Induction of DNA breaks and apoptosis in crosslink-hypersensitive V79 cells by the cytostatic drug beta-D-glucosyl-ifosfamide mustard

Rhabdomyosarcoma: Current Therapy, Challenges, and Future Approaches to Treatment Strategies

Rhabdomyosarcoma is a rare cancer arising in skeletal muscle that typically impacts children and young adults. It is a worldwide challenge in child health as treatment outcomes for metastatic and recurrent disease still pose a major concern for both basic and clinical scientists. The treatment strategies for rhabdomyosarcoma include multi-agent chemotherapies after surgical resection with or without ionization radiotherapy. In this comprehensive review, we first provide a detailed clinical understanding of rhabdomyosarcoma including its classification and subtypes, diagnosis, and treatment strategies. Later, we focus on chemotherapy strategies for this childhood sarcoma and discuss the impact of three mechanisms that are involved in the chemotherapy response including apoptosis, macro-autophagy, and the unfolded protein response. Finally, we discuss in vivo mouse and zebrafish models and in vitro three-dimensional bioengineering models of rhabdomyosarcoma to screen future therapeutic approaches and promote muscle regeneration.

An in vitro cytotoxicity of glufosfamide in HepG2 cells relative to its nonconjugated counterpart

BACKGROUND

Glufosfamide (β-D-glucosylisophosphoramide mustard, GLU) is an alkylating cytotoxic agent in which ifosforamide mustard (IPM) is glycosidically linked to the β-D-glucose molecule. GLU exerted its cytotoxic effect as a targeted chemotherapy. Although, its cytotoxic efficacy in a number of cell lines, there were no experimental or clinical data available on the oncolytic effect of oxazaphosphorine drugs in hepatocellular carcinoma. Therefore, the main objective of the current study is to assess the cytotoxic potential of GLU for the first time in the hepatocellular carcinoma HepG2 cell line model.

METHODS

Cytotoxicity was assayed by the MTT method, and half-maximal inhibitory concentration (IC₅₀) was calculated. Flow cytometric analysis of apoptosis frequencies was measured by using Annexin V/PI double stain, an immunocytochemical assay of caspase-9, visualization of caspase-3, and Bcl2 gene expression were undertaken as apoptotic markers. Mitochondrial membrane potential was measured using the potentiometric dye; JC-1, as a clue for early apoptosis as well as ATP production, was measured by the luciferase-chemiluminescence assay.

RESULTS

Glufosfamide induced cytotoxicity in HepG2 cells in a concentration- and time-dependent manner. The IC₅₀ values for glufosfamide were significantly lower compared to ifosfamide. The frequency of apoptosis was much higher for glufosfamide than that of ifosfamide. The contents of caspase-9 and caspase-3 were elevated following exposure to GLU more than IFO. The anti-apoptotic Bcl2 gene expression, the mitochondrial membrane potential, and the cellular ATP levels were significantly decreased than in case of ifosfamide.

CONCLUSIONS

The current study reported for the first time cytotoxicity activity of glufosfamide in HepG2 cells in vitro. The obtained results confirmed the higher oncolytic activity of glufosfamide than its aglycone ifosfamide. The generated data warrants further elucidations by in vivo study.

DNA Double Strand Breaks Repair Inhibitors: Relevance as Potential New Anticancer Therapeutics

DNA double-strand breaks are considered one of the most lethal forms of DNA damage. Many effective anticancer therapeutic approaches used chemical and physical methods to generate DNA double-strand breaks in the cancer cells. They include: IR and drugs which mimetic its action, topoisomerase poisons, some alkylating agents or drugs which affected DNA replication process. On the other hand, cancer cells are mostly characterized by highly effective systems of DNA damage repair. There are two main DNA repair pathways used to fix double-strand breaks: NHEJ and HRR. Their activity leads to a decreased effect of chemotherapy. Targeting directly or indirectly the DNA double-strand breaks response by inhibitors seems to be an exciting option for anticancer therapy and is a part of novel trends that arise after the clinical success of PARP inhibitors. These trends will provide great opportunities for the development of DNA repair inhibitors as new potential anticancer drugs. The main objective of this article is to address these new promising advances.

Oxazaphosphorine cytostatics: from serendipity to rational drug design

On the basis of the discovery that the proapoptotic aldehyde 3-hydroxypropanal is a cyclophosphamide metabolite, a novel mechanism of action of oxazaphosphorine cytostatics is presented and confirmed by animal experiments. Furthermore, it is shown that new oxazaphosphorine cytostatics, which are on orders of magnitude more effective than already existing, can be developed on the basis of the new model for the mechanism of action.

The chemomodulatory effects of glufosfamide on docetaxel cytotoxicity in prostate cancer cells

Background. Glufosfamide (GLU) is a glucose conjugate of ifosfamide in which isophosphoramide mustard is glycosidically linked to the β-D-glucose molecule. Based on GLU structure, it is considered a targeted chemotherapy with fewer side effects. The main objective of the current study is to assess the cytotoxic potential of GLU for the first time in prostate cancer (PC) cells representing different stages of the tumor. Furthermore, this study examined the potential synergistic activity of GLU in combination with docetaxel (DOC).

Methods. Two different cell lines were used, LNCaP and PC-3. Concentration-response curves were assessed. The tested groups per cell line were, control, GLU, DOC and combination. Treatment duration was 72 h. Cytotoxicity was assessed using sulforhodamine B (SRB) assay and half maximal inhibitory concentration (IC50) was calculated. Synergy analyses were performed using Calcusyn^®software. Subsequent mechanistic studies included β-glucosidase activity assay, glucose uptake and apoptosis studies, namely annexin V-FITC assay and the protein expression of mitochondrial pathway signals including Bcl-2, Bax, Caspase 9 and 3 were assessed. Data are presented as mean ± SD; comparisons were carried out using one way analysis of variance (ANOVA) followed by Tukey-Kramer’s test for post hoc analysis.

Results. GLU induced cytotoxicity in both cell lines in a concentration-dependent manner. The IC50 in PC-3 cells was significantly lower by 19% when compared to that of LNCaP cells. The IC50 of combining both drugs showed comparable effect to DOC in PC-3 but was tremendously lowered by 49% compared to the same group in LNCaP cell line. β-glucosidase activity was higher in LNCaP by about 67% compared to that determined in PC-3 cells while the glucose uptake in PC-3 cells was almost 2 folds that found in LNCaP cells. These results were directly correlated to the efficacy of GLU in each cell line. Treatment of PC cells with GLU as single agent or in combination with DOC induced significantly higher apoptosis as evidenced by Annexin V-staining. Apoptosis was significantly increased in combination group by 4.9 folds and by 2.1 Folds when compared to control in LNCaP cells and PC-3 cells; respectively. Similarly, the expression of Bcl-2 was significantly decreased while Bax, caspase 9 and 3 were significantly increased in the combined treatment groups compared to the control.

Conclusion. GLU has a synergistic effect in combination with DOC as it increases the cell kill which can be attributed at least partially to apoptosis in both the tested cell lines and it is suggested as a new combination regimen to be considered in the treatment of the prostate cancer. Further experiments and clinical investigations are needed for assessment of that regimen.

Small Molecules for Active Targeting in Cancer

For the purpose of this review, active targeting in cancer research encompasses strategies wherein a ligand for a cell surface receptor expressed on tumor cells is used to deliver a cytotoxic or imaging cargo. This area of research is more than two decades old, but in those 20 and more years, how many receptors have been studied extensively? What kinds of the ligands are used for active targeting? Are they mostly naturally occurring molecules such as folic acid, or synthetic substances developed in campaigns for medicinal chemistry efforts? This review outlines the most important receptor or ligand combinations that have been used in active targeting to answer these questions, and therefore to address the most important one of all: is research in active targeting affording diminishing returns, or is this an area for which the potential far exceeds progress made so far?

Combination of cyclophosphamide and double-stranded DNA demonstrates synergistic toxicity against established xenografts

BACKGROUND

Extracellular double-stranded DNA participates in various processes in an organism. Here we report the suppressive effects of fragmented human double-stranded DNA along or in combination with cyclophosphamide on solid and ascites grafts of mouse Krebs-2 tumor cells and DNA preparation on human breast adenocarcinoma cell line MCF-7.

METHODS

Apoptosis and necrosis were assayed by electrophoretic analysis (DNA nucleosomal fragmentation) and by measurements of LDH levels in ascitic fluid, respectively. DNA internalization into MCF-7 was analyzed by flow cytometry and fluorescence microscopy.

RESULTS

Direct cytotoxic activity of double-stranded DNA (along or in combination with cyclophosphamide) on a solid transplant was demonstrated. This resulted in delayed solid tumor proliferation and partial tumor lysis due to necrosis of the tumor and adjacent tissues. In the case of ascites form of tumor, extensive apoptosis and secondary necrosis were observed. Similarly, MCF-7 cells showed induction of massive apoptosis (up to 45%) as a result of treatments with double-stranded DNA preparation.

CONCLUSIONS

Double-stranded DNA (along or in combination with cyclophosphamide) induces massive apoptosis of Krebs-2 ascite cells and MCF-7 cell line (DNA only). In treated mice it reduces the integrity of gut wall cells and contributes to the development of systemic inflammatory reaction.

Downregulation of oxidative and nitrosative apoptotic signaling by L-carnitine in Ifosfamide-induced Fanconi syndrome rat model

It is well documented that ifosfamide (IFO) therapy is associated with sever nephropathy in the form of Fanconi syndrome. Although oxidative stress has been reported as a major player in IFO-induced Fanconi syndrome, no mechanism for this effect has been ascertained. Therefore, this study has been initiated to investigate, on gene expression level, the mechanism of IFO-induce nephrotoxicity and those whereby carnitine supplementation attenuates this serious side effect of IFO. To achieve the ultimate goals of this study, adult male rats were assigned to one of four treatment groups, namely, control, L-carnitine, IFO, and IFO plus L-carnitine. Administration of IFO for 5 days significantly increased serum creatinine, blood urea nitrogen (BUN), and total nitrate/nitrite (NOx) production in kidney tissues. In addition, IFO significantly increased mRNA expression of inducible nitric oxide synthase (iNOS), caspase-9, and caspase-3 and significantly decreased expression of glutathione peroxides (GPx), catalase (CAT), and Bcl2 in kidney tissues. Administration of L-carnitine to IFO-treated rats resulted in a complete reversal of the all biochemical and gene expression changes, induced by IFO, to the control values. Data from this study suggest that L-carnitine prevents the development of IFO-induced nephrotoxicity via downregulation of oxidative and nitrosative apoptotic signaling in kidney tissues.

Glufosfamide as a new oxazaphosphorine anticancer agent

Glufosfamide (β-D-glucose-isophosphoramide mustard, D-19575) belongs to the oxazaphosphorine class. Glufosfamide is a novel glucose conjugate of ifosfamide in which isophosphoramide mustard, the alkylating metabolite of ifosfamide, is glycosidically linked to the β-D-glucose molecule. Glufosfamide represents an attractive new agent for cancer therapy. Its mode of action on normal and pathological cells is still under experimental and clinical investigations. An assessment of the anticancer potential of glufosfamide is of key importance in therapy. The researchers reviewed the current knowledge available on glufosfamide tested in the preclinical studies/clinical trials, based on a collection of the original papers and conference abstracts published and relevant articles searched in the SCOPUS and MEDLINE database and websites.

The effect of N-acetylcysteine on the antitumor activity of ifosfamide

Ifosfamide-induced nephrotoxicity is a serious adverse effect in children undergoing chemotherapy. Our previous cell and rodent models have shown that the antioxidant N-acetylcysteine (NAC), used extensively as an antidote for acetaminophen poisoning, protects renal tubular cells from ifosfamide-induced nephrotoxicity at a clinically relevant concentration. For the use of NAC to be clinically relevant in preventing ifosfamide nephrotoxicity, we must ensure there is no effect of NAC on the antitumor activity of ifosfamide. Common pediatric tumors that are sensitive to ifosfamide, human neuroblastoma SK-N-BE(2) and rhabdomyosarcoma RD114-B cells, received either no pretreatment or pretreatment with 400 µmol/L of NAC, followed by concurrent treatment with NAC and either ifosfamide or the active agent ifosfamide mustard. Ifosfamide mustard significantly decreased the growth of both cancer cell lines in a dose-dependent manner (p < 0.001). The different combined treatments of NAC alone, sodium 2-mercaptoethanesulfonate alone, or NAC plus sodium 2-mercaptoethanesulfonate did not significantly interfere with the tumor cytotoxic effect of ifosfamide mustard. These observations suggest that NAC may improve the risk/benefit ratio of ifosfamide by decreasing ifosfamide-induced nephrotoxicity without interfering with its antitumor effect in cancer cells clinically treated with ifosfamide.

Possible contribution of beta-glucosidase and caspases in the cytotoxicity of glufosfamide in colon cancer cells

Glycoconjugates represent a recent trend in cancer chemotherapy that adopts the concept of selective prodrug/drug targeting of tumor cells by binding to specific transmembrane glucose transporters. Following preferential uptake of sugar conjugates into cancer cells, they are presumably subject to enzymatic cleavage by specific beta-glycosidases to liberate the free active cytotoxic aglycones that act selectively on cancer cells and spare other noncancerous ones. In this sense, the role of beta-glucosidase and caspases in the bioactivation and cytotoxicity of glufosfamide has been addressed in the current study. The cytotoxicity of glufosfamide has been investigated over 24-96 h in a panel of human colon cancer cells namely, Caco-2, HT29 and T84 using a tetrazole dye; 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; MTT assay technique. Apoptosis was assessed by fluorometric assay of caspase-3 and caspase-9 activities. Enzymatic cleavage of glufosfamide was accomplished using a host of hydrolytic enzymes and cleavage kinetics was determined using HPLC. Glufosfamide has proven cytotoxic efficacy in a concentration- and time-dependent manner. The sensitivity rank order of tumor cells towards the glycoconjugate was Caco-2>HT29>T84. This sensitivity ranking was well correlated with the enzymatic activity of beta-glucosidase assessed in these cell lines. Initiation and activation of apoptosis were increased in all colon cancer cells following exposure to glufosfamide and were well correlated with the cytotoxicity rank order of the glycoconjugate. Glufosfamide was cleaved by cytosolic and lysosomal beta-glucosidases but not by other hydrolytic enzymes such as cytosolic beta-galactosidase, pancreatic lipase or hepatic esterase. In conclusion, the current data could possibly unravel the mechanistic role of beta-glucosidase and apoptotic caspases in the bioactivation and cytotoxicity of glufosfamide within colon cancer cells.

A novel alkylating agent, glufosfamide, enhances the activity of gemcitabine in vitro and in vivo

Glufosfamide is an alkylating agent consisting of iphosphoramide mustard conjugated to glucose that is currently included in clinical studies of pancreatic cancer. We studied the effects of glufosfamide, in combination with gemcitabine, on in vitro and in vivo models of pancreatic cancer. In proliferation assays, glufosfamide and gemcitabine inhibited the growth of MiaPaCa-2, H766t, and PANC-1 cells, but the combination of the two agents provided greater effects. Apoptosis of MiaPaCa-2 cells, measured by fluorescence-activated cell sorting, was enhanced by the combination of the two drugs, compared to single-agent treatment. Glufosfamide alone inhibited the growth of red fluorescent protein-expressing MiaPaCa-2 tumors in an orthotopic nude mouse model in a dose-dependent manner. Combining glufosfamide (30 mg/kg) with gemcitabine resulted in enhanced inhibition of tumor growth and significantly prolonged survival. Immunohistochemistry of excised tumors revealed that both glufosfamide and gemcitabine increased levels of apoptosis (measured by terminal deoxynucleotidyl transferase-mediated nick end labeling staining) and reduced proliferation (measured by proliferating cell nuclear antigen staining). No effects on microvessel density were observed. These results support the use of the alkylating agent glufosfamide and the DNA synthesis inhibitor gemcitabine, rather than the use of either agent alone, to provide greater benefits and demonstrate that this combination treatment should be useful in the clinical treatment of pancreatic carcinoma.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

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

Various tumor-therapeutic drugs and environmental carcinogens alkylate DNA inducing O(6)-methylguanine (O(6)MeG) that provokes cell death by apoptosis. In rodent fibroblasts, apoptosis triggered by O(6)MeG is executed via the mitochondrial damage pathway. Conversion of O(6)MeG into critical downstream lesions requires mismatch repair (MMR). This is thought to signal apoptosis upon binding to O(6)MeG lesions mispaired with thymine. Alternatively, O(6)MeG lesions might be processed by MMR giving rise to DNA double-strand breaks (DSBs) during replication that finally provoke apoptosis. To test this, we examined apoptosis triggered by O(6)MeG in human peripheral lymphocytes in which O(6)-methylguanine-DNA methyltransferase (MGMT) had been inactivated by O(6)-benzylguanine (O(6)BG) and which were not proliferating or proliferating upon CD3/CD28 stimulation. Treatment with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) or the anticancer drug temozolomide induced apoptosis only in proliferating, but not resting cells. With exceptional high alkylation doses (>/=15 microM of MNNG), apoptosis was also observed in resting lymphocytes, albeit at a lower level than in proliferating cells. This response was not affected by O(6)BG, suggesting that replication-independent apoptosis at high dose levels is caused by lesions other than O(6)MeG. O(6)MeG-triggered apoptosis in proliferating lymphocytes was preceded by a wave of DSBs, which coincided with p53 and Fas receptor upregulation, while Fas ligand, Bax and Bcl-2 expression was not altered. Treatment with anti-Fas neutralizing antibody attenuated MNNG-induced apoptosis in MGMT-depleted proliferating lymphocytes. The data suggest that O(6)MeG is converted by MMR and DNA replication into DSBs that trigger apoptosis by p53 stabilization and Fas/CD95/Apo-1 upregulation. This is supported by the finding that ionizing radiation, inducing DSBs on its own, provokes apoptosis in lymphocytes in a replication-independent way. The strict proliferation dependence of apoptosis triggered by O(6)MeG may explain the specific killing response of MGMT-deficient proliferating cells, including tumors, to O(6)MeG generating anticancer drugs and suggests that tumor proliferation rate, Fas responsiveness, MGMT and MMR status are important prognosis parameters.

DNA damage-triggered apoptosis: critical role of DNA repair, double-strand breaks, cell proliferation and signaling

Genotoxic DNA damaging agents may activate both membrane death receptors and the endogenous mitochondrial damage pathway leading to cell death via apoptosis. Here, apoptotic responses in cells exhibiting a defect in various DNA repair pathways such as alkyltransferase, base excision repair, nucleotide excision repair and mismatch repair are reviewed. The HSVTk/ganciclovir and VZV/BVDU suicide system will also be discussed. Data are available to show that critical DNA damage triggers apoptosis in a DNA replication dependent way by activating the mitochondrial damage pathway in fibroblasts. It is proposed that DNA double-strand breaks (DSBs) are common ultimate apoptosis-triggering lesions arising from primary DNA lesions during DNA replication. Thus, DNA replication is a necessary component in DNA damage-triggered apoptosis, at least in fibroblasts treated with genotoxins not inducing DSBs themselves. For methylating agents inducing O(6)-methylguanine, an additional requirement is mismatch repair provoking DSB formation that triggers Bcl-2 decline and caspase-9/-3 activation. This occurs independent of p53 since most of the repair deficient cell lines under study were mutated for p53. Moreover, p53 knockout fibroblasts are more sensitive to methylating agents and UV light than p53 wt cells, suggesting p53 to play a protective rather than a pro-apoptotic role in this cell system, probably by its involvement in DNA repair. However, for lymphoblastoid cells p53 wt variants are more sensitive to DNA damage indicating that p53 participates in apoptotic signaling in a cell type-specific fashion. The role of topoisomerase II inhibitors and c-Fos/AP-1 in apoptosis will also be discussed.

Reduced DNA double strand breaks in chlorambucil resistant cells are related to high DNA-PKcs activity and low oxidative stress

Modulation of DNA repair represents a strategy to overcome acquired drug resistance of cells to genotoxic chemotherapeutic agents, including nitrogen mustards (NM). These agents induce DNA inter-strand cross-links, which in turn produce double strand breaks (dsbs). These breaks are primarily repaired via the nonhomologous end-joining (NHEJ) pathway. A DNA-dependent protein kinase (DNA-PK) complex plays an important role in NHEJ, and its increased level/activity is associated with acquired drug resistance of human tumors. We show in this report that the DNA-PK complex has comparable levels and kinase activity of DNA-PK catalytic subunit (DNA-PKcs) in a nearly isogenic pair of drug-sensitive (A2780) and resistant (A2780/100) cells; however, treatment with chlorambucil (Cbl), a NM-type of drug, induced differential effects in these cells. The kinase activity of DNA-PKcs was increased up to 2h after Cbl treatment in both cell types; however, it subsequently decreased only in sensitive cells, which is consistent with increased levels of DNA dsbs. The decreased kinase activity of DNA-PKcs was not due to a change in its amount or the levels of Ku70 and Ku86, their subcellular distribution, cell cycle progression or caspase-mediated degradation of DNA-PK. In addition to DNA cross-links, Cbl treatment of cells causes a 2.2-fold increase in the level of reactive oxygen species (ROS) in both cell types. However, the ROS in A2780/100 cells were reduced to the basal level after 3-4h, while sensitive cells continued to produce ROS and undergo apoptosis. Pre-treatment of A2780 cells with the glutathione (GSH) precursor, N-acetyl-L-cysteine prevented Cbl-induced increase in ROS, augmented the kinase activity of DNA-PKcs, decreased the levels of DNA dsbs and increased cell survival. Depletion in GSH from A2780/100 cells by L-buthionine sulfoximine (BSO) resulted in sustained production of ROS, lowered DNA-PKcs kinase activity, enhanced levels of DNA dsbs, and increased cell killing by Cbl. We propose that oxidative stress decreases repair of DNA dsbs via lowering kinase activity of DNA-PKcs and that induction of ROS could be the basis for adjuvant therapies for sensitizing tumor cells to nitrogen mustards and other DNA cross-linking drugs.

Resistance of p53 knockout cells to doxorubicin is related to reduced formation of DNA strand breaks rather than impaired apoptotic signaling

The anthracycline doxorubicin (adriamycin) is an important chemotherapeutic agent used in the treatment of solid epithelial and mesenchymal tumors as well as leukemias. A variety of mechanisms has been proposed to be involved in doxorubicin-induced cytotoxicity such as DNA intercalation, oxidative stress, DNA strand breakage by inhibition of topoisomerase II, activation of death receptors, and altered p53 expression. Concerning doxorubicin resistance and p53 status data reported are contradictory. Here, we show that mouse fibroblasts deficient in p53 (p53(-/-)) are more resistant to doxorubicin than p53 wild-type (p53 wt) cells. This is in contrast to other genotoxic agents (UV-light, alkylating drugs) for which p53(-/-) fibroblasts proved to be more sensitive. Resistance of p53(-/-) cells to doxorubicin is related to reduced induction of apoptosis. This is not likely to be due to altered apoptotic signaling since the expression of Bax and Bcl-2 was unchanged and the induction of Fas/CD95/APO-1 receptor and caspase-8 was the same in p53(-/-) and p53 wt cells on treatment with doxorubicin. However, we observed a clearly lower level of doxorubicin-induced DNA strand breaks in p53(-/-) cells compared to the wt. P170 glycoprotein was equally expressed and the accumulation and elimination of the drug occurred with identical kinetics in both cell types. p53 deficient cells were cross-resistant to another topoisomerase II inhibitor etoposide, which also provoked increased DNA strand breakage in p53 wt cells. Based on the data we conclude that the p53 status significantly impacts the generation of DNA strand breaks because of drug-induced topoisomerase inhibition rather than death receptor signaling. Since human tumors are frequently mutated in p53 the findings bear clinical implications.