Induction of biphasic DNA double strand breaks and activation of multiple repair protein complexes by DNA topoisomerase I drug 7-ethyl-10-hydroxy-camptothecin

Assay conditions for estimating differences in base excision repair activity with Fpg-modified comet assay

DNA repair is an essential agent in cancer development, progression, prognosis, and response to therapy. We have adapted a cellular repair assay based on the formamidopyrimidine DNA glycosylase (Fpg)-modified comet assay to assess DNA repair kinetics. The removal of oxidized nucleobases over time (0-480 min) was analyzed in peripheral blood mononuclear cells (PBMCs) and 8 cell lines. DNA damage was induced by exposure to either Ro19-8022 plus visible light or potassium bromate (KBrO3). The initial amount of damage induced by Ro 19-8022 plus light varied between cell lines, and this was apparently associated with the rate of repair. However, the amount of DNA damage induced by KBrO3 varied less between cell types, so we used this agent to study the kinetics of DNA repair. We found an early phase of ca. 60 min with fast removal of Fpg-sensitive sites, followed by slower removal over the following 7 h. In conclusion, adjusting the initial damage at T0 to an equal level can be achieved by the use of KBrO3, which allows for accurate analysis of subsequent cellular DNA repair kinetics in the first hour after exposure.

Camptothecin loaded casein nanosystem for tuning the therapeutic efficacy against highly metastatic triple-negative breast cancer cells

The heterogenic of TNBC and the side effects of chemo drugs lead to the failure of therapy. Protein-based nanoplatforms have emerged as an important domain in protein-engineered biomedicine for delivering anticancer therapeutics. Protein-based nanosystems are biocompatible and biodegradable, with a long half-life and high purity. TNBC is sensitive to DNA-damaging chemo drugs. In this study, we used 10-hydroxy camptothecin, which causes DNA damage in cancer cells. However, the inappropriate solubility and toxic side effects limit its application in cancer therapy. We encapsulated 10-Hydroxycamptothecin in biocompatible casein by synthesizing nanoparticles from it. The synthesized CS and CCS NPs showed excellent biocompatibility in fibroblast cell lines L929, NIH-3T3, and zebrafish embryos. Enhanced uptake of CCS NPs in zebrafish embryos and 4T1 cells, cancer cell toxicity of nearly 80-85%, sub-cellular mitochondrial localization, alterations of mitochondrial membrane potential, lysosomal localization, and reactive oxygen species generation that causes cancer cell apoptosis have been observed. Growth inhibition of 4T1 cell colonies and antimetastatic activity were also noted. Further upregulation of γ-H2AX which causes DNA damage, downregulation of the PARP protein related to DNA repair, and increased level of the CHOP protein marker for endoplasmic reticulum stress-mediated cell death were observed. The 3-D model of 4T1 cells exhibited deep tumor penetration with significant therapeutic efficacy for CCS NPs. These results imply that casein-based nanoformulation could open a new scope for safe and affordable cancer therapy in TNBC.

Recent advances in ATM inhibitors as potential therapeutic agents

ATM, a member of the PIKK-like protein family, plays a central role in responding to DNA double-strand breaks and other lesions to protect the genome against DNA damage. Loss of ATM's kinase function has been shown to increase the sensitivity of most cells to ionizing radiation. Therefore, ATM is thought to be a promising target for chemotherapy as a radiotherapy sensitizer. The mechanism of ATM in cancer treatment and the development of its inhibitors have become research hotspots. Here we present an overview of research concerning ATM protein domains, functions and inhibitors, as well as perspectives and insights for future development of ATM-targeting agents.

ATM kinase inhibitor AZD0156 in combination with irinotecan and 5-fluorouracil in preclinical models of colorectal cancer

Background

AZD0156 is an oral inhibitor of ATM, a serine threonine kinase that plays a key role in DNA damage response (DDR) associated with double-strand breaks. Topoisomerase-I inhibitor irinotecan is used clinically to treat colorectal cancer (CRC), often in combination with 5-fluorouracil (5FU). AZD0156 in combination with irinotecan and 5FU was evaluated in preclinical models of CRC to determine whether low doses of AZD0156 enhance the cytotoxicity of irinotecan in chemotherapy regimens used in the clinic.

Methods

Anti-proliferative effects of single-agent AZD0156, the active metabolite of irinotecan (SN38), and combination therapy were evaluated in 12 CRC cell lines. Additional assessment with clonogenic assay, cell cycle analysis, and immunoblotting were performed in 4 selected cell lines. Four colorectal cancer patient derived xenograft (PDX) models were treated with AZD0156, irinotecan, or 5FU alone and in combination for assessment of tumor growth inhibition (TGI). Immunofluorescence was performed on tumor tissues. The DDR mutation profile was compared across in vitro and in vivo models.

Results

Enhanced effects on cellular proliferation and regrowth were observed with the combination of AZD0156 and SN38 in select models. In cell cycle analysis of these models, increased G2/M arrest was observed with combination treatment over either single agent. Immunoblotting results suggest an increase in DDR associated with irinotecan therapy, with a reduced effect noted when combined with AZD0156, which is more pronounced in some models. Increased TGI was observed with the combination of AZD0156 and irinotecan as compared to single-agent therapy in some PDX models. The DDR mutation profile was variable across models.

Conclusions

AZD0156 and irinotecan provide a rational and active combination in preclinical colorectal cancer models. Variability across in vivo and in vitro results may be related to the variable DDR mutation profiles of the models evaluated. Further understanding of the implications of individual DDR mutation profiles may help better identify patients more likely to benefit from treatment with the combination of AZD0156 and irinotecan in the clinical setting.

Discovery of a Series of 3-Cinnoline Carboxamides as Orally Bioavailable, Highly Potent, and Selective ATM Inhibitors

We report the discovery of a novel series of 3-cinnoline carboxamides as highly potent and selective ataxia telangiectasia mutated (ATM) kinase inhibitors. Optimization of this series focusing on potency and physicochemical properties (especially permeability) led to the identification of compound 21, a highly potent ATM inhibitor (ATM cell IC₅₀ 0.0028 μM) with excellent kinase selectivity and favorable physicochemical and pharmacokinetics properties. In vivo, 21 in combination with irinotecan showed tumor regression in the SW620 colorectal tumor xenograft model, superior inhibition to irinotecan alone. Compound 21 was selected for preclinical evaluation alongside AZD0156.

The Cytotoxic Effects of Camptothecin and Mastoparan on the Unicellular Green Alga Chlamydomonas reinhardtii

We have recently reported that protease inhibitors affecting the activity of the proteasome cause necrotic cell death in Chlamydomonas reinhardtii instead of inducing apoptosis as shown for some mammalian cell lines. Therefore, we have studied other well-known inducers of apoptosis in mammalian cells for their effects on C. reinhardtii cells. Mastoparan caused rapid cell death without a prominent lag-phase under all growth conditions, whereas the cytotoxic effect of the topoisomerase I inhibitor camptothecin exclusively occurred during the cell-division phase. Essentially no differences between wall-deficient and wild-type cells were observed with respect to dose-response and time-course of camptothecin and mastoparan. In cultures of the wall-deficient strain, cell death was accompanied by swelling and subsequent disruption of the cells, established markers of necrosis. In case of the wild-type strain, camptothecin and mastoparan caused accumulation of apparently intact, but dead cells instead of cell debris due to the presence of the wall. Both in cultures of the wall-deficient and the wild-type strains, cell death was accompanied by an increase of the protein concentration in the culture medium indicating a lytic process like necrosis. Taking together, we have severe doubts on the existence of an apoptotic program in case of C. reinhardtii.

Discovery of Novel 3-Quinoline Carboxamides as Potent, Selective, and Orally Bioavailable Inhibitors of Ataxia Telangiectasia Mutated (ATM) Kinase

A novel series of 3-quinoline carboxamides has been discovered and optimized as selective inhibitors of the ataxia telangiectasia mutated (ATM) kinase. From a modestly potent HTS hit (4), we identified molecules such as 6-[6-(methoxymethyl)-3-pyridinyl]-4-{[(1R)-1-(tetrahydro-2H-pyran-4-yl)ethyl]amino}-3-quinolinecarboxamide (72) and 7-fluoro-6-[6-(methoxymethyl)pyridin-3-yl]-4-{[(1S)-1-(1-methyl-1H-pyrazol-3-yl)ethyl]amino}quinoline-3-carboxamide (74) as potent and highly selective ATM inhibitors with overall ADME properties suitable for oral administration. 72 and 74 constitute excellent oral tools to probe ATM inhibition in vivo. Efficacy in combination with the DSB-inducing agent irinotecan was observed in a disease relevant model.

Cyclo(phenylalanine-proline) induces DNA damage in mammalian cells via reactive oxygen species

Cyclo(phenylalanine‐proline) is produced by various organisms such as animals, plants, bacteria and fungi. It has diverse biological functions including anti‐fungal activity, anti‐bacterial activity and molecular signalling. However, a few studies have demonstrated the effect of cyclo(phenylalanine‐proline) on the mammalian cellular processes, such as cell growth and apoptosis. In this study, we investigated whether cyclo(phenylalanine‐proline) affects cellular responses associated with DNA damage in mammalian cells. We found that treatment of 1 mM cyclo(phenylalanine‐proline) induces phosphorylation of H2AX (S139) through ATM‐CHK2 activation as well as DNA double strand breaks. Gene expression analysis revealed that a subset of genes related to regulation of reactive oxygen species (ROS) scavenging and production is suppressed by the cyclo(phenylalanine‐proline) treatment. We also found that cyclo(phenylalanine‐proline) treatment induces perturbation of the mitochondrial membrane, resulting in increased ROS, especially superoxide, production. Collectively, our study suggests that cyclo(phenylalanine‐proline) treatment induces DNA damage via elevation of ROS in mammalian cells. Our findings may help explain the mechanism underlying the bacterial infection‐induced activation of DNA damage response in host mammalian cells.

The use of Olaparib (AZD2281) potentiates SN-38 cytotoxicity in colon cancer cells by indirect inhibition of Rad51-mediated repair of DNA double-strand breaks

Potent application of topoisomerase I inhibitor plus PARP inhibitor has been suggested to be an effective strategy for cancer therapy. Reportedly, mismatch repair (MMR)-deficient colon cancer cells are sensitive to topoisomerase I inhibitor, presumably due to microsatellite instability (MSI) of the MRE11 locus. We examined the synergy of SN-38, an active metabolite of irinotecan, in combination with the PARP inhibitor olaparib in colon cancer cells showing different MMR status, such as MSI or microsatellite stable (MSS) phenotype. Treatment with SN-38 and olaparib in combination almost halved the IC50 of SN-38 for a broad spectrum of colon cancer cells independent of the MMR status. Furthermore, olaparib potentiated S-phase-specific double-strand DNA breaks (DSB) induced by SN-38, which is followed by Rad51 recruitment. siRNA-mediated knockdown of Rad51, but not Mre11 or Rad50, increased the sensitivity to olaparib and/or SN-38 treatment in colon cancer cells. In vivo study using mouse xenograft demonstrated that olaparib was effective to potentiate the antitumor effect of irinotecan. In conclusion, olaparib shows a synergistic effect in colon cancer cells in combination with SN-38 or irinotecan, potentiated by the Rad51-mediated HR pathway, irrespective of the Mre11-mediated failure of the MRN complex. These results may contribute to future clinical trials using PARP inhibitor plus topoisomerase I inhibitor in combination. Furthermore, the synergistic effect comprising topoisomerase I-mediated DNA breakage-reunion reaction, PARP and Rad51-mediated HR pathway suggests the triple synthetic lethal pathways contribute to this event and are applicable as a potential target for future chemotherapy.

Acquired irinotecan resistance is accompanied by stable modifications of cell cycle dynamics independent of MSI status

Irinotecan is a major anticancer agent specifically targeting DNA topoisomerase I. Its cytotoxicity is mediated via a two-step process involving accumulation of reversible DNA‑topoisomerase I complexes associated with transient DNA single-strand breaks which subsequently are converted into permanent DNA double-strand breaks by the replication fork during S phase. Irinotecan may be selectively active for treatment of colorectal cancers that show microsatellite instability (MSI) due to deficiencies in mismatch repair enzymes, compared to tumors that are microsatellite stable but show chromosome instability (CIN). Although the clinical activity of irinotecan is principally limited by acquired drug resistance, surprisingly little is known about the influence of prolonged irinotecan exposure on the cell cycle dynamics. We have developed two colon cancer cell lines resistant to SN-38, the active metabolite of irinotecan, one derived from HT-29 (CIN), the other from HCT-116 (MSI). We here show that besides classical resistance mechanisms, SN-38 resistance is accompanied by an increased generation doubling time, a decreased S phase fraction and an increased G2 fraction in vitro as in tumor xenografts for both CIN and MSI models. As a consequence, SN-38-resistant cells and tumors show cross-resistance to the S-phase selective agent 5-fluorouracil. The resistance is accompanied by increased basal levels of γ-H2AX and phospho-Chk2 without notable changes in the levels of phospho-Chk1. Taken together, our results show that prolonged irinotecan exposure is accompanied by stable modifications of cell cycle dynamics which could have profound impact on tumor sensitivity to a wide range of antitumor agents and may influence tumor progression in patients.

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.

Telomerase inhibition potentiates the effects of genotoxic agents in breast and colorectal cancer cells in a cell cycle-specific manner

Previous studies have shown that telomerase facilitates DNA-damage repair and cell survival following stress. It is not clear how telomerase promotes DNA repair, or whether short-term telomerase inhibition, combined with genotoxic stress, can be exploited for cancer therapy. Here, we show that transient inhibition of telomerase activity by the specific inhibitor, GRN163L, increases the cytotoxicity of some, but not all, DNA-damaging agents. Such synergistic inhibition of growth requires the use of DNA-damaging agents that are toxic in the S/G(2) phase of the cell cycle. Notably, inhibition of Ataxia Telangiectasia Mutated (ATM) kinase, together with telomerase inhibition, synergistically increases the cytotoxicity induced by the G(2)-specific topoisomerase II inhibitor etoposide. By varying the timing of telomerase inhibition, relative to the timing of DNA damage, it is apparent that the prosurvival functions of telomerase occur at early stages of DNA damage recognition and repair. Our results suggest that the protective role of telomerase in cell cycle-restricted DNA damage repair could be exploited for combined anticancer chemotherapy.

Downregulation of cystine transporter xc by irinotecan in human head and neck cancer FaDu xenografts

BACKGROUND

The purpose of this study was: (1) to document the critical requirement of cystine for growth of human tumor cells in vitro, and (2) to determine the effect of the anticancer agent irinotecan on the cystine transporter x(c)(-) in head and neck FaDu xenografts.

METHODS

Cell growth was measured by sulforhodamine B assay. xCT protein, glutathione (GSH) and DNA damage were determined using Western blot, spectrophotometry, and immunohistochemistry, respectively.

RESULTS

Depletion of cystine from the medium inhibited tumor cell growth. Treatment of FaDu tumor with a therapeutic dose of irinotecan resulted in depression of xCT protein levels, leading to tumor growth retardation and downregulation of GSH with increased reactive oxygen species (ROS). The accumulation of ROS correlated with increased DNA damage as evidenced by increased H2AX.

CONCLUSION

Depression of xCT protein by irinotecan resulted in downregulation of GSH and increase in ROS, which could be the other possible mechanisms of DNA damage by irinotecan.

Phase II trial of docetaxel-irinotecan combination in advanced esophageal cancer

BACKGROUND

Preclinical evidence suggests synergy between docetaxel and irinotecan, two drugs active in esophagogastric cancer. We previously demonstrated the safety of docetaxel 35 mg/m2 and irinotecan 50 mg/m2 given on days 1 and 8 of a 21-day schedule.

MATERIALS AND METHODS

Patients who had unresectable/metastatic squamous cell carcinoma or adenocarcinoma of the esophagus, measurable disease, Eastern Cooperative Oncology Group performance status of zero to two, and normal bilirubin were eligible. Tumor assessment was carried out every three cycles.

RESULTS

We enrolled 29 chemotherapy-naive (CN) and 15 chemotherapy-exposed (CE) eligible patients. Principal toxic effects were diarrhea, neutropenia, and hyperglycemia. There were no toxic deaths. There was one early death, from myocardial infarction. Among 26 CN and assessable patients, there were seven (26.9%) with a partial response (PR) and one (3.8%) with a complete response (CR). There were two PRs and one CR among the patients with CE disease. Median time to progression for CN patients was 4.0 months and for CE patients 3.5 months. Median survival for CN eligible patients was 9.0 months and for CE patients 11.4 months.

CONCLUSIONS

Docetaxel-irinotecan combination given on a weekly x 2 of 3 schedule is promising in the treatment of advanced esophageal cancer.

Pharmacogenetic profiling in patients with advanced colorectal cancer treated with first-line FOLFIRI chemotherapy

The primary end point of the study was the analysis of associations between polymorphisms with putative influence on 5-fluorouracil/irinotecan activity and progression-free survival (PFS) of patients with advanced colorectal cancer treated with first-line FOLFIRI chemotherapy. Peripheral blood samples from 146 prospectively enrolled patients were used for genotyping polymorphisms in thymidylate synthase (TS), methylenetetrahydrofolate reductase (MTHFR), excision repair cross-complementation group-1 (ERCC 1) xeroderma pigmentosum group-D (XPD), X-ray cross-complementing-1 (XRCC 1), X-ray cross-complementing-3 (XRCC 3) and uridine diphosphate-glucuronosyltransferases-A1 (UGT1 A1). TS 3'-UTR 6+/6+ and XRCC3-241 C/C genotypes were associated with adverse PFS. Hazard ratio for PFS achieved 2.89 (95% confidence interval=1.56-5.80; P=0.002) in 30 patients (20%) with both risk genotypes. Risk for Grade III-IV neutropenia was significantly associated with UGT1A1*28 7/7 genotype. These promising findings deserve further investigations and their validation in independent prospective studies.

Camptothecin acts synergistically with imatinib and overcomes imatinib resistance through Bcr-Abl independence in human K562 cells

In this study, we have tried to find new targets and effective drugs for imatinib-resistant chronic myelogenous leukemia (CML) cells displaying loss of Bcr-Abl kinase target dependence. The imatinib-resistant K562/R1, -R2 and -R3 cells showed profound declines of Bcr-Abl level and concurrently exhibited up-regulation of Bcl-2 and Ku70/80, and down-regulation of Bax, DNA-PKcs and BRCA1, suggesting that loss of Bcr-Abl after exposure to imatinib might be accompanied by other cell survival mechanism. K562/R3 cells were more sensitive to camptothecin (CPT)- and radiation-induced apoptosis than K562 cells, indicating hypersensitivity of imatinib-resistant cells to DNA damaging agents. Moreover, when K562 cells were treated with the combination of imatinib with CPT, the level of Bax and the cleavage of PARP-1 and DNA-PK were significantly increased in comparison with the effects of each drug. Therefore, our study suggests that CPT can be used to treat CML with loss of Bcr-Abl expression.

DNA single-strand break repair and spinocerebellar ataxia with axonal neuropathy-1

DNA single-strand breaks (SSBs) are the commonest DNA lesions arising spontaneously in cells, and if not repaired may block transcription or may be converted into potentially lethal/clastogenic DNA double-strand breaks (DSBs). Recently, evidence has emerged that defects in the rapid repair of SSBs preferentially impact the nervous system. In particular, spinocerebellar ataxia with axonal neuropathy (SCAN1) is a human disease that is associated with mutation of TDP1 (tyrosyl DNA phosphodiesterase 1) protein and with a defect in repairing certain types of SSBs. Although SCAN1 is a rare neurodegenerative disorder, understanding the molecular basis of this disease will lead to better understanding of neurodegenerative processes. Here we review recent progress in our understanding of TDP1, single-strand break repair (SSBR), and neurodegenerative disease.

Synthesis and Biological Evaluation of 10,11-Methylenedioxy-14-azacamptothecin

[reaction: see text] 10,11-Methylenedioxy-14-azacamptothecin, a potent analogue of the antitumor agent camptothecin (CPT), has been prepared via a key condensation between AB and DE ring precursors. The biological testing of this compound validated a strategy for modulation of the off-rate of camptothecin analogues from the topoisomerase-DNA-CPT ternary complex via structural modification.

TDP1-dependent DNA single-strand break repair and neurodegeneration

DNA single-strand breaks (SSBs) are the commonest DNA lesions that arise spontaneously in living cells. Cells employ efficient processes for the rapid repair of these breaks and defects in these processes appear to preferentially impact on the nervous system, causing human ataxia. Spinocerebellar ataxia with axonal neuropathy (SCAN1) is a human disease that is associated with a defect in repairing certain types of SSBs. Although it is a rare neurodegenerative disease, understanding the molecular basis of SCAN1 will lead to better understanding of the mechanisms that underpin not only neurodegeneration but also cancer.

p21CDKN1A allows the repair of replication-mediated DNA double-strand breaks induced by topoisomerase I and is inactivated by the checkpoint kinase inhibitor 7-hydroxystaurosporine

This study provides evidence for the importance of p21(CDKN1A) for the repair of replication-mediated DNA double-strand breaks (DSBs) induced by topoisomerase I. We report that defects of p21(CDKN1A) and p53 enhance camptothecin-induced histone H2AX phosphorylation (gammaH2AX), a marker for DNA DSBs. In human colon carcinoma HCT116 cells with wild-type (wt) p53, gammaH2AX reverses after camptothecin removal. By contrast, gammaH2AX increases after camptothecin removal in HCT116 cells deficient for p53 (p53-/-) or p21(CDKN1A) (p21-/-) as the cells reach the late-S and G2 phases. Since p21-/- cells exhibit similar S-phase arrest as wt cells in response to camptothecin and aphidicolin does not abrogate the enhanced gammaH2AX formation in p21-/- cells, we conclude that enhanced gammaH2AX formation in p21-/- cells is not due to re-replication. The cell cycle checkpoint abrogator and Chk1/Chk2 inhibitor 7-hydroxystaurosporine (UCN-01) also increases camptothecin-induced gammaH2AX formation and inhibits camptothecin-induced p21(CDKN1A) upregulation in HCT116 wt cells. TUNEL (terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling) assays demonstrate that gammaH2AX formation in late S and G2 cells following CPT treatment corresponds to DNA breaks. However, these breaks are not related to apoptotic DNA fragmentation. We propose that p21(CDKN1A) prevents the collapse of replication forks damaged by stabilized topoisomerase I cleavage complexes.

Spontaneous homologous recombination is induced by collapsed replication forks that are caused by endogenous DNA single-strand breaks

Homologous recombination is vital to repair fatal DNA damage during DNA replication. However, very little is known about the substrates or repair pathways for homologous recombination in mammalian cells. Here, we have compared the recombination products produced spontaneously with those produced following induction of DNA double-strand breaks (DSBs) with the I-SceI restriction endonuclease or after stalling or collapsing replication forks following treatment with thymidine or camptothecin, respectively. We show that each lesion produces different spectra of recombinants, suggesting differential use of homologous recombination pathways in repair of these lesions. The spontaneous spectrum most resembled the spectra produced at collapsed replication forks formed when a replication fork runs into camptothecin-stabilized DNA single-strand breaks (SSBs) within the topoisomerase I cleavage complex. We found that camptothecin-induced DSBs and the resulting recombination repair require replication, showing that a collapsed fork is the substrate for camptothecin-induced recombination. An SSB repair-defective cell line, EM9 with an XRCC1 mutation, has an increased number of spontaneous gammaH2Ax and RAD51 foci, suggesting that endogenous SSBs collapse replication forks, triggering recombination repair. Furthermore, we show that gammaH2Ax, DSBs, and RAD51 foci are synergistically induced in EM9 cells with camptothecin, suggesting that lack of SSB repair in EM9 causes more collapsed forks and more recombination repair. Furthermore, our results suggest that two-ended DSBs are rare substrates for spontaneous homologous recombination in a mammalian fibroblast cell line. Interestingly, all spectra showed evidence of multiple homologous recombination events in 8 to 16% of clones. However, there was no increase in homologous recombination genomewide in these clones nor were the events dependent on each other; rather, we suggest that a first homologous recombination event frequently triggers a second event at the same locus in mammalian cells.

Reactive Oxygen Species Elicit Apoptosis by Concurrently Disrupting Topoisomerase II and DNA-Dependent Protein Kinase

Reactive oxygen species (ROS) are produced by all aerobic cells and have been implicated in the regulation of diverse cellular functions, including intracellular signaling, transcription activation, proliferation, and apoptosis. Salvicine, a novel diterpenoid quinone compound, demonstrates a broad spectrum of antitumor activities. Although salvicine is known to trap the DNA-topoisomerase II (Topo II) complex and induce DNA double-strand breaks (DSBs), its precise antitumor mechanisms remain to be clarified. In this study, we investigated whether salvicine altered the levels of ROS in breast cancer MCF-7 cells and whether these ROS contributed to the observed antitumoral activity. Our data revealed that salvicine stimulated intracellular ROS production and subsequently elicited notable DSBs. The addition of N-acetyl cysteine (NAC), an antioxidant, effectively attenuated the salvicine-induced ROS enhancement and subsequent DNA DSBs. Heat treatment reversed the accumulation of DNA DSBs, and the addition of NAC attenuated the Topo II-DNA cleavable complexes formation and the growth inhibition of salvicine-treated JN394top2-4 yeast cells, collectively indicating that Topo II is a target of the salvicine-induced ROS. On the other hand, when examining the impact of salvicine on DNA repair pathways, we unexpectedly observed that salvicine selectively down-regulated the catalytic subunit of DNA-dependent protein kinase (DNA-PK(CS)) protein levels and repressed DNA-PK kinase activity; both of these effects were attenuated by NAC pretreatment of MCF-7 cells. Finally and most importantly, NAC attenuated salvicine-induced apoptosis and cytotoxicity in MCF-7 cells. These results indicate that apart from its direct actions, salvicine generates ROS that modulate DNA damage and repair, contributing to the comprehensive biological consequences of salvicine treatment, such as DNA DSBs, apoptosis, and cytotoxicity in tumor cells. The finding of salvicine-induced ROS provides new evidence for the molecular mechanisms of this compound. Moreover, the effects of salvicine-induced ROS on Topo II and DNA-PK give new insights into the diverse biological activities of ROS.

Antitumor and neurotoxic effects of novel harmine derivatives and structure-activity relationship analysis

Beta-carboline alkaloids such as harmine are present in medicinal plants such as Peganum harmala that have been used as folk medicine in anticancer therapy. In our study, 9 harmine derivatives (including harmine) were investigated for their antitumor effects and acute toxicities in mice, and the structure-activity relationship (SAR) was also analyzed. Administration of these compounds resulted in tumor inhibition rates of 15.3-49.5% in mice bearing Lewis Lung Cancer, sarcoma180 or HepA tumor, with the highest value of 49.5% from compound 6. Acute toxicity studies showed that all these compounds except compounds 2 and 5 caused remarkable acute neurotoxicities manifested by tremble, twitch and jumping. SAR analysis indicated that the formate substitution at R3 of the tricyclic skeleton reduced their neurotoxicity, while the short alkyl or aryl substitution at R9 increased the antitumor activity. The harmine and its derivatives resulted in in vitro cytotoxicity (IC50) values of 0.011-0.021 micromol/ml in HepG2 cells, with compound 8 being the most potent among all agents tested. Compounds 1, 6, 7 and 8 induced apoptosis in HepG2 cells, with the highest apoptotic rate (55.34%) from compound 6. Western blotting analysis demonstrated that compound 6 completely inhibited the expression of Bcl-2 gene, and compounds 1 and 8 produced a significant inhibition by 40 and 60%, respectively, compared to the control, while compound 7 did not alter the level of Bcl-2. Compounds 1, 6, 7 and 8 upregulated the expression of death receptor Fas by approximately 50-120%. All these findings indicate that compounds with both substitutions at R3 and R9 (such as compound 5) have high antitumor activity and low toxicity, which might be chosen as lead molecules for further development. Further studies on the effects of harmine derivatives on key regulators for tumor cell apoptosis are needed.

Effects of camptothecin on double-strand break repair by non-homologous end-joining in DNA mismatch repair-deficient human colorectal cancer cell lines

Loss of a functional mismatch repair (MMR) system in colorectal cancer (CRC) cells is associated with microsatellite instability and increased sensitivity to topoisomerase inhibitors. In this study, we have investigated whether a defect in double-strand break (DSB) repair by non-homologous end-joining (NHEJ) could explain why MMR-deficient CRC cells are hypersensitive to camptothecin (CPT), a topoisomerase I inhibitor. To evaluate the efficiency and the fidelity of DSB repair, we have transiently transfected plasmids containing cohesive or non-complementary ends in cells with various MMR defects. We have observed that the repair efficiency of DSB with cohesive and non-complementary ends is comparable in all cell lines. In contrast to the MMR-proficient cell line HT29, the MMR-deficient cell lines were highly accurate in repairing DSB with cohesive ends, but this characteristic could not be directly assigned to the primary MMR deficiency. Furthermore, CPT treatment had no detectable effect on the repair of cohesive ends but significantly decreased the repair efficiency of non-complementary DSB. In conclusion, although our observations show that DSB repair efficiency by NHEJ decreases upon treatment with CPT, which possibly contributes to its cytotoxicity, it is quite unlikely that it accounts for the hypersensitivity of MMR-deficient cells to topoisomerase inhibitors.

Synergistic antitumor activity of troxacitabine and camptothecin in selected human cancer cell lines

Troxacitabine (L-OddC) is an L-configuration deoxycytidine analog currently in phase II trials for the treatment of cancer. The cytotoxicity of L-OddC in combination with other anticancer agents has not been studied systematically. In the present study, we assessed the cytotoxic effects produced by the combinations of L-OddC and several commonly used chemotherapy drugs in a panel of cultured human cancer cell lines. Growth inhibition resulting from simultaneous exposure to two-drug combinations was determined using the methylene blue staining method. Camptothecin (CPT) and analogs exhibited additives to synergistic interactions with L-OddC by isobologram analysis. These effects were cell type-specific, with the most pronounced synergism being observed in KB oropharyngeal carcinoma and CPT-resistant KB100 cell lines. In KB cells, the total cellular uptake and DNA incorporation of L-OddC were increased by the addition of CPT. One explanation that emerged from enzyme assays of deoxycytidine kinase (dCK) and deoxycytidine monophosphate kinase (dCMPK), key enzymes involved in L-OddC phosphorylation, was that CPT protected against L-OddC-induced reduction in dCK and dCMPK activity. The resulting increase in l-OddC metabolites and incorporation into DNA was associated with enhanced L-OddC cytotoxicity. These findings will be useful in designing future clinical trials of combination chemotherapy with l-OddC and CPT analogs with the potential for a broad use against both hematological and solid tumors.

Irod/Ian5: an inhibitor of gamma-radiation- and okadaic acid-induced apoptosis

Protein phosphatase-directed toxins such as okadaic acid (OA) are general apoptosis inducers. We show that a protein (inhibitor of radiation- and OA-induced apoptosis, Irod/Ian5), belonging to the family of immune-associated nucleotide binding proteins, protected Jurkat T-cells against OA- and gamma-radiation-induced apoptosis. Unlike previously described antiapoptotic proteins Irod/Ian5 did not protect against anti-Fas, tumor necrosis factor-alpha, staurosporine, UV-light, or a number of chemotherapeutic drugs. Irod antagonized a calmodulin-dependent protein kinase II-dependent step upstream of activation of caspase 3. Irod has predicted GTP-binding, coiled-coil, and membrane binding domains. Irod localized to the centrosomal/Golgi/endoplasmic reticulum compartment. Deletion of either the C-terminal membrane binding domain or the N-terminal GTP-binding domain did not affect the antiapoptotic function of Irod, nor the centrosomal localization. The middle part of Irod, containing the coiled-coil domain, was therefore responsible for centrosomal anchoring and resistance toward death. Being widely expressed and able to protect also nonimmune cells, the function of Irod may not be limited to the immune system. The function and localization of Irod indicate that the centrosome and calmodulin-dependent protein kinase II may have important roles in apoptosis signaling.

Nucleolytic processing of a protein-bound DNA end by the E. coli SbcCD (MR) complex

SbcCD and other Mre11/Rad50 (MR) complexes are implicated in the metabolism of DNA ends. They cleave ends sealed by hairpin structures and have been postulated to play roles in removing protein bound to DNA termini. Here we provide direct evidence that the Escherichia coli MR complex (SbcCD) removes protein from a protein-bound DNA end by inserting a double-strand break (DSB). These observations indicate a more complex biochemical action than has been assumed previously and argue that this class of protein has the potential to play a direct role in deprotecting protein-bound DNA ends in vivo.

Tracking the cell cycle origins for escape from topotecan action by breast cancer cells

The anticancer agent topotecan is considered to be S-phase specific. This implies that cancer cells that are not actively replicating DNA could resist the effects of the drug. The cycle specificity of topotecan action was investigated in MCF-7 cells, using time-lapse microscopy to link the initial cell cycle position during acute exposures to topotecan with the antiproliferative consequences for individual cells. The bioactive dose range (0.5-10 microM) for 1-h topotecan exposures was defined by rapid drug delivery and topoisomerase I trapping. Topotecan caused pan-cycle induction and activation of p53. Lineage analysis of the time-lapse sequences identified cells initially in S-phase and G2, and defined the time to mitosis for cells originating from G2, S-phase and G1. Topotecan prevented all mitoses from S-phase cells and G1 cells (half-maximal effects at 0.14 microM and 0.96 microM, respectively). No dose of topotecan completely prevented mitosis among G2 cells, and at saturating doses of topotecan about half the cells of G2 origin continued dividing (the half-maximal effects was at 0.31 microM). Overall, topotecan differentially targeted G1-, S- and G2-phase cells, but many G2 cells were resistant to topotecan, presenting a clear route for cell cycle-mediated drug resistance.

Chk1 signaling pathways that mediated G(2)M checkpoint in relation to the cellular resistance to the novel topoisomerase I poison BNP1350

A novel karenitecin, BNP1350, is a topoisomerase I-targeting anticancer agent with significant antitumor activity in vitro and in vivo. A BNP1350-resistant human head and neck carcinoma A253 cell line, denoted A253/BNPR, was developed. The A253/BNPR cell line was approximately 9-fold resistant to BNP1350 and 4-fold cross-resistant to another topoisomerase I inhibitor SN-38, the active metabolite of irinotecan. After drug treatment with equimolar concentrations of BNP1350 (0.7 microM) for 2h, activation of the DNA double-strand break repair protein complexes was similar in the two cell lines, suggesting that DNA dsb repair is not attributable to resistance to BNP1350 in the A253/BNPR cells. Cell cycle analysis indicates that the A253 cell line accumulated primarily in S phase, but G(2) phase accumulation was observed in the A253/BNPR cell line at 48 h after drug removal. Elevated chk1 phosphorylation at Ser(345) following DNA damage induced by BNP1350 was accompanied by G(2) accumulation in the A253/BNPR cell line, while exposure to equimolar concentrations of BNP1350 (0.7 microM) induced S-phase arrest and no increased phosphorylation of chk1 at Ser(345) in the A253 cell line. Under the same conditions, increased chk1 activity was observed in the A253/BNPR cell line, but not in the A253 cell line. Moreover, stimulated binding of 14-3-3 proteins to chk1 was observed in BNP1350-treated A253/BNPR cells. To confirm relationship between chk1 expression/phosphorylation and drug resistance to topo I poisons, we examined the effects of chk1 or chk2 antisense oligonucleotides on the cellular growth inhibition. Chk1 antisense oligonucleotide can sensitize the A253/BNPR cells to killing by topo I inhibitor BNP1350, but no significant sensitization of BNP1350-induced growth inhibition was observed in the drug-sensitive cell line. Chk2 antisense oligonucleotide has only a small sensitization effect on BNP1350-induced growth inhibition in both cell lines. The data indicate that the chk1 signaling pathways that mediate cell cycle checkpoint are associated with cellular resistance to BNP1350 in the A253/BNPR cell line.