA-PK and other proteins that impact on drug resistance, and evaluated the effects of a DNA-PK inhibitor (NU7441) on mitoxantrone-induced cytotoxicity in CLL cells. NU7441 sensitized cells from 42/49 CLL samples to mitoxantrone, with sensitization ranging from 2- to 200-fold Co-culture of CLL cells in conditioned stromal medium increased chemoresistance but did not reduce sensitization by NU7441. Mitoxantrone treatment induced γH2AX foci and NU7441 increased their longevity (24 h). NU7441 prevented mitoxantrone-induced autophosphorylation of the DNA-PK catalytic subunit (DNA-PKcs) at Ser 2056, confirming that DNA-PK participates in repair of mitoxantrone-induced DNA damage. del(17p) cases were more resistant to mitoxantrone than del(13q) cases, but were resensitized (7-16 fold) by co-incubation with NU7441. Expression of DNA-PKcs, Ku80, P-glycoprotein and topoisomerase IIβ were significantly higher in del(17p) cases. PRKDC mRNA levels correlated with DNA-PKcs protein expression, which predicted shorter survival. These data confirm the potential of DNA-PK as a therapeutic target in poor prognosis CLL.

Radiosensitization and growth inhibition of cancer cells mediated by an scFv antibody gene against DNA-PKcs in vitro and in vivo

BACKGROUND

Overexpression of DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is commonly occurred in cancers and causes radioresistance and poor prognosis. In present study, the single-chain variable antibody fragments (scFv) targeting DNA-PKcs was developed for the application of radiosensitization in vitro and in vivo. A humanized semisynthetic scFv library and the phage-display antibodies technology were employed to screen DNA-PKcs scFv antibody.

METHODS

DNA-PKcs epitopes were predicted and cloned. A humanized semisynthetic scFv library and the phage-display antibodies technology were employed to screen DNA-PKcs scFv antibody. DNA damage repair was analyzed by comet assay and immunofluorescence detection of gammaH2AX foci. The radiosensitization in vivo was determined on Balb/c athymic mice transplanted tumours of HeLa cells.

RESULTS

Four epitopes of DNA-PKcs have been predicted and expressed as the antigens, and a specific human anti-DNA-PKcs scFv antibody gene, anti-DPK3-scFv, was obtained by screening the phage antibody library using the DNA-PKcs peptide DPK3. The specificity of anti-DPK3-scFv was verified, in vitro. Transfection of HeLa cells with the anti-DPK3-scFv gene resulted in an increased sensitivity to IR, decreased repair capability of DNA double-strand breaks (DSB) detected by comet assay and immunofluorescence detection of gammaH2AX foci. Moreover, the kinase activity of DNA-PKcs was inhibited by anti-DPK3-scFv, which was displayed by the decreased phosphorylation levels of its target Akt/S473 and the autophosphorylation of DNA-PKcs on S2056 induced by radiation. Measurement of the growth and apoptosis rates showed that anti-DPK3-scFv enhanced the sensitivity of tumours transplanted in Balb/c athymic mice to radiation therapy.

CONCLUSION

The antiproliferation and radiosensitizing effects of anti-DPK3-scFv via targeting DNA-PKcs make it very appealing for the development as a novel biological radiosensitizer for cancer therapeutic potential.

Thymoquinone induces telomere shortening, DNA damage and apoptosis in human glioblastoma cells

Background

A major concern of cancer chemotherapy is the side effects caused by the non-specific targeting of both normal and cancerous cells by therapeutic drugs. Much emphasis has been placed on discovering new compounds that target tumour cells more efficiently and selectively with minimal toxic effects on normal cells.

Methodology/Principal Findings

The cytotoxic effect of thymoquinone, a component derived from the plant Nigella sativa, was tested on human glioblastoma and normal cells. Our findings demonstrated that glioblastoma cells were more sensitive to thymoquinone-induced antiproliferative effects. Thymoquinone induced DNA damage, cell cycle arrest and apoptosis in the glioblastoma cells. It was also observed that thymoquinone facilitated telomere attrition by inhibiting the activity of telomerase. In addition to these, we investigated the role of DNA-PKcs on thymoquinone mediated changes in telomere length. Telomeres in glioblastoma cells with DNA-PKcs were more sensitive to thymoquinone mediated effects as compared to those cells deficient in DNA-PKcs.

Conclusions/Significance

Our results indicate that thymoquinone induces DNA damage, telomere attrition by inhibiting telomerase and cell death in glioblastoma cells. Telomere shortening was found to be dependent on the status of DNA-PKcs. Collectively, these data suggest that thymoquinone could be useful as a potential chemotherapeutic agent in the management for brain tumours.

Prospects for the Use of ATR Inhibitors to Treat Cancer

ATR is an apical kinase in one of the DNA-damage induced checkpoint pathways. Despite the development of inhibitors of kinases structurally related to ATR, as well as inhibitors of the ATR substrate Chk1, no ATR inhibitors have yet been developed. Here we review the effects of ATR downregulation in cancer cells and discuss the potential for development of ATR inhibitors for clinical use.

Cohesin promotes the repair of ionizing radiation-induced DNA double-strand breaks in replicated chromatin

The cohesin protein complex holds sister chromatids together after synthesis until mitosis. It also contributes to post-replicative DNA repair in yeast and higher eukaryotes and accumulates at sites of laser-induced damage in human cells. Our goal was to determine whether the cohesin subunits SMC1 and Rad21 contribute to DNA double-strand break repair in X-irradiated human cells in the G2 phase of the cell cycle. RNA interference-mediated depletion of SMC1 sensitized HeLa cells to X-rays. Repair of radiation-induced DNA double-strand breaks, measured by gammaH2AX/53BP1 foci analysis, was slower in SMC1- or Rad21-depleted cells than in controls in G2 but not in G1. Inhibition of the DNA damage kinase DNA-PK, but not ATM, further inhibited foci loss in cohesin-depleted cells in G2. SMC1 depletion had no effect on DNA single-strand break repair in either G1 or late S/G2. Rad21 and SMC1 were recruited to sites of X-ray-induced DNA damage in G2-phase cells, but not in G1, and only when DNA damage was concentrated in subnuclear stripes, generated by partially shielded ultrasoft X-rays. Our results suggest that the cohesin complex contributes to cell survival by promoting the repair of radiation-induced DNA double-strand breaks in G2-phase cells in an ATM-dependent pathway.

New molecular targets in radiotherapy: DNA damage signalling and repair in targeted and non-targeted cells

Ionising radiation plays a key role in therapy due to its ability to directly induce DNA damage, in particular DNA double-strand breaks leading to cell death. Cells have multiple repair pathways which attempt to maintain genomic stability. DNA repair proteins have become key targets for therapy, using small molecule inhibitors, in combination with radiation and or chemotherapeutic agents as a means of enhancing cell killing. Significant advances in our understanding of the response of cells to radiation exposures has come from the observation of non-targeted effects where cells respond via mechanisms other than those which are a direct consequence of energy-dependent DNA damage. Typical of these is bystander signalling where cells respond to the fact that their neighbours have been irradiated. Bystander cells show a DNA damage response which is distinct from directly irradiated cells. In bystander cells, ATM- and Rad3-related (ATR) protein kinase-dependent signalling in response to stalled replication forks is an early event in the DNA damage response. The ATM protein kinase is activated downstream of ATR in bystander cells. This offers the potential for differential approaches for the modulation of bystander and direct effects with repair inhibitors which may impact on the response of tumours and on the protection of normal tissues during radiotherapy.

Targeting DNA repair pathways: a novel approach to reduce cancer therapeutic resistance

Increased chemo-resistance and radio-resistance of cancer cells is a major obstacle in the treatment and management of malignant cancers. An important mechanism that underlies the development of such therapeutic resistance is that cancer cells recognize DNA lesions induced by DNA-damaging agents and by ionizing radiation, and repair these lesions by activating various DNA repair pathways. Therefore, Use of pharmacological agents that can inhibit certain DNA repair pathways in cancer cells has the potential for enhancing the targeted cytotoxicity of anticancer treatments and reversing the associated therapeutic resistance associated with DNA repair; such agents, offering a promising opportunity to achieve better therapeutic efficacy. Here we review the major DNA repair pathways and discuss recent advances in the development of novel inhibitors of DNA repair pathways; many of these agents are under preclinical/clinical investigation.

DNA-dependent protein kinase (DNA-PK)-dependent cisplatin-induced loss of nucleolar facilitator of chromatin transcription (FACT) and regulation of cisplatin sensitivity by DNA-PK and FACT

Both the Ku subunit of the DNA-dependent protein kinase (DNA-PK) and the facilitator of chromatin transcription (FACT) complex reportedly bind cisplatin-DNA adducts. For this study, we developed an immunocytochemical assay based on detergent extraction allowing unveiling nucleolar subpopulations of proteins present in both the nucleoplasm and the nucleolus. Immunofluorescence analysis in various human cancer cell lines and immunoblotting of isolated nucleoli show that DNA-PK catalytic subunit (DNA-PKcs), Ku86, the Werner syndrome protein (WRN), and the structure-specific recognition protein 1 (SSRP1) subunit of FACT colocalize in the nucleolus and exit the nucleolus after cisplatin treatment. Nucleolar localization of Ku is also lost after gamma or UV irradiation and exposure to DNA-damaging drugs, such as actinomycin D, mitomycin C, hydroxyurea, and doxorubicin. Ku86 and WRN leave the nucleolus after exposure to low (>1 microg/mL) doses of cisplatin. In contrast, the SSRP1 association with the nucleolus was disrupted only by high (50-100 microg/mL) doses of cisplatin. Both cisplatin-induced loss of nucleolar SSRP1 and DNA-PK activation are suppressed by pretreatment of the cells with wortmannin or the DNA-PK inhibitor NU7026 but not by the phosphatidylinositol 3-kinase inhibitor LY294002. In the same conditions, kinase inhibitors did not alter the exit of DNA-PKcs and WRN, suggesting that different mechanisms regulate the exit of DNA-PK/WRN and FACT from the nucleolus. Furthermore, RNA silencing of DNA-PKcs blocked the cisplatin-induced exit of nucleolar SSRP1. Finally, silencing of DNA-PKcs or SSRP1 by short hairpin RNA significantly increased the sensitivity of cancer cells to cisplatin.

Targeting DNA topoisomerase II in cancer chemotherapy

Recent molecular studies have expanded the biological contexts in which topoisomerase II (TOP2) has crucial functions, including DNA replication, transcription and chromosome segregation. Although the biological functions of TOP2 are important for ensuring genomic integrity, the ability to interfere with TOP2 and generate enzyme-mediated DNA damage is an effective strategy for cancer chemotherapy. The molecular tools that have allowed an understanding of the biological functions of TOP2 are also being applied to understanding the details of drug action. These studies promise refined targeting of TOP2 as an effective anticancer strategy.

KU70/80, DNA-PKcs, and Artemis are essential for the rapid induction of apoptosis after massive DSB formation

KU70(-/-) and DNA-PKcs(-/-/-)chicken DT40 cells are reportedly highly sensitive to the DNA topoisomerase II inhibitor etoposide. Here we report that KU70 and DNA-PKcs unexpectedly function together during the induction of apoptosis after exposure to high levels of etoposide. In the presence of 100 microM etoposide, apoptosis was induced within 1 h in wild type DT40 cells but not in KU70(-/-) and DNA-PKcs(-/-/-) cells. In addition, the DNA-PK inhibitors NU7026 and wortmannin, as well as the caspase inhibitor Z-VAD-FMK, inhibited etoposide-induced apoptosis in wild type cells. Although Artemis(-/-) cells also showed defects in the etoposide-induced apoptosis, the other mutants defective in nonhomologous end-joining (NHEJ), LIG4(-/-), XRCC4(-), and XLF(-/-) cells were capable to induce apoptosis. When cells were treated with high doses of etoposide, the chromatin binding of DNA-PKcs was impaired by deletion of KU70 but not of Artemis, suggesting that KU70 acts upstream of DNA-PKcs and Artemis acts downstream of DNA-PKcs in the apoptotic pathway like the NHEJ pathway. These results suggest that the proteins involved in the early stage of NHEJ pathway including Artemis but not the downstream factors decide the cell fate by selecting apoptosis or DNA repair according to the degree of DNA damage.

DNA repair proteins as molecular targets for cancer therapeutics

Cancer therapeutics include an ever-increasing array of tools at the disposal of clinicians in their treatment of this disease. However, cancer is a tough opponent in this battle and current treatments which typically include radiotherapy, chemotherapy and surgery are not often enough to rid the patient of his or her cancer. Cancer cells can become resistant to the treatments directed at them and overcoming this drug resistance is an important research focus. Additionally, increasing discussion and research is centering on targeted and individualized therapy. While a number of approaches have undergone intensive and close scrutiny as potential approaches to treat and kill cancer (signaling pathways, multidrug resistance, cell cycle checkpoints, anti-angiogenesis, etc.), much less work has focused on blocking the ability of a cancer cell to recognize and repair the damaged DNA which primarily results from the front line cancer treatments; chemotherapy and radiation. More recent studies on a number of DNA repair targets have produced proof-of-concept results showing that selective targeting of these DNA repair enzymes has the potential to enhance and augment the currently used chemotherapeutic agents and radiation as well as overcoming drug resistance. Some of the targets identified result in the development of effective single-agent anti-tumor molecules. While it is inherently convoluted to think that inhibiting DNA repair processes would be a likely approach to kill cancer cells, careful identification of specific DNA repair proteins is increasingly appearing to be a viable approach in the cancer therapeutic cache.

DNA-ligase IV and DNA-protein kinase play a critical role in deficient caspases activation in apoptosis-resistant cancer cells by using doxorubicin

Resistance toward cytotoxic drugs is one of the primary causes for therapeutic failure in cancer therapy. DNA repair mechanisms as well as deficient caspases activation play a critical role in apoptosis resistance of tumor cells toward anticancer drug treatment. Here, we discovered that deficient caspases activation in apoptosis-resistant cancer cells depends on DNA-ligase IV and DNA-protein kinase (DNA-PK), playing crucial roles in the nonhomologous end joining (NHEJ) pathway, which is the predominant pathway for DNA double-strand break repair (DNA-DSB-repair) in mammalian cells. DNA-PK(+/+) as well as DNA-ligase IV (+/+) cancer cells were apoptosis resistant and deficient in activation of caspase-3, caspase-9, and caspase-8 and in cleavage of poly(ADP-ribose) polymerase after doxorubicin treatment. Inhibition of NHEJ by knocking out DNA-PK or DNA-ligase IV restored caspases activation and apoptosis sensitivity after doxorubicin treatment. In addition, inhibition of caspases activation prevented doxorubicin-induced apoptosis but could not prevent doxorubicin-induced DNA damage, indicating that induction of DNA damage is independent of caspases activation. However, caspases activation depends on induction of DNA damage left unrepaired by NHEJ-DNA-DSB-repair. We conclude that DNA damage left unrepaired by DNA-ligase IV or DNA-PK might be the initiator for caspases activation by doxorubicin in cancer cells. Failure in caspases activation using doxorubicin depends on loss of DNA damage and is due to higher rates of NHEJ-DNA-DBS-repair.

Ataxia telangiectasia mutated and p21CIP1 modulate cell survival of drug-induced senescent tumor cells: implications for chemotherapy

PURPOSE

Premature or stress-induced senescence is a major cellular response to chemotherapy in solid tumors and contributes to successful treatment. However, senescent tumor cells are resistant to apoptosis and may also reenter the cell cycle. We set out to find a means to specifically induce senescent tumor cells to undergo cell death and not to reenter the cell cycle that may have general application in cancer therapy.

EXPERIMENTAL DESIGN

We investigated the mechanisms regulating cell survival in drug-induced senescent tumor cells. Using immunofluorescence and flow cytometry-based techniques, we established the status of the ataxia telangiectasia mutated (ATM) signaling pathway in these cells. We assayed the requirement of ATM signaling and p21(CIP1) expression for survival in premature senescent tumor cells using pharmacologic inhibitors and antisense oligonucleotides.

RESULTS

The ATM/ATR (ATM- and Rad3-related) signaling pathway was found to be constitutively active in drug-induced senescent tumor cells. We found that blocking ATM/ATR signaling with pharmacologic inhibitors, including the novel ATM inhibitors KU55933 and CGK733, induced senescent breast, lung, and colon carcinoma cells to undergo cell death. We show that the mechanism of action of this effect is directly via p21(CIP1), which acts downstream of ATM. This is in contrast to the effects of ATM inhibitors on normal, untransformed senescent cells.

CONCLUSIONS

Blocking ATM and/or p21(CIP1) following initial treatment with a low dose of senescence-inducing chemotherapy is a potentially less toxic and highly specific treatment for carcinomas.

Endosomal translocation of CpG-oligodeoxynucleotides inhibits DNA-PKcs-dependent IL-10 production in macrophages

Synthetic oligodeoxynucleotides containing unmethylated CpG motifs (CpG-ODNs) function as powerful immune adjuvants by activating macrophages, dendritic cells, and B cells. However, the molecular recognition mechanism that initiates signaling in response to CpG-ODN has not fully been identified. We show in this study that peritoneal macrophages from SCID mice having mutations in the catalytic subunit of DNA-protein kinase (DNA-PKcs) were almost completely defective in the production of IL-10 and in ERK activation when treated with CpG-ODN. In contrast, IL-12 p70 production significantly increased. Furthermore, small interfering RNA (siRNA)-mediated knockdown of DNA-PKcs expression in the mouse monocyte/macrophage cell line RAW264.7 led to reduced IL-10 production and ERK activation by CpG-ODN. IL-10 and IL-12 p70 production, but not ERK activation, are blocked by chloroquine, an inhibitor of endosomal acidification. Endosomal translocation of CpG-ODN in a complex with cationic liposomes consisting of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) (CpG-DOTAP-liposomes) decreased IL-10 production and ERK activation, whereas the endosomal escape of CpG-ODN in a complex with cationic liposomes consisting of DOTAP and dioleyl-phosphatidylethanolamine (DOPE) (CpG-DOTAP/DOPE-liposomes) increased. In contrast, IL-12 p70 production was increased by CpG-DOTAP-liposomes and decreased by CpG-DOTAP/DOPE-liposomes. IL-10 production induced by CpG-DOTAP/DOPE-liposomes was not observed in macrophages from SCID mice. Thus, our findings suggest that DNA-PKcs in the cytoplasm play an important role in CpG-ODN-induced production of IL-10 in macrophages. In addition, DNA-PKcs-mediated production of IL-10 and IL-12 p70 can be regulated by manipulating the intracellular trafficking of CpG-ODN in macrophages.

Bcl2 negatively regulates DNA double-strand-break repair through a nonhomologous end-joining pathway

Bcl2 can enhance susceptibility to carcinogenesis, but the mechanism(s) remains fragmentary. Here we discovered that Bcl2 suppresses DNA double-strand-break (DSB) repair and V(D)J recombination by downregulating Ku DNA binding activity, which is associated with increased genetic instability. Exposure of cells to ionizing radiation enhances Bcl2 expression in the nucleus, which interacts with both Ku70 and Ku86 via its BH1 and BH4 domains. Removal of the BH1 or BH4 domain abrogates the inhibitory effect of Bcl2 on Ku DNA binding, DNA-PK, and DNA end-joining activities, which results in the failure of Bcl2 to block DSB repair as well as V(D)J recombination. Intriguingly, Bcl2 directly disrupts the Ku/DNA-PKcs complex in vivo and in vitro. Thus, Bcl2 suppression of the general DSB repair and V(D)J recombination may occur in a mechanism by inhibiting the nonhomologous end-joining pathway, which may lead to an accumulation of DNA damage and genetic instability.

DNA protein kinase-dependent G2 checkpoint revealed following knockdown of ataxia-telangiectasia mutated in human mammary epithelial cells

Members of the phosphatidylinositol 3-kinase-related kinase family, in particular the ataxia-telangiectasia mutated (ATM) kinase and the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs), regulate cellular responses to DNA double-strand breaks. Increased sensitivity to ionizing radiation (IR) in DNA-PKcs- or ATM-deficient cells emphasizes their important roles in maintaining genome stability. Furthermore, combined knockout of both kinases is synthetically lethal, suggesting functional complementarity. In the current study, using human mammary epithelial cells with ATM levels stably knocked down by >90%, we observed an IR-induced G(2) checkpoint that was only slightly attenuated. In marked contrast, this G(2) checkpoint was significantly attenuated with either DNA-PK inhibitor treatment or RNA interference knockdown of DNA-PKcs, the catalytic subunit of DNA-PK, indicating that DNA-PK contributes to the G(2) checkpoint in these cells. Furthermore, in agreement with the checkpoint attenuation, DNA-PK inhibition in ATM-knockdown cells resulted in reduced signaling of the checkpoint kinase CHK1 as evidenced by reduced CHK1 phosphorylation. Taken together, these results show a DNA-PK-dependent component to the IR-induced G(2) checkpoint, in addition to the well-defined ATM-dependent component. This may have important implications for chemotherapeutic strategies for breast cancers.

Targeting DNA repair as a promising approach in cancer therapy

An increased DNA-repair activity in tumour cells has been associated with resistance to treatment to DNA-directed drugs, while defects in DNA repair pathways result in hypersensitivity to these agents. In the past years the unravelling of the molecular basis of these DNA pathways, with a better understanding of the DNA damage caused by different anticancer agents, has provided the rationale for the use of some DNA repair inhibitors to optimise the therapeutic use of DNA-damaging agents currently used in the treatment of tumours. In addition, the possibility to specifically target the differences in DNA repair capacity between normal and tumour cells has recently emerged as an exciting possibility. The present review will mainly cover those approaches that are currently under clinical investigation.

Induction of apoptosis and autophagic cell death by the vanillin derivative 6-bromine-5-hydroxy-4-methoxybenzaldehyde is accompanied by the cleavage of DNA-PKcs and rapid destruction of c-Myc oncoprotein in HepG2 cells

Autophagy is a regulated lysosomal pathway involving the bulk degradation of cytoplasmic contents, and is an emerging attractive therapeutic approach for treating cancers. In the present study, we demonstrates that bromovanin (6-bromine-5-hydroxy-4-methoxybenzaldehyde), a vanillin derivative, exhibits a potent antiproliferative effect on a broad spectrum of cancer cell lines, but it induces apoptosis with a large variation in extent on different cancer cell lines. Ultrastructural observation in transmission electron microscopy reveals that autophagy is another type of cell death induced by bromovanin in HepG2 cells. Treatment with bromovanin significantly increases cellular ROS level as well as elicits DNA double-strand breaks as indicated by comet assay and the increased phosphorylated H2AX. Cleavage and inactivation of DNA-PKcs induced by bromovanin is found to occur concurrently with a rapid destruction of c-Myc oncoprotein. These multiple effects of bromovanin, especially the induction of both apoptosis and autophagy, make it very appealing for the development as a novel anticancer drug.

Therapeutic potential of drugs to modulate DNA repair in cancer

Most established cancer therapy regimes involve DNA-damaging chemotherapy or radiotherapy. The DNA repair capacity of the tumour, therefore, represents a mechanism of therapeutic resistance. Drugs to inhibit DNA repair pathways have been developed and they demonstrate good chemosensitisation and radiosensitisation activity in preclinical models. Two classes of DNA repair inhibitors have entered clinical trial and show promising activity. Genetic instability in tumours may be at least partially due to defects in DNA repair pathways; such defects may underlie the inherent sensitivity of some tumours to certain classes of anticancer agent. DNA repair defects may also make the tumour dependent on complimentary or back-up pathways; laboratory evidence shows that targeting these complimentary pathways results in tumour-selective therapy.

Long-term XPC silencing reduces DNA double-strand break repair

To study the relationships between different DNA repair pathways, we established a set of clones in which one specific DNA repair gene was silenced using long-term RNA interference in HeLa cell line. We focus here on genes involved in either nucleotide excision repair (XPA and XPC) or nonhomologous end joining (NHEJ; DNA-PKcs and XRCC4). As expected, XPA(KD) (knock down) and XPC(KD) cells were highly sensitive to UVC. DNA-PKcs(KD) and XRCC4(KD) cells presented an increased sensitivity to various inducers of double-strand breaks (DSBs) and a 70% to 80% reduction of in vitro NHEJ activity. Long-term silencing of XPC gene expression led to an increased sensitivity to etoposide, a topoisomerase II inhibitor that creates DSBs through the progression of DNA replication forks. XPC(KD) cells also showed intolerance toward acute gamma-ray irradiation. We showed that XPC(KD) cells exhibited an altered spectrum of NHEJ products with decreased levels of intramolecular joined products. Moreover, in both XPC(KD) and DNA-PKcs(KD) cells, XRCC4 and ligase IV proteins were mobilized on damaged nuclear structures at lower doses of DSB inducer. In XPC-proficient cells, XPC protein was released from nuclear structures after induction of DSBs. By contrast, silencing of XPA gene expression did not have any effect on sensitivity to DSB or NHEJ. Our results suggest that XPC deficiency, certainly in combination with other genetic defects, may contribute to impair DSB repair.

Chlorambucil cytotoxicity in malignant B lymphocytes is synergistically increased by 2-(morpholin-4-yl)-benzo[h]chomen-4-one (NU7026)-mediated inhibition of DNA double-strand break repair via inhibition of DNA-dependent protein kinase

Chlorambucil (CLB) treatment is used in chronic lymphocytic leukemia (CLL) but resistance to CLB develops in association with accelerated repair of CLB-induced DNA damage. Phosphorylated histone H2AX (gammaH2AX) is located at DNA double-strand break (DSB) sites; furthermore, it recruits and retains damage-responsive proteins. This damage can be repaired by nonhomologous DNA end-joining (NHEJ) and/or homologous recombinational repair (HR) pathways. A key component of NHEJ is the DNA-dependent protein kinase (DNA-PK) complex. Increased DNA-PK activity is associated with resistance to CLB in CLL. We used the specific DNA-PK inhibitor 2-(morpholin-4-yl)-benzo[h]chomen-4-one (NU7026) to sensitize CLL cells to chlorambucil. Our results indicate that in a CLL cell line (I83) and in primary CLL-lymphocytes, chlorambucil plus NU7026 has synergistic cytotoxic activity at nontoxic doses of NU7026. CLB treatment results in G(2)/M phase arrest, and NU7026 increases this CLB-induced G(2)/M arrest. Moreover, a kinetic time course demonstrates that CLB-induced DNA-PK activity was inhibited by NU7026, providing direct evidence of the ability of NU7026 to inhibit DNA-PK function. DSBs, visualized as gammaH2AX, were enhanced 24 to 48 h after CLB and further increased by CLB plus NU7026, suggesting that the synergy of the combination is mediated by NU7026 inhibition of DNA-PK with subsequent inhibition of DSB repair.

t(8;21) acute myeloid leukaemia cells are dependent on vascular endothelial growth factor (VEGF)/VEGF receptor type2 pathway and phosphorylation of Akt

Several anti-angiogenic drugs have recently been clinically tested for haematological malignancies. To improve the efficacy of molecular target therapy against angiogenic molecules in acute myeloid leukaemia (AML), we examined the dependency of AML cells on the vascular endothelial growth factor (VEGF)/VEGF receptor type2 (VEGFR2) system by using VEGFR2 kinase inhibitor. Nineteen patient AML samples were cultured with or without VEGFR2 kinase inhibitor. All four t(8;21) viable AML cells showed significant reductions when treated with VEGFR2 kinase inhibitor, although VEGFR2 kinase inhibitor did not affect the cell proliferation of five t(15;17) AML samples. Other AML cases showed variable responses. VEGFR2 kinase inhibitor greatly suppressed the growth of Kasumi-1, a t(8;21) cell line in a dose-dependent manner through induction of apoptosis, but did not show any significant influence on NB4, a t(15;17) cell line. In addition, VEGFR2 kinase inhibitor potentiated the growth inhibitory effect of cytarabine in Kasumi-1. Finally, it was shown that the Akt phosphorylation was augmented by VEGF(165) in Kasumi-1, which was abrogated by VEGFR2 kinase inhibitor. NB4 showed undetectable Akt phosphorylation even with VEGF(165). These data demonstrated that t(8;21) AML cells are dependent on VEGF through VEGFR2, resulting in the phosphorylation of Akt.

New tricks for old drugs: the anticarcinogenic potential of DNA repair inhibitors

Defective or abortive repair of DNA lesions has been associated with carcinogenesis. Therefore it is imperative for a cell to accurately repair its DNA after damage if it is to return to a normal cellular phenotype. In certain circumstances, if DNA damage cannot be repaired completely and with high fidelity, it is more advantageous for an organism to have some of its more severely damaged cells die rather than survive as neoplastic transformants. A number of DNA repair inhibitors have the potential to act as anticarcinogenic compounds. These drugs are capable of modulating DNA repair, thus promoting cell death rather than repair of potentially carcinogenic DNA damage mediated by error-prone DNA repair processes. In theory, exposure to a DNA repair inhibitor during, or immediately after, carcinogenic exposure should decrease or prevent tumorigenesis. However, the ability of DNA repair inhibitors to prevent cancer development is difficult to interpret depending upon the system used and the type of genotoxic stress. Inhibitors may act on multiple aspects of DNA repair as well as the cellular signaling pathways activated in response to the initial damage. In this review, we summarize basic DNA repair mechanisms and explore the effects of a number of DNA repair inhibitors that not only potentiate DNA-damaging agents but also decrease carcinogenicity. In particular, we focus on a novel anti-tumor agent, beta-lapachone, and its potential to block transformation by modulating poly(ADP-ribose) polymerase-1.

Preclinical evaluation of a potent novel DNA-dependent protein kinase inhibitor NU7441

DNA double-strand breaks (DSB) are the most cytotoxic lesions induced by ionizing radiation and topoisomerase II poisons, such as etoposide and doxorubicin. A major pathway for the repair of DSB is nonhomologous end joining, which requires DNA-dependent protein kinase (DNA-PK) activity. We investigated the therapeutic use of a potent, specific DNA-PK inhibitor (NU7441) in models of human cancer. We measured chemosensitization by NU7441 of topoisomerase II poisons and radiosensitization in cells deficient and proficient in DNA-PK(CS) (V3 and V3-YAC) and p53 wild type (LoVo) and p53 mutant (SW620) human colon cancer cell lines by clonogenic survival assay. Effects of NU7441 on DSB repair and cell cycle arrest were measured by gammaH2AX foci and flow cytometry. Tissue distribution of NU7441 and potentiation of etoposide activity were determined in mice bearing SW620 tumors. NU7441 increased the cytotoxicity of ionizing radiation and etoposide in SW620, LoVo, and V3-YAC cells but not in V3 cells, confirming that potentiation was due to DNA-PK inhibition. NU7441 substantially retarded the repair of ionizing radiation-induced and etoposide-induced DSB. NU7441 appreciably increased G(2)-M accumulation induced by ionizing radiation, etoposide, and doxorubicin in both SW620 and LoVo cells. In mice bearing SW620 xenografts, NU7441 concentrations in the tumor necessary for chemopotentiation in vitro were maintained for at least 4 hours at nontoxic doses. NU7441 increased etoposide-induced tumor growth delay 2-fold without exacerbating etoposide toxicity to unacceptable levels. In conclusion, NU7441 shows sufficient proof of principle through in vitro and in vivo chemosensitization and radiosensitization to justify further development of DNA-PK inhibitors for clinical use.

DNA end joining activity is reduced in Alzheimer's disease

Evidence indicates that oxidative stress-induced damage to DNA, protein, and other cellular components contributes to the progression of Alzheimer's disease (AD). Several studies indicate that postmitotic neurons have a reduced capacity for some types of DNA repair, which is further compromised by aging. Thus in AD, the cellular response to increased oxidative DNA damage may be inadequate to protect the genome. Mammalian cells use several mechanisms to repair DNA damage generated during normal oxidative metabolism or by genotoxic insults. The predominant mechanism to repair double strand breaks is non-homologous end joining (NHEJ) which utilizes the DNA-dependent protein kinase (DNA-PK) complex. A cell-free DNA end joining assay was employed to determine if NHEJ was reduced in nuclear cortical extracts from brains of AD versus normal subjects. This report demonstrates that end joining activity and protein levels of DNA-PK catalytic subunit are significantly lower in AD brains compared to normal controls. The amount of end joining activity correlates with the expression of DNA-PK and is dependent on DNA-PK catalytic activity. This indicates that repair of DNA double-strand breaks by the DNA-PK-dependent NHEJ pathway may be deficient in AD.