Apoptosis triggered by DNA damage O6-methylguanine in human lymphocytes requires DNA replication and is mediated by p53 and Fas/CD95/Apo-1

The DNA Alkyltransferase Family of DNA Repair Proteins: Common Mechanisms, Diverse Functions

DNA alkyltransferase and alkyltransferase-like family proteins are responsible for the repair of highly mutagenic and cytotoxic O6-alkylguanine and O4-alkylthymine bases in DNA. Their mechanism involves binding to the damaged DNA and flipping the base out of the DNA helix into the active site pocket in the protein. Alkyltransferases then directly and irreversibly transfer the alkyl group from the base to the active site cysteine residue. In contrast, alkyltransferase-like proteins recruit nucleotide excision repair components for O6-alkylguanine elimination. One or more of these proteins are found in all kingdoms of life, and where this has been determined, their overall DNA repair mechanism is strictly conserved between organisms. Nevertheless, between species, subtle as well as more extensive differences that affect target lesion preferences and/or introduce additional protein functions have evolved. Examining these differences and their functional consequences is intricately entwined with understanding the details of their DNA repair mechanism(s) and their biological roles. In this review, we will present and discuss various aspects of the current status of knowledge on this intriguing protein family.

Modulating glioblastoma chemotherapy response: Evaluating long non-coding RNA effects on DNA damage response, glioma stem cell function, and hypoxic processes

Glioblastoma (GBM) is the most common and aggressive primary adult brain tumor, with an estimated annual incidence of 17 000 new cases in the United States. Current treatments for GBM include chemotherapy, surgical resection, radiation therapy, and antiangiogenic therapy. However, despite the various therapeutic options, the 5-year survival rate remains at a dismal 5%. Temozolomide (TMZ) is the first-line chemotherapy drug for GBM; however, poor TMZ response is one of the main contributors to the dismal prognosis. Long non-coding RNAs (lncRNAs) are nonprotein coding transcripts greater than 200 nucleotides that have been implicated to mediate various GBM pathologies, including chemoresistance. In this review, we aim to frame the TMZ response in GBM via exploration of the lncRNAs mediating three major mechanisms of TMZ resistance: (1) regulation of the DNA damage response, (2) maintenance of glioma stem cell identity, and (3) exploitation of hypoxia-associated responses.

Temozolomide – Just a Radiosensitizer?

Radiation concomitant with the DNA methylating drug temozolomide (TMZ) is the gold standard in the treatment of glioblastoma. In this adjuvant setting, TMZ is regarded to be a radiation sensitizer. However, similar to ionising radiation, TMZ induces DNA double-strand breaks and is itself a potent trigger of apoptosis, cellular senescence and autophagy, suggesting that radiation and TMZ act independently. Although cell culture experiments yielded heterogeneous results, some data indicate that the cytotoxic effect of radiation was only enhanced when TMZ was given before radiation treatment. Based on the molecular mechanism of action of TMZ, the importance of specific TMZ and radiation-induced DNA lesions, their repair as well as their interactions, possible scenarios for an additive or synergistic effect of TMZ and radiation are discussed, and suggestions for an optimal timing of radio-chemical treatments are proposed.

Translesion DNA synthesis mediates acquired resistance to olaparib plus temozolomide in small cell lung cancer

In small cell lung cancer (SCLC), acquired resistance to DNA-damaging therapy is challenging to study because rebiopsy is rarely performed. We used patient-derived xenograft models, established before therapy and after progression, to dissect acquired resistance to olaparib plus temozolomide (OT), a promising experimental therapy for relapsed SCLC. These pairs of serial models reveal alterations in both cell cycle kinetics and DNA replication and demonstrate both inter- and intratumoral heterogeneity in mechanisms of resistance. In one model pair, up-regulation of translesion DNA synthesis (TLS) enabled tolerance of OT-induced damage during DNA replication. TLS inhibitors restored sensitivity to OT both in vitro and in vivo, and similar synergistic effects were seen in additional SCLC cell lines. This represents the first described mechanism of acquired resistance to DNA damage in a patient with SCLC and highlights the potential of the serial model approach to investigate and overcome resistance to therapy in SCLC.

Class I HDAC overexpression promotes temozolomide resistance in glioma cells by regulating RAD18 expression

Overexpression of histone deacetylases (HDACs) in cancer commonly causes resistance to genotoxic-based therapies. Here, we report on the novel mechanism whereby overexpressed class I HDACs increase the resistance of glioblastoma cells to the S_N1 methylating agent temozolomide (TMZ). The chemotherapeutic TMZ triggers the activation of the DNA damage response (DDR) in resistant glioma cells, leading to DNA lesion bypass and cellular survival. Mass spectrometry analysis revealed that the catalytic activity of class I HDACs stimulates the expression of the E3 ubiquitin ligase RAD18. Furthermore, the data showed that RAD18 is part of the O⁶-methylguanine-induced DDR as TMZ induces the formation of RAD18 foci at sites of DNA damage. Downregulation of RAD18 by HDAC inhibition prevented glioma cells from activating the DDR upon TMZ exposure. Lastly, RAD18 or O⁶-methylguanine-DNA methyltransferase (MGMT) overexpression abolished the sensitization effect of HDAC inhibition on TMZ-exposed glioma cells. Our study describes a mechanism whereby class I HDAC overexpression in glioma cells causes resistance to TMZ treatment. HDACs accomplish this by promoting the bypass of O⁶-methylguanine DNA lesions via enhancing RAD18 expression. It also provides a treatment option with HDAC inhibition to undermine this mechanism.

XPA Enhances Temozolomide Resistance of Glioblastoma Cells by Promoting Nucleotide Excision Repair

Glioblastoma is the most frequent, as well as aggressive kind of high-grade malignant glioma. Chemoresistance is posing a significant clinical barrier to the efficacy of temozolomide-based glioblastoma treatment. By suppressing xeroderma pigmentosum group A (XPA), a pivotal DNA damage recognition protein implicated in nucleotide excision repair (NER), we devised a novel method to enhance glioblastoma therapy and alleviate temozolomide resistance. On the basis of preliminary assessment, we found that XPA dramatically increased in glioblastoma compared with normal cells and contributed to temozolomide resistance. By constructing XPA stably knockdown cells, we illustrate that XPA protects glioma cells from temozolomide-triggered reproductive cell death, apoptosis, as well as DNA repair. Besides, XPA silencing remarkably enhances temozolomide efficacy in vivo. This study revealed a crucial function of XPA-dependent NER in the resistance of glioma cells to temozolomide.

RECQ1 Promotes Stress Resistance and DNA Replication Progression Through PARP1 Signaling Pathway in Glioblastoma

Glioblastoma (GBM) is the most common aggressive primary malignant brain tumor, and patients with GBM have a median survival of 20 months. Clinical therapy resistance is a challenging barrier to overcome. Tumor genome stability maintenance during DNA replication, especially the ability to respond to replication stress, is highly correlated with drug resistance. Recently, we identified a protective role for RECQ1 under replication stress conditions. RECQ1 acts at replication forks, binds PCNA, inhibits single-strand DNA formation and nascent strand degradation in GBM cells. It is associated with the function of the PARP1 protein, promoting PARP1 recruitment to replication sites. RECQ1 is essential for DNA replication fork protection and tumor cell proliferation under replication stress conditions, and as a target of RECQ1, PARP1 effectively protects and restarts stalled replication forks, providing new insights into genomic stability maintenance and replication stress resistance. These findings indicate that tumor genome stability targeting RECQ1-PARP1 signaling may be a promising therapeutic intervention to overcome therapy resistance in GBM.

Methadone-mediated sensitization of glioblastoma cells is drug and cell line dependent

PURPOSE

D,L-methadone (MET), an analgesic drug used for pain treatment and opiate addiction, has achieved attention from oncologists and social media as possible chemoensitizing agent in cancer therapy, notably brain cancer (glioblastoma multiforme, GBM). MET has been reported to enhance doxorubicin-induced cytotoxicity in GBM cells via activation of the µ-opioid receptor (MOR). Here, we extended this work and quantified the toxic effect of MET in comparison to other opioids alone and in combination with doxorubicin and the clinically more relevant alkylating drug temozolomide (TMZ), using a set of GBM cell lines and primary GBM cells.

METHODS

MOR expression in GBM cells was investigated by immunofluorescence and immunoblotting. Resistance to drugs alone and in combination with anticancer drugs was assessed by MTT assays. Concentration effect curves were fitted by nonlinear regression analysis and IC50 values were calculated. Apoptosis and necrosis rates were determined by annexin V/propidium iodide (PI)-flow cytometry.

RESULTS

MET alone was cytotoxic in all GBM cell lines and primary GBM cells at high micromolar concentrations (IC50 ~ 60-130 µM), observed both in the metabolic MTT assay and by quantifying apoptosis and necrosis, while morphine and oxycodone were not cytotoxic in this concentration range. Naloxone was not able to block MET-induced cytotoxicity, indicating that cell death-inducing effects of MET are not MOR-dependent. We recorded doxorubicin and TMZ concentration- response curves in combination with fixed MET concentrations. MET enhanced doxorubicin-induced cytotoxicity in only one cell line, and in primary cells it was observed only in a particular MET concentration range. In all assays, MET was not effective in sensitizing cells to TMZ. In two cell lines, MET even decreased the cell's sensitivity to TMZ.

CONCLUSION

MET was found to be cytotoxic in GBM cells in vitro only at high, clinically not relevant concentrations, where it was effective in inducing apoptosis and necrosis. Sensitizing effects were only observed in combination with doxorubicin, but not with TMZ, and are dependent on cell line and the applied drug concentration. Therefore, our findings do not support the use of MET in the treatment of GBM in combination with TMZ, as no sensitizing effect of MET was observed.

DNA alkylation lesion repair: outcomes and implications in cancer chemotherapy

Alkylated DNA lesions, induced by both exogenous chemical agents and endogenous metabolites, represent a major form of DNA damage in cells. The repair of alkylation damage is critical in all cells because such damage is cytotoxic and potentially mutagenic. Alkylation chemotherapy is a major therapeutic modality for many tumors, underscoring the importance of the repair pathways in cancer cells. Several different pathways exist for alkylation repair, including base excision and nucleotide excision repair, direct reversal by methyl-guanine methyltransferase (MGMT), and dealkylation by the AlkB homolog (ALKBH) protein family. However, maintaining a proper balance between these pathways is crucial for the favorable response of an organism to alkylating agents. Here, we summarize the progress in the field of DNA alkylation lesion repair and describe the implications for cancer chemotherapy.

Arsenic exposure increased expression of HOTAIR and LincRNA-p21 in vivo and vitro

Arsenic is an environmental contaminant, its multiple effects on human tend to increase the rate of disease, cancer and other health problems. Some of long non-coding RNAs (lncRNAs) can be induced in major cellular processes such as necrosis, proliferation, and mutation. While the toxicity of arsenic is well established, the association between arsenic exposure and long non-coding RNAs has not been studied enough. This study investigated the association between arsenic and the expression of HOTAIR and LincRNA-p21 in vivo and vitro. In epidemiological studies, the expression of HOTAIR and LincRNA-p21 was increased after long-term arsenic exposure. HOTAIR and LincRNA-p21 expression were positively linked to monomethylarsenic acid (MMA), dimethylarsenic acid (DMA), inorganic arsenic (iAs), total arsenic (tAs), and MMA% and negatively linked to secondary methylation index (SMI). In A549 cells, arsenic exposure resulted in enhanced HOTAIR and LincRNA-p21 expression dose-dependently. The expression of HOTAIR was considerably high in the presence of NaAsO₂ and MMA but showed no difference in DMA compared with control group. And LincRNA-p21 expression was increased in the presence of NaAsO₂, MMA, and DMA. The expression of HOTAIR and LincRNA-p21 induced by iAs was much higher than that induced by MMA and DMA. Compared with the control group, treatment of A549 cells with NaAsO₂/S-adenosylmethionine (SAM) and NaAsO₂/glutathione (GSH) combination increased HOTAIR and LincRNA-p21 expression. The expression of LincRNA-p21 in combination of NaAsO₂/GSH was significantly decreased compared with NaAsO₂ alone. Besides, in the presence of arsenic, both of HOTAIR and LincRNA-p21 were upregulated significantly when P53 was knocked down. We revealed that inorganic arsenic, its methylated metabolites, and arsenic metabolism efficiency affect the expression of HOTAIR and LincRNA-p21.

MiR-320b/RAD21 axis affects hepatocellular carcinoma radiosensitivity to ionizing radiation treatment through DNA damage repair signaling

Hepatocellular carcinoma (HCC) is one of the most common malignancies in the world and is associated with high mortality. Ionizing radiation (IR)-based therapy causes DNA damage, exerting a curative effect; however, DNA damage repair signaling pathways lead to HCC resistance to IR-based therapy. RAD21 is a component of the cohesion complex, crucial for chromosome segregation and DNA damage repair, while it is still unclear whether RAD21 is implicated in DNA damage and influences IR sensitivity in HCC. The current research explores the effect and upstream regulatory mechanism of RAD21 on IR sensitivity in HCC. In the present study, RAD21 mRNA and protein expression were increased within HCC tissue samples, particularly within IR-insensitive HCC tissues. The overexpression of RAD21 partially attenuated the roles of IR in HCC by promoting the viability and suppressing the apoptosis of HCC cells. RAD21 overexpression reduced the culture medium 8-hydroxy-2-deoxyguanosine concentration and decreased the protein levels of γH2AX and ATM, suggesting that RAD21 overexpression attenuated IR treatment-induced DNA damage to HCC cells. miR-320b targeted RAD21 3'-UTR to inhibit RAD21 expression. In HCC tissues, particularly in IR-insensitive HCC tissues, miR-320b expression was significantly downregulated. miR-320b inhibition also attenuated IR treatment-induced DNA damage to HCC cells; more importantly, RAD21 silencing significantly attenuated the effects of miR-320b inhibition on IR treatment-induced DNA damage, suggesting that miR-320b plays a role through targeting RAD21. In conclusion, an miR-320b/RAD21 axis modulating HCC sensitivity to IR treatment through acting on IR-induced DNA damage was demonstrated. The miR-320b/RAD21 axis could be a novel therapeutic target for further study of HCC sensitivity to IR treatment.

Cytotoxic and Senolytic Effects of Methadone in Combination with Temozolomide in Glioblastoma Cells

Methadone is an analgesic drug used for pain treatment and heroin substitution. Recently, methadone has been proposed to be useful also for cancer therapy, including glioblastoma multiforme (GBM), the most severe form of brain cancer, because experiments on cultured glioma cells treated with doxorubicin showed promising results. Doxorubicin, however, is not used first-line in GBM therapy. Therefore, we analyzed the cytotoxic effect of methadone alone and in combination with temozolomide, a DNA-alkylating drug that is first-line used in GBM treatment, utilizing GBM-derived cell lines and a human fibroblast cell line. We show that methadone is cytotoxic on its own, inducing apoptosis and necrosis, which was observed at a concentration above 20 µg/mL. Methadone was similar toxic in isogenic MGMT expressing and non-expressing cells, and in LN229 glioblastoma and VH10T human fibroblasts. The apoptosis-inducing activity of methadone is not bound on the opioid receptor (OR), since naloxone, a competitive inhibitor of OR, did not attenuate methadone-induced apoptosis/necrosis. Administrating methadone and temozolomide together, temozolomide had no impact on methadone-induced apoptosis (which occurred 3 days after treatment), while temozolomide-induced apoptosis (which occurred 5 days after treatment) was unaffected at low (non-toxic) methadone concentration (5 µg/mL), and at high (toxic) methadone concentration (20 µg/mL) the cytotoxic effects of methadone and temozolomide were additive. Methadone is not genotoxic, as revealed by comet and γH2AX assay, and did not ameliorate the genotoxic effect of temozolomide. Further, methadone did not induce cellular senescence and had no effect on temozolomide-induced senescence. Although methadone was toxic on senescent cells, it cannot be considered a senolytic drug since cytotoxicity was not specific for senescent cells. Finally, we show that methadone had no impact on the MGMT promoter methylation. Overall, the data show that methadone on glioblastoma cells in vitro is cytotoxic and induces apoptosis/necrosis at doses that are above the level that can be achieved in vivo. It is not genotoxic, and does not ameliorate the cell killing or the senescence-inducing effect of temozolomide (no synergistic effect), indicating it has no impact on temozolomide-induced signaling pathways. The data do not support the notion that concomitant methadone treatment supports temozolomide-based chemotherapy.

Delivery of Temozolomide and N3-Propargyl Analog to Brain Tumors Using an Apoferritin Nanocage

Glioblastoma multiforme (GBM) is a grade IV astrocytoma, which is the most aggressive form of brain tumor. The standard of care for this disease includes surgery, radiotherapy and temozolomide (TMZ) chemotherapy. Poor accumulation of TMZ at the tumor site, tumor resistance to drug, and dose-limiting bone marrow toxicity eventually reduce the success of this treatment. Herein, we have encapsulated >500 drug molecules of TMZ into the biocompatible protein nanocage, apoferritin (AFt), using a "nanoreactor" method (AFt-TMZ). AFt is internalized by transferrin receptor 1-mediated endocytosis and is therefore able to facilitate cancer cell uptake and enhance drug efficacy. Following encapsulation, the protein cage retained its morphological integrity and surface charge; hence, its cellular recognition and uptake are not affected by the presence of this cargo. Additional benefits of AFt include maintenance of TMZ stability at pH 5.5 and drug release under acidic pH conditions, encountered in lysosomal compartments. MTT assays revealed that the encapsulated agents displayed significantly increased antitumor activity in U373V (vector control) and, remarkably, the isogenic U373M (MGMT expressing TMZ-resistant) GBM cell lines, with GI₅₀ values <1.5 μM for AFt-TMZ, compared to 35 and 376 μM for unencapsulated TMZ against U373V and U373M, respectively. The enhanced potency of AFt-TMZ was further substantiated by clonogenic assays. Potentiated G2/M cell cycle arrest following exposure of cells to AFt-TMZ indicated an enhanced DNA damage burden. Indeed, increased O6-methylguanine (O6-MeG) adducts in cells exposed to AFt-TMZ and subsequent generation of γH2AX foci support the hypothesis that AFt significantly enhances the delivery of TMZ to cancer cells in vitro, overwhelming the direct O6-MeG repair conferred by MGMT. We have additionally encapsulated >500 molecules of the N3-propargyl imidazotetrazine analog (N3P), developed to combat TMZ resistance, and demonstrated significantly enhanced activity of AFt-N3P against GBM and colorectal carcinoma cell lines. These studies support the use of AFt as a promising nanodelivery system for targeted delivery, lysosomal drug release, and enhanced imidazotetrazine potency for treatment of GBM and wider-spectrum malignancies.

p53-Mediated Activities in NS-5 Neural Stem Cells: Effects of Ethanol

BACKGROUND

Transforming growth factor (TGF) β1 and ethanol (EtOH) powerfully inhibit the proliferation, DNA repair, and survival of neural stem cells (NSCs). The present study tests the hypothesis that the EtOH-induced DNA damage response is mediated through p53 pathways and influenced by growth factor signals.

METHODS

Cultures of nonimmortalized NSCs, NS-5 cells, were transfected with p53 siRNA, exposed to either the mitogenic fibroblast growth factor (FGF) 2 or antimitogenic TGFβ1, and to EtOH. Stage-specific cellular and genomic responses were examined.

RESULTS

p53 status, EtOH exposure, and growth factor significantly affected the expression of transcripts related to the DNA damage response (including those coding for excision repair proteins), mitotic promoters, and regulators of cell death via the tumor necrosis factor pathway. There were significant compensatory increases in p53 family members, p63 and p73, notably in regard to the regulation of cell cycle restriction and apoptosis. Treatment with p53 siRNA potentiated EtOH- and TGFβ1-induced changes in the numbers of proliferating NSCs and increased the proportion of NSCs expressing the apoptotic marker annexin V.

CONCLUSIONS

Thus, it appears that EtOH and TGFβ1 affect proliferation, DNA repair, and survival of NSCs via p53-mediated activities.

The SIAH1-HIPK2-p53ser46 Damage Response Pathway is Involved in Temozolomide-Induced Glioblastoma Cell Death

Patients suffering from glioblastoma have a dismal prognosis, indicating the need for new therapeutic targets. Here we provide evidence that the DNA damage kinase HIPK2 and its negative regulatory E3-ubiquitin ligase SIAH1 are critical factors controlling temozolomide-induced cell death. We show that HIPK2 downregulation (HIPK2kd) significantly reduces the level of apoptosis. This was not the case in glioblastoma cells expressing the repair protein MGMT, suggesting that the primary DNA lesion responsible for triggering HIPK2-mediated apoptosis is O⁶ -methylguanine. Upon temozolomide treatment, p53 becomes phosphorylated whereby HIPK2kd had impact exclusively on ser46, but not ser15. Searching for the transcriptional target of p-p53ser46, we identified the death receptor FAS (CD95, APO-1) being involved. Thus, the expression of FAS was attenuated following HIPK2kd, supporting the conclusion that HIPK2 regulates temozolomide-induced apoptosis via p-p53ser46-driven FAS expression. This was substantiated in chromatin-immunoprecipitation experiments, in which p-p53ser46 binding to the Fas promotor was regulated by HIPK2. Other pro-apoptotic proteins such as PUMA, NOXA, BAX, and PTEN were not affected in HIPK2kd, and also double-strand breaks following temozolomide remained unaffected. We further show that downregulation of the HIPK2 inactivator SIAH1 significantly ameliorates temozolomide-induced apoptosis, suggesting that the ATM/ATR target SIAH1 together with HIPK2 plays a proapoptotic role in glioma cells exhibiting p53wt status. A database analysis revealed that SIAH1, but not SIAH2, is significantly overexpressed in glioblastomas. IMPLICATIONS: The identification of a novel apoptotic pathway triggered by the temozolomide-induced DNA damage O⁶ -methylguanine supports the role of p53 in the decision between survival and death and suggests SIAH1 and HIPK2 as new therapeutic targets.

Sensitivity of CD3/CD28-stimulated versus non-stimulated lymphocytes to ionizing radiation and genotoxic anticancer drugs: key role of ATM in the differential radiation response

Activation of T cells, a major fraction of peripheral blood lymphocytes (PBLCS), is essential for the immune response. Genotoxic stress resulting from ionizing radiation (IR) and chemical agents, including anticancer drugs, has serious impact on T cells and, therefore, on the immune status. Here we compared the sensitivity of non-stimulated (non-proliferating) vs. CD3/CD28-stimulated (proliferating) PBLC to IR. PBLCs were highly sensitive to IR and, surprisingly, stimulation to proliferation resulted in resistance to IR. Radioprotection following CD3/CD28 activation was observed in different T-cell subsets, whereas stimulated CD34+ progenitor cells did not become resistant to IR. Following stimulation, PBLCs showed no significant differences in the repair of IR-induced DNA damage compared with unstimulated cells. Interestingly, ATM is expressed at high level in resting PBLCs and CD3/CD28 stimulation leads to transcriptional downregulation and reduced ATM phosphorylation following IR, indicating ATM to be key regulator of the high radiosensitivity of resting PBLCs. In line with this, pharmacological inhibition of ATM caused radioresistance of unstimulated, but not stimulated, PBLCs. Radioprotection was also achieved by inhibition of MRE11 and CHK1/CHK2, supporting the notion that downregulation of the MRN-ATM-CHK pathway following CD3/CD28 activation results in radioprotection of proliferating PBLCs. Interestingly, the crosslinking anticancer drug mafosfamide induced, like IR, more death in unstimulated than in stimulated PBLCs. In contrast, the bacterial toxin CDT, damaging DNA through inherent DNase activity, and the DNA methylating anticancer drug temozolomide induced more death in CD3/CD28-stimulated than in unstimulated PBLCs. Thus, the sensitivity of stimulated vs. non-stimulated lymphocytes to genotoxins strongly depends on the kind of DNA damage induced. This is the first study in which the killing response of non-proliferating vs. proliferating T cells was comparatively determined. The data provide insights on how immunotherapeutic strategies resting on T-cell activation can be impacted by differential cytotoxic effects resulting from radiation and chemotherapy.

XRCC3 contributes to temozolomide resistance of glioblastoma cells by promoting DNA double-strand break repair

Glioblastoma is the most frequent and aggressive form of high-grade malignant glioma. Due to the dismal prognosis faced by patients suffering from this disease, there is a need for identifying new targets that might improve therapy. The aim of this study was to determine the contribution of the DNA double-strand break (DSB) repair protein X-ray repair cross-complementing 3 (XRCC3) to the resistance of glioma cells to the chemotherapeutic drug temozolomide. Analysis of a publicly available database, E-GEOD-4290, showed that gliomas overexpress XRCC3 (NM_005432) compared to normal brain tissue. Using an isogenic glioma cell system, in which XRCC3 was downregulated by interference RNA, we demonstrate that XRCC3 protects glioma cells against temozolomide-induced reproductive cell death, apoptosis and cell cycle inhibition. Furthermore, XRCC3 knockdown significantly reduced the rate of repair of DSBs following TMZ treatment, which results in increased drug sensitivity. This study confirms the importance of homologous recombination in the resistance of glioma cells to the methylating drug temozolomide and adds XRCC3 to the list of homology-directed DNA repair proteins as possible targets for therapeutic intervention.

Effect of the lymphocyte-to-monocyte ratio on the clinical outcome of chemotherapy administration in advanced melanoma patients

Skin cancer affects more individuals in the USA than any other malignancy and malignant melanoma is particularly deadly because of its metastatic potential. Melanoma has been recognized as one of the most immunogenic malignancies; therefore, understanding the mechanisms of tumor-immune interaction is key for developing more efficient treatments. As the tumor microenvironment shows an immunosuppressive action, immunotherapeutic agents promoting endogenous immune response to cancer have been tested (interleukin-2, anticytotoxic-T-lymphocyte-associated antigen 4, and antiprogrammed cell death protein 1 monoclonal antibodies) as well as combinations of cytotoxic chemotherapy agents and inhibitors of angiogenesis (taxol/carboplatin/avastin). However, clinical outcomes are variable, with only a minority of patients achieving durable complete responses. The variability of immune homeostasis, which may be more active or more tolerant at any given time, in cancer patients and the interaction of the immune system with the tumor could explain the inconsistency in clinical outcomes among these patients. Recently, the role of the lymphocyte-to-monocyte-ratio (LMR) in the peripheral blood has been investigated and has been proven to be an independent predictor of survival in different hematological malignancies and in solid tumors. In melanoma, our group has validated the significance of LMR as a predictor of relapse after resection of advanced melanoma. In this study, we examined the dynamics in the immune system of patients with advanced melanoma by performing serial multiday concentration measurements of cytokines and immune cell subsets in the peripheral blood. The analysis of outcomes of chemotherapy administration as related to LMR on the day of treatment initiation showed that progression-free survival was improved in the patients who received chemotherapy on the day when LMR was elevated.

Synthesis and biological evaluation of glycosides containing triazene-chalcones

By combining triazenes with chalcones, we designed and synthesized 12 novel glycosides. The antiproliferative activity of all products was screened using an MTT assay against MGC803 cells and PC-3 cells. Compound [Formula: see text] displayed more potent antiproliferative activity than dacarbazine. Furthermore, we explored the preliminary structure activity relationship of all target compounds. The derivatives in this work might serve as bioactive fragments and lead compounds for developing more potent cytotoxic agents.

Cell cycle-tailored targeting of metastatic melanoma: Challenges and opportunities

The advent of targeted therapies of metastatic melanoma, such as MAPK pathway inhibitors and immune checkpoint antagonists, has turned dermato-oncology from the "bad guy" to the "poster child" in oncology. Current targeted therapies are effective, although here is a clear need to develop combination therapies to delay the onset of resistance. Many antimelanoma drugs impact on the cell cycle but are also dependent on certain cell cycle phases resulting in cell cycle phase-specific drug insensitivity. Here, we raise the question: Have combination trials been abandoned prematurely as ineffective possibly only because drug scheduling was not optimized? Firstly, if both drugs of a combination hit targets in the same melanoma cell, cell cycle-mediated drug insensitivity should be taken into account when planning combination therapies, timing of dosing schedules and choice of drug therapies in solid tumors. Secondly, if the combination is designed to target different tumor cell subpopulations of a heterogeneous tumor, one drug effective in a particular subpopulation should not negatively impact on the other drug targeting another subpopulation. In addition to the role of cell cycle stage and progression on standard chemotherapeutics and targeted drugs, we discuss the utilization of cell cycle checkpoint control defects to enhance chemotherapeutic responses or as targets themselves. We propose that cell cycle-tailored targeting of metastatic melanoma could further improve therapy outcomes and that our real-time cell cycle imaging 3D melanoma spheroid model could be utilized as a tool to measure and design drug scheduling approaches.

Reviving the guardian of the genome: Small molecule activators of p53

The tumor suppressor p53 is one of the most important proteins for protection of genomic stability and cancer prevention. Cancers often inactivate it by either mutating its gene or disabling its function. Thus, activating p53 becomes an attractive approach for the development of molecule-based anti-cancer therapy. The past decade and half have witnessed tremendous progress in this area. This essay offers readers with a grand review on this progress with updated information about small molecule activators of p53 either still at bench work or in clinical trials.

Intravenous Formulation of HET0016 Decreased Human Glioblastoma Growth and Implicated Survival Benefit in Rat Xenograft Models

Glioblastoma (GBM) is a hypervascular primary brain tumor with poor prognosis. HET0016 is a selective CYP450 inhibitor, which has been shown to inhibit angiogenesis and tumor growth. Therefore, to explore novel treatments, we have generated an improved intravenous (IV) formulation of HET0016 with HPßCD and tested in animal models of human and syngeneic GBM. Administration of a single IV dose resulted in 7-fold higher levels of HET0016 in plasma and 3.6-fold higher levels in tumor at 60 min than that in IP route. IV treatment with HPßCD-HET0016 decreased tumor growth, and altered vascular kinetics in early and late treatment groups (p < 0.05). Similar growth inhibition was observed in syngeneic GL261 GBM (p < 0.05). Survival studies using patient derived xenografts of GBM811, showed prolonged survival to 26 weeks in animals treated with focal radiation, in combination with HET0016 and TMZ (p < 0.05). We observed reduced expression of markers of cell proliferation (Ki-67), decreased neovascularization (laminin and αSMA), in addition to inflammation and angiogenesis markers in the treatment group (p < 0.05). Our results indicate that HPßCD-HET0016 is effective in inhibiting tumor growth through decreasing proliferation, and neovascularization. Furthermore, HPßCD-HET0016 significantly prolonged survival in PDX GBM811 model.

Enhanced Histone Deacetylase Activity in Malignant Melanoma Provokes RAD51 and FANCD2-Triggered Drug Resistance

DNA-damaging anticancer drugs remain a part of metastatic melanoma therapy. Epigenetic reprogramming caused by increased histone deacetylase (HDAC) activity arising during tumor formation may contribute to resistance of melanomas to the alkylating drugs temozolomide, dacarbazine, and fotemustine. Here, we report on the impact of class I HDACs on the response of malignant melanoma cells treated with alkylating agents. The data show that malignant melanomas in situ contain a high level of HDAC1/2 and malignant melanoma cells overexpress HDAC1/2/3 compared with noncancer cells. Furthermore, pharmacologic inhibition of class I HDACs sensitizes malignant melanoma cells to apoptosis following exposure to alkylating agents, while not affecting primary melanocytes. Inhibition of HDAC1/2/3 caused sensitization of melanoma cells to temozolomide in vitro and in melanoma xenografts in vivo HDAC1/2/3 inhibition resulted in suppression of DNA double-strand break (DSB) repair by homologous recombination because of downregulation of RAD51 and FANCD2. This sensitized cells to the cytotoxic DNA lesion O(6)-methylguanine and caused a synthetic lethal interaction with the PARP-1 inhibitor olaparib. Furthermore, knockdown experiments identified HDAC2 as being responsible for the regulation of RAD51. The influence of class I HDACs on DSB repair by homologous recombination and the possible clinical implication on malignant melanoma therapy with temozolomide and other alkylating drugs suggests a combination approach where class I HDAC inhibitors such as valproic acid or MS-275 (entinostat) appear to counteract HDAC- and RAD51/FANCD2-mediated melanoma cell resistance. Cancer Res; 76(10); 3067-77. ©2016 AACR.

The multidrug-resistance transporter Abcc3 protects NK cells from chemotherapy in a murine model of malignant glioma

Abcc3, a member of the ATP-binding cassette transporter superfamily, plays a role in multidrug resistance. Here, we found that Abcc3 is highly expressed in blood-derived NK cells but not in CD8(+) T cells. In GL261 glioma-bearing mice treated with the alkylating agent temozolomide (TMZ) for 5 d, an early increased frequency of NK cells was observed. We also found that Abcc3 is strongly upregulated and functionally active in NK cells from mice treated with TMZ compared to controls. We demonstrate that Abcc3 is critical for NK cell survival during TMZ administration; more importantly, Akt, involved in lymphocyte survival, is phosphorylated only in NK cells expressing Abcc3. The resistance of NK cells to chemotherapy was accompanied by increased migration and homing in the brain at early time points. Cytotoxicity, evaluated by IFNγ production and specific lytic activity against GL261 cells, increased peripherally in the later phases, after conclusion of TMZ treatment. Intra-tumor increase of the NK effector subset as well as in IFNγ, granzymes and perforin-1 expression, were found early and persisted over time, correlating with a profound modulation on glioma microenvironment induced by TMZ. Our findings reveal an important involvement of Abcc3 in NK cell resistance to chemotherapy and have important clinical implications for patients treated with chemo-immunotherapy.

Cell Cycle Phase-Specific Drug Resistance as an Escape Mechanism of Melanoma Cells

The tumor microenvironment is characterized by cancer cell subpopulations with heterogeneous cell cycle profiles. For example, hypoxic tumor zones contain clusters of cancer cells that arrest in G1 phase. It is conceivable that neoplastic cells exhibit differential drug sensitivity based on their residence in specific cell cycle phases. In this study, we used two-dimensional and organotypic melanoma culture models in combination with fluorescent cell cycle indicators to investigate the effects of cell cycle phases on clinically used drugs. We demonstrate that G1-arrested melanoma cells, irrespective of the underlying cause mediating G1 arrest, are resistant to apoptosis induced by the proteasome inhibitor bortezomib or the alkylating agent temozolomide. In contrast, G1-arrested cells were more sensitive to mitogen-activated protein kinase pathway inhibitor-induced cell death. Of clinical relevance, pretreatment of melanoma cells with a mitogen-activated protein kinase pathway inhibitor, which induced G1 arrest, resulted in resistance to temozolomide or bortezomib. On the other hand, pretreatment with temozolomide, which induced G2 arrest, did not result in resistance to mitogen-activated protein kinase pathway inhibitors. In summary, we established a model to study the effects of the cell cycle on drug sensitivity. Cell cycle phase-specific drug resistance is an escape mechanism of melanoma cells that has implications on the choice and timing of drug combination therapies.

B-Raf inhibitor vemurafenib in combination with temozolomide and fotemustine in the killing response of malignant melanoma cells

In the treatment of metastatic melanoma, a highly therapy-refractory cancer, alkylating agents are used and, for the subgroup of BRAF^V600E cancers, the B-Raf inhibitor vemurafenib. Although vemurafenib is initially beneficial, development of drug resistance occurs leading to tumor relapse, which necessitates the requirement for combined or sequential therapy with other drugs, including genotoxic alkylating agents. This leads to the question whether vemurafenib and alkylating agents act synergistically and whether chronic vemurafenib treatment alters the melanoma cell response to alkylating agents. Here we show that a) BRAF^V600E melanoma cells are killed by vemurafenib, driving apoptosis, b) BRAF^V600E melanoma cells are neither more resistant nor sensitive to temozolomide/fotemustine than non-mutant cells, c) combined treatment with vemurafenib plus temozolomide or fotemustine has an additive effect on cell kill, d) acquired vemurafenib resistance of BRAF^V600E melanoma cells does not affect MGMT, MSH2, MSH6, PMS2 and MLH1, nor does it affect the resistance to temozolomide and fotemustine, e) metastatic melanoma biopsies obtained from patients prior to and after vemurafenib treatment did not show a change in the MGMT promoter methylation status and MGMT expression level. The data suggest that consecutive treatment with vemurafenib and alkylating drugs is a reasonable strategy for metastatic melanoma treatment.

Functional interaction between hMYH and hTRADD in the TNF-α-mediated survival and death pathways of HeLa cells

The tumor necrosis factor (TNF) signaling pathway is a classical immune system pathway that plays a key role in regulating cell survival and apoptosis. The TNF receptor-associated death domain (TRADD) protein is recruited to the death domain of TNF receptor 1 (TNFR1), where it interacts with TNF receptor-associated factor 2 (TRAF2) and receptor-interacting protein (RIP) for the induction of apoptosis, necrosis, nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB), and mitogen-activated protein (MAP) kinase activation. In this study, we found that the human MutY homolog (hMYH) interacted with human TRADD (hTRADD) via the C-terminal domain of hMYH. Moreover, under conditions promoting TNF-α-induced cell death or survival in HeLa cells, this interaction was weakened or enhanced, respectively. The interaction between hMYH and hTRADD was important for signaling pathways mediated by TNF-α. Our results also suggested that the hTRADD-hMYH association was involved in the nuclear translocation of NFκB and formation of the TNFR1-TRADD complex. Thus, this study identified a novel mechanism through which the hMYH-hTRADD interaction may affect the TNF-α signaling pathway.

IMPLICATIONS

In HeLa cells, the hTRADD-hMYH interaction functioned in both cell survival and apoptosis pathways following TNF-α stimulation.

Theoretical considerations for thresholds in chemical carcinogenesis

There is increasing evidence for non-linear relationships for gene mutations, chromosomal aberrations and even tumor incidences in response to low doses of genotoxic carcinogens. To attain the biological relevance of such non-linear responses, there is a need to identify the underlying defense mechanisms that allow tolerance to low doses of genotoxicants. This communication discusses presumptive cancer prevention mechanisms that may contribute to thresholds, i.e. points of departure, for each endpoint, from initial DNA lesion to tumor formation. We discuss a sequential order of genome protection during carcinogenesis where genotoxicant scavenging, cellular efflux, DNA repair, elimination of damaged cells by apoptosis, autophagy, silencing by DNA damage-triggered replicative senescence, and finally, elimination of transformed (premalignant) cells by the immune system are thought to be responsible for a threshold in tumor formation. We highlight DNA repair, for which experimental evidence has been recently provided to dictate a role in PoDs. In conclusion, from a theoretical perspective it is reasonable to posit that tolerance to low dose levels exists for each requisite step of tumor formation and these tolerance mechanisms are critical in determining thresholds in chemical carcinogenesis.

Severity, etiology and possible consequences of treatment-related lymphopenia in patients with newly diagnosed high-grade gliomas

Lymphopenia is a common consequence of therapy for malignant glioma. Current standard therapy includes corticosteroids, temozolomide and radiation therapy, all of which are toxic to lymphocytes. The resulting immunosuppression has serious clinical consequences. Decreased lymphocyte counts can result in opportunistic infections, decreased efficacy of immunotherapy and reduced overall survival. The exact mechanisms underlying the association between decreased survival and lymphopenia in malignant glioma patients are unclear. However, as lymphocytes are key effector cells in the immune response to cancer, it is likely that depleting their numbers renders the immune system less effective at eliminating malignant cells. Currently, no strategies exist for the prevention or reversal of treatment-related immunosuppression in malignant glioma patients, although there are several promising theoretical approaches. This article reviews the current state of knowledge regarding the severity, etiology and possible consequences of treatment-related lymphopenia in patients with malignant glioma.

Molecular signalling pathways in canine gliomas

In this study, we determined the expression of key signalling pathway proteins TP53, MDM2, P21, AKT, PTEN, RB1, P16, MTOR and MAPK in canine gliomas using western blotting. Protein expression was defined in three canine astrocytic glioma cell lines treated with CCNU, temozolamide or CPT-11 and was further evaluated in 22 spontaneous gliomas including high and low grade astrocytomas, high grade oligodendrogliomas and mixed oligoastrocytomas. Response to chemotherapeutic agents and cell survival were similar to that reported in human glioma cell lines. Alterations in expression of key human gliomagenesis pathway proteins were common in canine glioma tumour samples and segregated between oligodendroglial and astrocytic tumour types for some pathways. Both similarities and differences in protein expression were defined for canine gliomas compared to those reported in human tumour counterparts. The findings may inform more defined assessment of specific signalling pathways for targeted therapy of canine gliomas.

Bak and Mcl-1 are essential for Temozolomide induced cell death in human glioma

Temozolomide (TMZ) is an alkylating agent used for the treatment of glioblastoma multiforme (GBM), the main form of human brain tumours in adults. It has been reported that TMZ induced DNA lesions that subsequently trigger cell death but the actual mechanisms involved in the process are still unclear. We investigated the implication of major proteins of the Bcl-2 family in TMZ-induced cell death in GBM cell lines at concentrations closed to that reached in the brain during the treatments. We did not observe modulation of autophagy at these concentrations but we found an induction of apoptosis. Using RNA interference, we showed that TMZ induced apoptosis is dependent on the pro-apoptotic protein Bak but independent of the pro-apoptotic protein Bax. Apoptosis was not enhanced by ABT-737, an inhibitor of Bcl-2/Bcl-Xl/Bcl-W but not Mcl-1. The knock-down of Mcl-1 expression increased TMZ induced apoptosis. Our results identify a Mcl-1/Bak axis for TMZ induced apoptosis in GBM and thus unravel a target to overcome therapeutic resistance toward TMZ.

Haematological toxicity of Valproic acid compared to Levetiracetam in patients with glioblastoma multiforme undergoing concomitant radio-chemotherapy: a retrospective cohort study

Patients with glioblastoma multiforme (GBM) and symptomatic seizures are in need of a sufficient antiepileptic treatment. Haematological toxicity is a limiting side effect of both, first line radio-chemotherapy with temozolomide (TMZ) and co-medication with antiepileptic drugs. Valproic acid (VPA) and levetiracetam (LEV) are considered favourable agents in brain tumor patients with seizures, but are commonly reported to induce haematological side effects on their own. We hypothesized, that antiepileptic treatment with these agents has no increased impact on haematological side effects during radio-chemotherapy in the first line setting. We included 104 patients from two neuro-oncologic centres with GBM and standard radio-chemotherapy in a retrospective cohort study. Patients were divided according to their antiepileptic treatment with either VPA, LEV or without antiepileptic drug therapy (control group). Declines in haemoglobin levels and absolute blood cell counts for neutrophil granulocytes, lymphocytes and thrombocytes were analyzed twice during concomitant and once during adjuvant phase. A comparison between the examined groups was performed, using a linear mixed model. Neutrophil granulocytes, lymphocytes and thrombocytes significantly decreased over time in all three groups (all p < 0.012), but there was no significant difference between the compared groups. A significant decline in haemoglobin was observed in the LEV treated group (p = 0.044), but did not differ between the compared groups. As a novel finding, this study demonstrates that co-medication either with VPA or LEV in GBM patients undergoing first line radio-chemotherapy with TMZ has no additional impact on medium-term haematological toxicity.

Translesion polymerase η is upregulated by cancer therapeutics and confers anticancer drug resistance

DNA repair processes are a key determinant of the sensitivity of cancer cells to DNA-damaging chemotherapeutics, which may induce certain repair genes as a mechanism to promote resistance. Here, we report the results of a screen for repair genes induced in cancer cells treated with DNA crosslinking agents, which identified the translesion polymerase η (PolH) as a p53-regulated target acting as one defense against interstrand crosslink (ICL)-inducing agents. PolH was induced by fotemustine, mafosfamide, and lomustine in breast cancer, glioma, and melanoma cells in vitro and in vivo, with similar inductions observed in normal cells such as lymphocytes and diploid fibroblasts. PolH contributions to the protection against ICL-inducing agents were evaluated by its siRNA-mediated attenuation in cells, which elevated sensitivity to these drugs in all tumor cell models. Conversely, PolH overexpression protected cancer cells against these drugs. PolH attenuation reduced repair of ICL lesions as measured by host cell reactivation assays and enhanced persistence of γH2AX foci. Moreover, we observed a strong accumulation of PolH in the nucleus of drug-treated cells along with direct binding to damaged DNA. Taken together, our findings implicated PolH in ICL repair as a mechanism of cancer drug resistance and normal tissue protection.

Mechanism of cadmium induced apoptosis in human peripheral blood lymphocytes: the role of p53, Fas and Caspase-3

Cadmium (Cd) is a major pollutant of environment. It can be fatal to human. In spite of bulk of research and literatures, the mechanism of a fatality against human is still not understood completely. Toxic and carcinogenic effects of Cd in rodents and humans are well known. However, effects of Cd on induction of apoptosis are still elusive. This study indicates immunosuppression and immunotoxicity due to Cd exposure. Present study was undertaken to determine the mechanism of cell death in vitro in human peripheral blood lymphocytes induced by Cd. Our findings suggest the toxicity due to Cd is attributed to programmed cell death-apoptosis. IC₅₀ was calculated at 21.74 μM. A significant increase of expression of the pro-apoptotic genep53, Fas and Caspase-3 in human lymphocytes was found. Cd induced p53-dependent apoptosis through cooperation between Bak upregulation without changing the Bcl-2 and Bax expression. Data of this study compel to speculate that apoptosis may also be attributed to CD95/Fas complex formation, and p53 direct apoptogenic potential at mitochondria. It was confirmed by the increased expression of Caspase-3. Although, this work does not address all the questions regarding the mechanism of Cd induced apoptosis, but these findings establish an important role of p53 and mitochondrial function during apoptosis in human lymphocyte. Moreover, based upon our findings, the role of Fas in Cd induced apoptosis is also undeniable. Hence further investigations are required to understand the different mechanism involved into apoptosis of lymphocytes due to Cd exposure.

Alkylating chemotherapy may exert a uniquely deleterious effect upon neo-antigen-targeting anticancer vaccination

Alkylating chemotherapy exerts both antineoplastic and immunostimulatory effects. However, in addition to depleting regulatory T cells (Treg), alkylating agents also mediate a long lasting antiproliferative effect on responder lymphocytes. Our recent findings indicate that this antiproliferative effect profoundly impairs vaccination-induced immune responses, especially in the case of vaccines that target specific tumor-associated neo-antigens that do not require Treg depletion.

Contribution of ATM and ATR to the resistance of glioblastoma and malignant melanoma cells to the methylating anticancer drug temozolomide

The major cytotoxic DNA adduct induced by temozolomide and other methylating agents used in malignant glioma and metastasized melanoma therapy is O(6)-methylguanine (O(6)-MeG). This primary DNA damage is converted by mismatch repair into secondary lesions, which block replication and in turn induce DNA double-strand breaks that trigger the DNA damage response (DDR). Key upstream players in the DDR are the phosphoinositide 3-kinases ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3 related (ATR). Here, we addressed the question of the importance of ATM and ATR in the cell death response following temozolomide. We show that (i) ATM- and ATR-mutated cells are hypersensitive to temozolomide, (ii) O(6)-MeG triggers ATM and ATR activation, (iii) knockdown of ATM and ATR enhances cell kill in gliobalstoma and malignant melanoma cells with a stronger and significant effect in ATR knockdown cells, (iv) ATR, but not ATM, knockdown abolished phosphorylation of H2AX, CHK1, and CHK2 in glioma cells, and (v) temozolomide-induced cell death was more prominently enhanced by pharmacologic inhibition of CHK1 compared with CHK2. The data suggest that ATM and, even better, ATR inhibition is a useful strategy in sensitizing cancer cells to temozolomide and presumably also other anticancer drugs.

Suppression of alkylating agent induced cell transformation and gastric ulceration by low-dose alkylating agent pretreatment

Exposure to mild stress by chemicals and radiation causes DNA damage and leads to acquired stress resistance. Although the linear no-threshold (LNT) model of safety assessment assumes risk from any dose, evidence from radiological research demonstrates a conflicting hormetic phenomenon known as the hormesis effect. However, the mechanisms underlying radiation hormesis have not yet been clarified, and little is known about the effects of low doses of chemical carcinogens. We analyzed the efficacy of pretreatment with low doses of the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on the subsequent induction of cell transformation and gastric ulceration by high-dose MNNG. We used an in vitro Balb/3T3 A31-1-1 cell transformation test and monitored the formation of gastric ulcers in 5-week-old male ICR mice that were administered MNNG in drinking water. The treatment concentrations of MNNG were determined by the cell survival rate and past reports. For low-dose in vitro and in vivo experiments, MNNG was used at 0.028 μM, and 2.8 μg/mL, respectively. The frequency of cell transformation induced by 10 μm MNNG was decreased by low-dose MNNG pretreatment to levels similar to that of spontaneous transformation. In addition, reactive oxygen species (ROS) and mutation frequencies induced by 10 μm MNNG were decreased by low-dose MNNG pretreatment. Importantly, low-dose MNNG pretreatment had no effect on cell proliferation. In vivo studies showed that the number of gastric ulcers induced by 1 mg/mL MNNG decreased after low-dose MNNG pretreatment. These data indicate that low-dose pretreatment with carcinogens may play a beneficial role in the prevention of chemical toxicity under specified conditions.

Profound impairment of adaptive immune responses by alkylating chemotherapy

Overall, cancer vaccines have had a record of failure as an adjuvant therapy for malignancies that are treated with alkylating chemotherapy, and the contribution of standard treatment to that failure remains unclear. Vaccines aim to harness the proliferative potential of the immune system by expanding a small number of tumor-specific lymphocytes into a large number of antitumor effectors. Clinical trials are often conducted after treatment with alkylating chemotherapy, given either as standard therapy or for immunomodulatory effect. There is mounting evidence for synergy between chemotherapy and adoptive immunotherapy or vaccination against self-Ags; however, the impact of chemotherapy on lymphocytes primed against tumor neoantigens remains poorly defined. We report that clinically relevant dosages of standard alkylating chemotherapies, such as temozolomide and cyclophosphamide, significantly inhibit the proliferative abilities of lymphocytes in mice. This proliferative impairment was long-lasting and led to quantitative and qualitative defects in B and T cell responses to neoantigen vaccines. High-affinity responder lymphocytes receiving the strongest proliferative signals from vaccines experienced the greatest DNA damage responses, skewing the response toward lower-affinity responders with inferior functional characteristics. Together, these defects lead to inferior efficacy and overall survival in murine tumor models treated by neoantigen vaccines. These results suggest that clinical protocols for cancer vaccines should be designed to avoid exposing responder lymphocytes to alkylating chemotherapy.

Decoupling of DNA damage response signaling from DNA damages underlies temozolomide resistance in glioblastoma cells

Glioblastoma multiforme (GBM) is the most aggressive primary brain tumor in adults. Current therapy includes surgery, radiation and chemotherapy with temozolomide (TMZ). Major determinants of clinical response to TMZ include methylation status of the O6-methylguanine-DNA methyltransferase (MGMT) promoter and mismatch repair (MMR) status. Though the MGMT promoter is methylated in 45% of cases, for the first nine months of follow-up, TMZ does not change survival outcome. Furthermore, MMR deficiency makes little contribution to clinical resistance, suggesting that there exist unrecognized mechanisms of resistance. We generated paired GBM cell lines whose resistance was attributed to neither MGMT nor MMR. We show that, responding to TMZ, these cells exhibit a decoupling of DNA damage response (DDR) from ongoing DNA damages. They display methylation-resistant synthesis in which ongoing DNA synthesis is not inhibited. They are also defective in the activation of the S and G2 phase checkpoint. DDR proteins ATM, Chk2, MDC1, NBS1 and gammaH2AX also fail to form discrete foci. These results demonstrate that failure of DDR may play an active role in chemoresistance to TMZ. DNA damages by TMZ are repaired by MMR proteins in a futile, reiterative process, which activates DDR signaling network that ultimately leads to the onset of cell death. GBM cells may survive genetic insults in the absence of DDR. We anticipate that our findings will lead to more studies that seek to further define the role of DDR in ultimately determining the fate of a tumor cell in response to TMZ and other DNA methylators.

Temozolomide-mediated DNA methylation in human myeloid precursor cells: differential involvement of intrinsic and extrinsic apoptotic pathways

PURPOSE

An understanding of how hematopoietic cells respond to therapy that causes myelosuppression will help develop approaches to prevent this potentially life-threatening toxicity. The goal of this study was to determine how human myeloid precursor cells respond to temozolomide (TMZ)-induced DNA damage.

EXPERIMENTAL DESIGN

We developed an ex vivo primary human myeloid precursor cells model system to investigate the involvement of cell-death pathways using a known myelosuppressive regimen of O(6)-benzylguanine (6BG) and TMZ.

RESULTS

Exposure to 6BG/TMZ led to increases in p53, p21, γ-H2AX, and mitochondrial DNA damage. Increases in mitochondrial membrane depolarization correlated with increased caspase-9 and -3 activities following 6BG/TMZ treatment. These events correlated with decreases in activated AKT, downregulation of the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT), and increased cell death. During myeloid precursor cell expansion, FAS/CD95/APO1(FAS) expression increased over time and was present on approximately 100% of the cells following exposure to 6BG/TMZ. Although c-flipshort, an endogenous inhibitor of FAS-mediated signaling, was decreased in 6BG/TMZ-treated versus control, 6BG-, or TMZ alone-treated cells, there were no changes in caspase-8 activity. In addition, there were no changes in the extent of cell death in myeloid precursor cells exposed to 6BG/TMZ in the presence of neutralizing or agonistic anti-FAS antibodies, indicating that FAS-mediated signaling was not operative.

CONCLUSIONS

In human myeloid precursor cells, 6BG/TMZ-initiated apoptosis occurred by intrinsic, mitochondrial-mediated and not extrinsic, FAS-mediated apoptosis. Human myeloid precursor cells represent a clinically relevant model system for gaining insight into how hematopoietic cells respond to chemotherapeutics and offer an approach for selecting effective chemotherapeutic regimens with limited hematopoietic toxicity.

Human monocytes undergo excessive apoptosis following temozolomide activating the ATM/ATR pathway while dendritic cells and macrophages are resistant

Immunodeficiency is a severe therapy-limiting side effect of anticancer chemotherapy resulting from sensitivity of immunocompetent cells to DNA damaging agents. A central role in the immune system is played by monocytes that differentiate into macrophages and dendritic cells (DCs). In this study we compared human monocytes isolated from peripheral blood and cytokine matured macrophages and DCs derived from them and assessed the mechanism of toxicity of the DNA methylating anticancer drug temozolomide (TMZ) in these cell populations. We observed that monocytes, but not DCs and macrophages, were highly sensitive to the killing effect of TMZ. Studies on DNA damage and repair revealed that the initial DNA incision was efficient in monocytes while the re-ligation step of base excision repair (BER) can not be accomplished, resulting in an accumulation of DNA single-strand breaks (SSBs). Furthermore, monocytes accumulated DNA double-strand breaks (DSBs) following TMZ treatment, while DCs and macrophages were able to repair DSBs. Monocytes lack the DNA repair proteins XRCC1, ligase IIIα and PARP-1 whose expression is restored during differentiation into macrophages and DCs following treatment with GM-CSF and GM-CSF plus IL-4, respectively. These proteins play a key role both in BER and DSB repair by B-NHEJ, which explains the accumulation of DNA breaks in monocytes following TMZ treatment. Although TMZ provoked an upregulation of XRCC1 and ligase IIIα, BER was not enhanced likely because PARP-1 was not upregulated. Accordingly, inhibition of PARP-1 did not sensitize monocytes, but monocyte-derived DCs in which strong PARP activation was observed. TMZ induced in monocytes the DNA damage response pathways ATM-Chk2 and ATR-Chk1 resulting in p53 activation. Finally, upon activation of the Fas-receptor and the mitochondrial pathway apoptosis was executed in a caspase-dependent manner. The downregulation of DNA repair in monocytes, resulting in their selective killing by TMZ, might impact on the immune response during cancer chemotherapy.

DNA damage-induced cell death: from specific DNA lesions to the DNA damage response and apoptosis

DNA damaging agents are potent inducers of cell death triggered by apoptosis. Since these agents induce a plethora of different DNA lesions, it is firstly important to identify the specific lesions responsible for initiating apoptosis before the apoptotic executing pathways can be elucidated. Here, we describe specific DNA lesions that have been identified as apoptosis triggers, their repair and the signaling provoked by them. We discuss methylating agents such as temozolomide, ionizing radiation and cisplatin, all of them are important in cancer therapy. We show that the potentially lethal events for the cell are O(6)-methylguanine adducts that are converted by mismatch repair into DNA double-strand breaks (DSBs), non-repaired N-methylpurines and abasic sites as well as bulky adducts that block DNA replication leading to DSBs that are also directly induced following ionizing radiation. Transcriptional inhibition may also contribute to apoptosis. Cells are equipped with sensors that detect DNA damage and relay the signal via kinases to executors, who on their turn evoke a process that inhibits cell cycle progression and provokes DNA repair or, if this fails, activate the receptor and/or mitochondrial apoptotic cascade. The main DNA damage recognition factors MRN and the PI3 kinases ATM, ATR and DNA-PK, which phosphorylate a multitude of proteins and thus induce the DNA damage response (DDR), will be discussed as well as the downstream players p53, NF-κB, Akt and survivin. We review data and models describing the signaling from DNA damage to the apoptosis executing machinery and discuss the complex interplay between cell survival and death.

Certain imidazotetrazines escape O6-methylguanine-DNA methyltransferase and mismatch repair

Resistance to temozolomide (TMZ), conferred by O6-methylguanine-DNA methyltransferase (MGMT) or mismatch repair (MMR) deficiency, presents obstacles to successful glioblastoma multiforme (GBM) treatment. Activities of novel TMZ analogs, designed to overcome resistance, were tested against isogenic SNB19 and U373 GBM cell lines (V = vector control, low MGMT; M = MGMT overexpression). TMZ and triazene MTIC demonstrated >9-fold resistance in SNB19M cells (cf SNB19V). N-3 methyl ester analog 11 and corresponding triazene 12 inhibited growth of TMZ-sensitive (V) and TMZ-resistant (M) cells (GI(50) <50 μM). Ethyl ester 13 and triazene 14 gave similar profiles. MMR-deficient colorectal carcinoma cells, resistant to TMZ (GI(50) >500 μM), responded to analog 11 and 13 treatment. Cross-resistance to these agents was not observed in cell lines possessing acquired TMZ resistance (SNB19VR; U373VR). Methyl ester 11 blocked SNB19V, SNB19M and SNB19VR cells in S and G(2)/M, causing dose- and time-dependent apoptosis. DNA damage, recruiting excision repair was detected by alkaline comet assay; H2AX phosphorylation indicated a lethal DNA double-strand break formation following analog 11 exposure. Compounds 11 and 13 demonstrated 3.7- and 5.1-fold enhanced activity in base excision repair-deficient Chinese hamster ovary cells; furthermore, poly (ADP-ribose) polymerase-1 inhibition potentiated HCT-116 cells' sensitivity to analog 11. In conclusion, analogs 11 and 13 exert anticancer activity irrespective of MGMT and MMR.

Mutagenicity associated with O6-methylguanine-DNA damage and mechanism of nucleotide flipping by AGT during repair

Methylated guanine damage at O6 position (i.e. O6MG) is dangerous due to its mutagenic and carcinogenic character that often gives rise to G:C-A:T mutation. However, the reason for this mutagenicity is not known precisely and has been a matter of controversy. Further, although it is known that O6-alkylguanine-DNA alkyltransferase (AGT) repairs O6MG paired with cytosine in DNA, the complete mechanism of target recognition and repair is not known completely. All these aspects of DNA damage and repair have been addressed here by employing high level density functional theory in gas phase and aqueous medium. It is found that the actual cause of O6MG mediated mutation may arise due to the fact that DNA polymerases incorporate thymine opposite to O6MG, misreading the resulting O6MG:T complex as an A:T base pair due to their analogous binding energies and structural alignments. It is further revealed that AGT mediated nucleotide flipping occurs in two successive steps. The intercalation of the finger residue Arg128 into the DNA double helix and its interaction with the O6MG:C base pair followed by rotation of the O6MG nucleotide are found to be crucial for the damage recognition and nucleotide flipping.