Alteration in DNA cross-linking and sequence selectivity of a series of aziridinylbenzoquinones after enzymic reduction by DT-diaphorase

[(2R,3R)-3-(4-Nitrophenyl)aziridin-2-yl]methanol monohydrate

The title monohydrate, C₉H₁₀N₂O₃·H₂O, contains an aziridine ring including two contiguous stereocenters, both of which exhibit an R configuration. The methyl­hydroxy and nitro­phenyl groups are cis-disposed about the aziridine ring. The mean plane of the benzene ring is tilted to the aziridine ring by 66.65 (8)°. The nitro group is nearly coplanar with the benzene ring [dihedral angle = 2.5 (2)°]. In the crystal, the components are linked by N—H⋯O, O—H⋯N and O—H⋯O hydrogen bonds, generating supra­molecular layers parallel to (001).

DT-diaphorase: a target for new anticancer drugs

DT-diaphorase (DTD) is an obligate two-electron reductase which bioactivates chemotherapeutic quinones. DTD levels are elevated in a number of tumour types, including non-small cell lung carcinoma, colorectal carcinoma, liver cancers and breast carcinomas, when compared to the surrounding normal tissue. The differential in DTD between tumour and normal tissue should allow targeted activation of chemotherapeutic quinones in the tumour whilst minimising normal tissue toxicity. The prototypical bioreductive drug is Mitomycin C (MMC) which is widely used in clinical practice. However, MMC is actually a relatively poor substrate for DTD and its metabolism is pH-dependent. Other bioreductive drugs have failed because of poor solubility and inability to surpass other agents in use. RH1, a novel diaziridinylbenzoquinone, is a more efficient substrate for DTD. It has been demonstrated to have anti-tumour effects both in vitro and in vivo and demonstrates a relationship between DTD expression levels and drug response. RH1 has recently entered a phase I clinical trial in solid tumours under the auspices of Cancer Research UK. Recent work has demonstrated that DTD is present in the nucleus and is associated with both p53 and the heat shock protein, HSP-70. Furthermore, DTD is inducible by several non-toxic compounds and therefore much interest has focussed on increasing the differential in DTD levels between tumour and normal tissues.

Structure-activity study with bioreductive benzoquinone alkylating agents: effects on DT-diaphorase-mediated DNA crosslink and strand break formation in relation to mechanisms of cytotoxicity

PURPOSE

Structure-activity studies were carried out with the model bioreductive alkylating agent benzoquinone mustard (BM) and its structural analogs. The specific objectives were: (1) to investigate the effects of functional group substitutions to the benzoquinone ring on DNA crosslink and strand break formation subsequent to reduction of the analogs by DT-diaphorase (DTD) in vitro, (2) to correlate DNA crosslink and strand break formation by the analogs with anaerobic reduction of the BM analogs by DTD and their redox cycling in vitro, and (3) to correlate DNA crosslink and strand break formation by the BM analogs with their cytotoxic effects in cancer cells.

METHODS

DNA interstrand crosslink and single-strand break formation were assessed using agarose gel assays. To determine DNA interstrand crosslinks or single-strand breaks, linearized or supercoiled plasmid DNA, respectively, were incubated with purified human DTD and increasing concentrations of each BM analog. Subsequently, DNA was electrophoresed on an agarose gel and DNA crosslink and strand break formation were quantified using densitometry. The rates of reduction of the BM analogs by purified human DTD were measured in vitro under hypoxic conditions, and the redox cycling potential was determined under aerobic conditions using HPLC analysis. The cytotoxic activities of these agents in human tumor cell lines were measured by the MTT assay, with and without the DTD inhibitor, dicoumarol.

RESULTS

BM analogs with electron-donating groups (MeBM, MBM, m-MeBM), electron-withdrawing groups (CBM, FBM), sterically bulky groups (PBM, m-PBM, m-TBM) and positional isomers (MeBM, m-MeBM, PBM, m-PBM) were synthesized. After reduction by DTD, the BM analogs produced a concentration-dependent increase in DNA crosslink and DNA strand break formation. The E(10) (extent of DNA crosslink formation produced by 10 micro M BM analog) for DNA crosslink formation displayed the rank order MeBM approximately MBM>m-MeBM approximately PBM approximately BM>CBM>FBM>m-PBM approximately m-TBM. For DNA strand break formation, the E(10) values (extent of DNA strand break formation produced by 10 micro M BM analog) displayed the rank order MeBM>MBM>m-MeBM>PBM>BM approximately CBM>FBM>m-PBM approximately m-TBM. Importantly, the cytotoxic activity of the BM analogs in SK-Mel-28 human melanoma cells correlated positively with the E(10) values for DTD-mediated DNA crosslink formation ( r(s)=0.87, P<0.05) and DNA strand break formation ( r(s)=0.95, P<0.05). Similar correlations were observed in NCI-H661 human lung carcinoma cells. Furthermore, the D(10) values (concentration of BM analog that decreased the surviving cell fraction to 0.1) for cytotoxic activity of the BM analogs correlated with the maximum levels of DNA crosslinks formed with each BM analog, with r(s) values of -0.85 ( P<0.05) for the NCI-H661 cell line, and -0.81 ( P<0.05) for the SK-MEL-28 cell line. The half-time of reduction (t(1/2)) of the BM analogs by DTD did not correlate with DNA crosslink formation, DNA strand break formation, or cytotoxic potency of the analogs.

CONCLUSIONS

Functional groups on the benzoquinone ring affect the ability of BM to produce DNA crosslinks and strand breaks following reduction by DTD. Electron-donating groups increased DNA damage, whereas electron-withdrawing groups and sterically bulky groups at the C6 position had no effect or decreased the ability of the compounds to produce DNA damage compared to BM. Moreover, both DNA crosslink and strand break formation appear to have an important impact on the cytotoxicity of the BM analogs. These results may have significance for optimal use of BM-based antitumor agents and for rationalization of the development of novel therapeutic compounds that require bioactivation by DTD.

Kinetics of NAD(P)H:quinone oxidoreductase I (NQO1) inhibition by mitomycin C in vitro and in vivo

The bioreductive activation of the antitumor quinone mitomycin C (MMC) by NAD(P)H: quinone oxidoreductase 1 (NQO1) is complicated by the ability of MMC to also act as a mechanism-based inhibitor of NQO1 in a pH dependent manner. Inhibition of NQO1 by MMC has been studied in purified enzyme preparations and in cultured cells but has not determined in vivo. In the studies presented here, NQO1 activity was measured in mouse tissues following treatment with MMC or the potent mechanism-based human NQO1 inhibitor 5-methoxy-1,2-dimethyl-[(4-nitrophenoxy)methyl]indole-4,7-dione (ES936). NQO1 activity was significantly decreased at 1, 2, and 4 h following MMC (10 or 20 mg/kg) treatment in kidney and lung but was unchanged in brain, heart, liver, and bladder. ES936 (1 mg/kg) treatment led to a significant and much more potent inhibition of NQO1 in all murine tissues analyzed except for bladder. To extrapolate these in vivo results from mice to humans, the species-specific kinetics of NQO1 inactivation by MMC was determined in vitro using mouse, rat, and human recombinant NQO1 proteins. Results showed the inactivation kinetics of mouse and human proteins by MMC were similar. Treatment of human and murine endothelial cells with MMC or ES936 showed similar inhibition of NQO1 activity. The aforementioned results clearly demonstrate that MMC can serve as a substrate for NQO1 in vivo; however, the metabolism resulting in enzyme inactivation is possibly tissue-specific. Furthermore, the kinetic similarities for inactivation between murine and human forms of NQO1 show these results are apropos to clinical use of MMC.

Cytotoxicity and apoptosis of benzoquinones: redox cycling, cytochrome c release, and BAD protein expression

The metabolic activation of a variety of quinone-based anticancer agents occurs, in part, as a result of the bioreductive activation by the flavoprotein NAD(P)H:quinone-acceptor oxidoreductase (NQO1) (EC 1.6.99.2). Using the COMPARE algorithm (http://dtp.nci.nih.gov), a significant statistical correlation has been found in the NCI in vitro anticancer drug screen between high endogenous expression of the pro-apoptotic protein BAD, NQO1 enzymatic activity, and the cytotoxicity of certain antitumor quinones. Two statistically correlated groups of quinones can be discerned: positive-correlated compounds, which are more active in cell lines expressing high baseline levels of BAD protein and NQO1 activity (e.g. the MCF-7 breast carcinoma), and negative-correlated compounds, which are more active in cell lines with undetectable levels of BAD and NQO1 activity (e.g. the HL-60 myeloid leukemia). In the present study, the relationship between quinone structure, redox cycling, and cytotoxicity in the MCF-7 and HL-60 cell lines was investigated. A good biological correlation exists between cytotoxicity and NQO1 activity, BAD protein levels and apoptosis, but not always between cytotoxicity and intracellular reactive oxygen species levels. The overall markedly increased cytotoxicity of the aziridinylbenzoquinone compounds used in this study is accompanied by apoptosis, which occurs mostly through a cytochrome c-independent pathway.

Prevention of the areca nut extract-induced unscheduled DNA synthesis of gingival keratinocytes by vitamin C and thiol compounds

There are about 600 million betel quid (BQ) chewers in the world. BQ chewing is the major risk factor of oral cancer in India, Taiwan, South Africa and numerous other countries. Areca nut (AN) extract, the main component of BQ, exerts cytotoxicity and genotoxicity to several types of cells. In the present study, AN extract induced the unscheduled DNA synthesis (UDS) of gingival keratinocytes (GK). Vitamin C, at concentration of 50 and 200 microg/ml prevented the AN-induced UDS by 41 and 56%, respectively. Glutathione (GSH, 1-3 mM) and N-acetyl-L-cysteine (NAC, 1-3 mM) also protected the AN-induced UDS by 89-100 and 76-90%. These preventive effects were not due to cytotoxicity as analyzed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. Deferoxamine (20 and 30 mM), an iron chelator and a free radical scavenger, also prevented AN extract induced UDS of GK by 30-55%. On the contrary, banthocuproine (50-200 microM, a copper chelator) and 1,10-phenanthroline (50, 100 microM, a lipid permeable iron chelator), lacked preventive effects. Specific reactive oxygen species scavengers such as dimethyl-sulfoxide (2%), mannitol (10-20 mM), dimethylthiourea (10-20 mM), pyruvate (10 mM), catalase (200 and 400 U/ml), and superoxide dismutase (50 and 200 U/ml) also lacked these preventive effects. Moreover, higher concentrations of H(2)O(2) (0.5-1 mM) inhibited the basal levels of UDS by 19-37%. Interestingly, NAC, GSH, Vitamin C and deferoxamine cannot prevent the AN-induced morphological changes of GK at similar concentrations. These results reveal that AN extract-induced UDS of GK is associated with free radical reactions. Possibly different ingredients of AN is responsible for genotoxicity and cytotoxicity. Vitamin C, GSH and NAC may be potentially used in the future for chemoprevention of BQ chewing related oral mucosal lesions.

High throughput studies of gene expression using green fluorescent protein-oxidative stress promoter probe constructs: the potential for living chips

Green fluorescent protein fusions were constructed with several oxidative stress promoters from Escherichia coli. These promoters were chosen for their induction by reactive oxygen species (ROS) such as superoxide, hydrogen peroxide, and hydroxyl radicals. When exposed to various free radical insults, the cells fluoresced with great specificity based on the corresponding ROS. In this work, we propose a way in which these constructs could be used to study the mode of action of a variety of antitumor drugs. This approach offers the possibility of complementing gene chip technology by the creation of living chips for high throughput screening as well as studying differential gene expression.

Structure-based development of anticancer drugs: complexes of NAD(P)H:quinone oxidoreductase 1 with chemotherapeutic quinones

BACKGROUND

NAD(P)H:quinone acceptor oxidoreductase (QR1) protects animal cells from the deleterious and carcinogenic effects of quinones and other electrophiles. Remarkably, the same enzyme activates cancer prodrugs that become cytotoxic only after two-electron reduction. QR1's ability to bioactivate quinones and its elevated expression in many human solid tumors makes this protein an excellent target for enzyme-directed drug development. Until now, structural analysis of the mode of binding of chemotherapeutic compounds to QR1 was based on model building using the structures of complexes with simple substrates; no structure of complexes of QR1 with chemotherapeutic prodrugs had been reported.

RESULTS

Here we report the high-resolution crystal structures of complexes of QR1 with three chemotherapeutic prodrugs: RH1, a water-soluble homolog of dimethylaziridinylbenzoquinone; EO9, an aziridinylindolequinone; and ARH019, another aziridinylindolequinone. The structures, determined to resolutions of 2.0 A, 2.5 A, and 1.86 A, respectively, were refined to R values below 21% with excellent geometry.

CONCLUSIONS

The structures show that compounds can bind to QR1 in more than one orientation. Surprisingly, the two aziridinylindolequinones bind to the enzyme in different orientations. The results presented here reveal two new factors that must be taken into account in the design of prodrugs targeted for activation by QR1: the enzyme binding site is highly plastic and changes to accommodate binding of different substrates, and homologous drugs with different substituents may bind to QR1 in different orientations. These structural insights provide important clues for the optimization of chemotherapeutic compounds that utilize this reductive bioactivation pathway.

Rational Design of Pyrrolo[1,2-a]benzimidazole-Based Antitumor Agents Targeting the DNA Major Groove

Models have been developed for the interaction of the pyrrolo[1,2-a]benzimidazole (PBI) antitumor agents with the two-electron activating enzyme DT-diaphorase and the DNA major groove. The DT-diaphorase model and experimental results indicate that the S-enantiomer of 3-carbamido PBI can enantioselect ovarian cancers. The reduced PBI interacts with the DNA major groove at AT base pairs by forming Hoogsteen-like hydrogen bonds. The reduced 3-amino PBI forms three hydrogen bonds in the major groove with the amino group acting as an H-bond donor to the thymine carbonyl. The DNA-binding model will permit the design of major groove recognition agents.

NAD(P)H:quinone oxidoreductase 1 (NQO1): chemoprotection, bioactivation, gene regulation and genetic polymorphisms

NAD(P)H:quinone oxidoreductase 1 (NQO1) is an obligate two-electron reductase that is involved in chemoprotection and can also bioactivate certain antitumor quinones. This review focuses on detoxification reactions catalyzed by NQO1 and its role in antioxidant defense via the generation of antioxidant forms of ubiquinone and vitamin E. Bioactivation reactions catalyzed by NQO1 are also summarized and the development of new antitumor agents for the therapy of solid tumors with marked NQO1 content is reviewed. NQO1 gene regulation and the role of the antioxidant response element and the xenobiotic response element in transcriptional regulation is summarized. An overview of genetic polymorphisms in NQO1 is presented and biological significance for chemoprotection, cancer susceptibility and antitumor drug action is discussed.

A survey of the sequence-specific interaction of damaging agents with DNA: emphasis on antitumor agents

This article reviews the literature concerning the sequence specificity of DNA-damaging agents. DNA-damaging agents are widely used in cancer chemotherapy. It is important to understand fully the determinants of DNA sequence specificity so that more effective DNA-damaging agents can be developed as antitumor drugs. There are five main methods of DNA sequence specificity analysis: cleavage of end-labeled fragments, linear amplification with Taq DNA polymerase, ligation-mediated polymerase chain reaction (PCR), single-strand ligation PCR, and footprinting. The DNA sequence specificity in purified DNA and in intact mammalian cells is reviewed for several classes of DNA-damaging agent. These include agents that form covalent adducts with DNA, free radical generators, topoisomerase inhibitors, intercalators and minor groove binders, enzymes, and electromagnetic radiation. The main sites of adduct formation are at the N-7 of guanine in the major groove of DNA and the N-3 of adenine in the minor groove, whereas free radical generators abstract hydrogen from the deoxyribose sugar and topoisomerase inhibitors cause enzyme-DNA cross-links to form. Several issues involved in the determination of the DNA sequence specificity are discussed. The future directions of the field, with respect to cancer chemotherapy, are also examined.

Spermine inhibition of the 2,5-diaziridinyl-1,4-benzoquinone (DZQ) crosslinking reaction with DNA duplexes containing poly(purine). poly(pyrimidine) tracts

Upon reduction, 2,5-diaziridinyl-1,4-benzoquinone (DZQ) can form an interstrand guanine to guanine crosslink with DNA duplexes containing a d(GC).d(GC) dinucleotide step. The reaction is enhanced by a thymine positioned 5[prime] to each guanine [i.e. in a d(TGCA). d(TGCA) duplex fragment]. Here we show that spermine can inhibit DZQ crosslink formation in duplexes of sequence d[C(N6)TGCA(M6)C]. d[G(M[prime]6)TG-CA(N[prime]6)G]. For N6= M6= GGGGGG, N6= M6= a 'random' sequence and N6= GGGGGG and M6= a 'random' sequence, spermine concentrations of 20, 1 and 3 microM, respectively, were required for 50% inhibition of the DZQ crosslink. This suggests that spermine is more strongly bound to the polyguanosine tract than the random sequence, making it less available for crosslink inhibition. When the polyguanosine tract is interrupted by N 7-deazaguanine (D) located three bases, d(CGGGDGGTGCAGGDGGGC), and four bases, d(CG-GDGGGTGCAGGGDGGC), from the d(TGCA).d(TGCA) site, 30 and 3 microM spermine, respectively, were required for 50% crosslink inhibition. We suggest that this difference is due to the relative proximity of the three-guanosine tract to the d(TGCA).d(TGCA) site. We were able to confirm these conclusions with further experiments using duplexes containing three-guanosine and two-guanosine tracts and from computer simulations of the spermine-DNA complexes.

Reactions of halogen-substituted aziridinylbenzoquinones with glutathione. Formation of diglutathionyl conjugates and semiquinones

The reaction between glutathione and 2,5-diaziridinyl-1,4-benzoquinones bearing halogen substituents at C3 and C6 was examined in terms of the formation of glutathionyl-quinone conjugates and semiquinones by HPLC with UV detection, mass spectroscopy and EPR. The reactivity of the halogen atoms toward sulfur substitution is the primary reaction leading to the formation of mono- and di-glutathionyl-substituted quinones. The relative formation of these conjugates depended on the GSH/quinone molar ratios. At GSH/quinone molar ratios below unity, the products observed were the reduced form of the parent quinone, a dithioether derivative and GSSG. Disulfide formation accounted for 60-68% of total GSH consumed. EPR analysis of these reaction mixtures showed a 5-line spectrum (1:2:3:2:1 relative intensities) with 2 equivalent N (aN = 1.98 G) and assigned to the semiquinone form of dichloro- diaziridinylbenzoquinone. Semiquinone quantification by double integration of the EPR signals and interpolation with an adequate standard revealed that the amount of semiquinone formed per GSH consumed was 0.98. At GSH/quinone molar ratios above unity (4, 10 and 100 molar excess of GSH) a pattern of products emerged consisting of 3,6-diglutathionyl quinones with two, one and no aziridinyl moieties, identified by mass spectral analysis. EPR studies revealed that these compounds were minor components of a composite EPR spectrum (a 3-line signal with 1:1:1 relative intensities, 1 equivalent N (aN = 1.73 G) and 1 H (aH = 1.45 G) or a 3-line signal with 1:2:1 relative intensities and 2 equivalent H (aH = 1.4 G). These minor components were assigned to the diglutathionyl conjugates bearing one- or no aziridinyl moiety, respectively. The major component in the EPR signal showed a 3-line spectrum (1:1:1 relative intensity) with 1 equivalent N (aN = 1.7 G) and a g shift of -0.96 G. This spectrum was assigned to a triglutathionyl conjugate of a monoaziridinylbenzoquinone. This major component was also observed when GSH/quinone mixtures were incubated with the two-electron transfer flavoprotein NAD(P)H:quinone oxidoreductase. The semiquinone signals were abolished by superoxide dismutase. In the presence of catalase, the contribution of these components to the overall EPR spectrum was equal. These data are discussed in terms of the one-electron transfer steps encompassed by thiol oxidation and semiquinone formation and the two-electron transfers inherent in sulfur substitution and aziridinyl group loss.

Involvement of DT-diaphorase (EC 1.6.99.2) in the DNA cross-linking and sequence selectivity of the bioreductive anti-tumour agent EO9

The chemistry of the mitomycin C-related drug indoloquinone EO9 would suggest that its mechanism of action is likely to involve DNA damage after reductive activation. The ability of this agent to induce DNA damage in intact cells has been examined using alkaline filter elution. After treatment with pharmacologically relevant concentrations of EO9, both DNA strand breaks and interstrand cross-links were detected in rat Walker tumour cells and human HT29 colon carcinoma cells. These cell lines express relatively high levels of DT-diaphorase (NAD(P)H: quinone acceptor oxidoreductase), which is believed to be involved in EO9 activation. The extent of DNA damage was increased by approximately 30-fold under hypoxia in BE colon carcinoma cells that express non-functional DT-diaphorase, but this dramatic hypoxia enhancement was not seen in HT-29 cells. These data are consistent with cytotoxicity studies that indicate that DT-diaphorase appears to be important in EO9 activation under aerobic conditions, but other enzymes may be more relevant under hypoxia. The involvement of DT-diaphorase in DNA damage induction was further investigated using cell-free assays. DNA cross-links were detectable in plasmid DNA co-incubated with EO9, cofactor and DT-diaphorase but not in the absence of this enzyme. In contrast, using a Taq polymerase stop assay, monofunctional alkylation was detected in plasmid DNA without metabolic activation, although the sequence selectivity was altered after reduction catalysed by DT-diaphorase.

Bioreductive activation of a series of analogues of 5-aziridinyl-3-hydroxymethyl-1-methyl-2-[1H-indole-4, 7-dione] prop-beta-en-alpha-ol (EO9) by human DT-diaphorase

The enzyme DT-diaphorase (NAD(P)H:quinone acceptor oxidoreductase, EC 1.6.99.2.; DTD) is believed to be a good target for enzyme-directed bioreductive drug development because elevated levels of enzyme activity have been described in several human tumour types and it plays a key role in the bioreductive activation of several quinone-based anticancer drugs. As part of an ongoing program to develop new bioreductive drugs, the ability of a series of indoloquinone compounds to serve as substrates for and to be bioreductively activated by purified recombinant human DTD was investigated. Of the seven compounds evaluated, EO9, EO68 and EO4 were substrates for human DTD, but only EO4 was reduced to a DNA cross-linking species, and this DNA damage was both concentration dependent and inhibited by dicoumarol. A broad spectrum of chemosensitivity was observed in the H460 non-small cell lung cancer cell line, with the most potent compounds being EO4 (IC50 = 23.9 nM), EO9 (IC50 = 34.5 nM) and EO68 (IC50 = 37.8 nM). Relatively minor structural changes resulted in major changes in both substrate specificity and cytotoxic potency. Comparative chemosensitivity studies demonstrated that EO4, EO9 and EO68 are preferentially toxic towards DTD-rich H460 cells compared with DTD-deficient H596 cells (ratio of IC50 values for H596 cells to H460 cells were 113.8, 92.2 and 103.9 respectively). In conclusion, this study has identified two new compounds that are substrates for human DTD, one of which (EO4) is reduced to a DNA cross-linking species. Further studies in a broad panel of cell lines and human tumour xenografts are warranted for EO4 and EO68 based upon the result of this study.

Induction of p21 mediated by reactive oxygen species formed during the metabolism of aziridinylbenzoquinones by HCT116 cells

Aziridinylbenzoquinones are a group of antitumor agents that elicit cytotoxicity by generating either alkylating intermediates or reactive oxygen species. The mechanism of toxicity may not always, however, involve profound damage of cellular constituents, but may involve a cytostatic effect through interference with the cell cycle. In this context, we have examined the induction of the cell cycle inhibitor p21 (WAF1, CIP1, or sdi1), whose overexpression suppresses the growth of various tumor cells, in human tumor cells metabolizing 3,6-diaziridinyl-1,4-benzoquinone (DZQ) and its C2,C5-substituted derivatives: 2,5-bis-(carboethoxyamino) (AZQ) and 2, 5-bis-2(-hydroxyethylamino) (BZQ). Both DZQ and AZQ were effectively activated by HCT116 human colonic carcinoma cells; the activation of the former involved largely a dicoumarol-sensitive activity, whereas that of the latter appeared to be accomplished primarily by one-electron transfer reductases. BZQ was not a substrate for the dicoumarol-sensitive enzyme in HCT116 cells. Cellular activation of the first two quinones was associated with formation of oxygen-centered radicals as detected by EPR in conjunction with the spin trap 5,5'-dimethyl-1-pyrroline-N-oxide. The redox transitions of DZQ involved hydroxyl radical formation and were strongly inhibited by catalase, whereas those of AZQ showed a strong superoxide anion component sensitive to superoxide dismutase. These signals were suppressed by N-acetylcysteine with concomitant production of a thiyl radical adduct. This suggests an effective electron transfer between the thiol and free radicals formed during the activation of these quinones. DZQ and AZQ induced significantly the expression of p21 in HCT116 cells, but a 10-fold higher concentration of AZQ was required to achieve the level of induction elicited by DZQ. BZQ had little effect on p21 expression. p21 induction at both mRNA and protein levels correlated with the inhibition of either cyclin-dependent kinase activity or cell proliferation. p21 induction elicited by the above quinones was inhibited by N-acetylcysteine, whereas the non-sulfur analog, N-acetylalanine, was without effect. Catalase and superoxide dismutase did not effect p21 induction by aziridinylbenzoquinones in HCT116 cells, thus suggesting that extracellular sources of oxygen radicals generated by plasma membrane reductases have no influence in the expression of this gene. Hydrogen peroxide, a product of quinone redox cycling, elicited an increase of p21 mRNA levels in HCT116 and K562 human chronic myelogenous leukemia cells. The latter lacks p53, one of the activators of p21 transcription, thus suggesting that p21 expression can be accomplished in a p53-independent manner in these cells. This study suggests that p21 induction is mediated by an increase in the cellular steady-state concentration of oxygen radicals and that the greater effectiveness in p21 induction by DZQ may be related to its efficient metabolism by NAD(P)H:quinone oxidoreductase activity in HCT116 cells.

Contribution of phenols, quinones and reactive oxygen species to the mutagenicity of white grape juice in the Ames test

The purpose of this study was to evaluate the role of phenols, quinones and reactive oxygen species in the mutagenicity of white grape juice in the Ames mutagenicity test. Mutagenicity was markedly suppressed by reduced glutathione but was not influenced by superoxide dismutase or catalase. In the presence of grape polyphenol oxidase, the mutagenicity of grape juice was markedly increased. When hepatic cytosol from Aroclor 1254-induced rats, supplemented with a reduced nicotinamide adenine dinucleotide phosphate-generating system. served as an activation system, an increase in the mutagenicity of grape juice was observed. The cytosol-induced mutagenicity of grape juice was attenuated in the presence of superoxide dismutase, catalase and glutathione. It is concluded that polyphenol oxidase-catalysed oxidation of phenolic compounds generates genotoxic species that are, at least partly, responsible for the mutagenicity of grape juice. In the presence of hepatic cytosol, one-electron reduction of grape juice quinones leads to the production of reactive oxygen species resulting in an increase in the mutagenic response.

Role of NAD(P)H:quinone oxidoreductase (DT-diaphorase) in cytotoxicity and induction of DNA damage by streptonigrin

The metabolism, cytotoxicity, and genotoxicity of streptonigrin (SN) w ere determined in two human colon carcinoma cell lines: HT-29 with high NAD(P)H:quinone oxidoreductase (EC 1.6.99.2, DTD) activity and BE with undetectable DTD activity. Dicumarol-sensitive oxidation of NADH was observed with HT-29 cytosol, but not with BE cytosol. Oxygen consumption was also observed using HT-29 cytosol, but was absent with BE cytosol. Dicumarol inhibited oxygen consumption with HT-29 cytosol, but deferoxamine had no effect, suggesting that divalent metal cations were not necessary for efficient auto-oxidation of SN hydroquinone. In cytotoxicity studies, SN was much more toxic to the DTD-rich HT-29 cells than to the DTD-deficient BE cells. Deferoxamine decreased toxicity in both cell lines, implicating hydroxyl radicals produced during Fenton-type reactions as the toxic species. In the genotoxicity assay, SN induced a much higher incidence of DNA strand breaks in HT-29 cells than in BE cells, and deferoxamine protected against DNA strand breaks in both cell lines. Some evidence of DNA repair was also observed in the two cell lines. These results support an important role for DTD in the cytotoxicity of SN in the high DTD HT-29 colon carcinoma cell line.

Evidence for DNA phosphate backbone alkylation and cleavage by pyrrolo[1,2-a]benzimidazoles: small molecules capable of causing base-pair-specific phosphodiester bond hydrolysis

This report presents evidence that a reduced pyrrolo[1,2-a]benzimidazole (PBI) cleaves DNA as a result of phosphate alkylation followed by hydrolysis of the resulting phosphate triester. The base-pair specificity of the phosphate alkylation results from Hoogsteen-type hydrogen bonding of the reduced PBI in the major groove at only A.T and G.C base pairs. Alkylated phosphates were detected by 31P NMR and the cleavage products were detected by 1H NMR and HPLC. Evidence is also presented that a reduced PBI interacts with DNA in the major groove rather than in the minor groove or by intercalation.

Enzymic- and thiol-mediated activation of halogen-substituted diaziridinylbenzoquinones: redox transitions of the semiquinone and semiquinone-thioether species

Activation of 2,5-diaziridinyl-1,4-benzoquinones bearing halogen (Cl, Br, or F) substituents at C3 and C6 by NADPH-cytochrome P450 reductase and glutathione nucleophilic substitution was examined in terms of free radical production and DNA strand scission. A semiquinone species was observed by direct ESR in aerobic conditions during: (a) NADPH-cytochrome P450 reductase-catalyzed reduction of the above quinones. (b) The interaction of these quinones with GSH entailing primarily reactivity of halogen substituents toward sulfur substitution. (c) NADPH-cytochrome P450 reductase-catalyzed activation of products resulting from the quinone/GSH interaction. The semiquinone ESR signal observed during enzymic catalysis was suppressed by superoxide dismutase and was not affected by catalase. ESR studies in conjunction with the spin trapping technique on the autoxidation of the semiquinones formed by the above reaction pathways indicated the formation of superoxide radicals. In addition, thiyl radicals were formed during the reactions following glutathione necleophilic substitution of the above quinones. The ESR signals of both superoxide and thiyl radicals were abolished by superoxide dismutase. No hydroxyl radicals were formed in solution during the redox transitions of these halogen-containing diaziridinylbenzoquinones. Bioreductive activation of these compounds via NADPH-cytochrome P450 reductase or sulfur nucleophilic substitution was associated with the formation of DNA strand breaks. This process was substantially inhibited (74-86%) by superoxide dismutase and to a lesser extent (23-31%) by catalase. It is suggested that DNA strand breakage proceeds in a manner entailing a semiquinone-dependent reduction of metal-ligands bound at the DNA surface and leading to site-specific, hydroxyl radical production.

Molecular dynamics simulations of chlorambucil/DNA adducts. A structural basis for the 5'-GNC interstrand DNA crosslink formed by nitrogen mustards

The alkylation of DNA by chlorambucil has been studied using a computational approach. Molecular dynamics simulations were performed on the fully solvated non-covalent complex, two monoadducts and a crosslinked diadduct of chlorambucil with the d(CGG3G2CGC).-d(GCG1CCCG) duplex, in which the N7 atoms of G1, G2 and G3 are potential alkylation sites. The results provide a structural basis for the preference of nitrogen mustards to crosslink DNA duplexes at a 5'-GNC site (a 1,3 crosslink, G1-G3) rather than at a 5'-GC sites (a 1,2 crosslink, G1-G2). In the non-covalent complex simulation the drug reoriented from a non-interstrand crosslinking location to a position favorable for G1-G3 diadduct formation. It proved possible to construct a G1-G3 diadduct from a structure from the non-covalent simulation, and continue the molecular dynamics calculation without further disruption of the DNA structure. A crosslinked diadduct developed with four BII conformations on the 3' side of each alkylated guanine and of their respective complementary cytosine. In the first monoadduct simulation the starting point was the same DNA conformation used in the crosslinked diadduct simulation with alkylation at G1. In this simulation the DNA deformation was reduced, with the helix returning to a more canonical form. A second monoadduct simulation was started from a canonical DNA conformation alkylated at G3. Here, no significant motion towards a potential crosslinking conformation occurred. Collectively, the results suggest that crosslink formation is dependent upon the drug orientation prior to alkylation and the required deformation of the DNA to permit 1,3 crosslinking can largely be achieved in the non-covalent complex.

Contribution of phenolic and quinonoid structures in the mutagenicity of the edible mushroom Agaricus bisporus

The objectives of this work were to establish the contribution of agaritine in the mutagenicity of ethanolic extracts from Agaricus bisporus and to examine the possible involvement of phenolic and quinonoid compounds in the mutagenic response to mushrooms. The mutagenic profile of agaritine in the Ames test, in the absence of an activation system, was different from that of the mushroom ethanolic extracts. Incorporation of rat hepatic cytosolic fractions as the activation system increased the mutagenicity of the mushroom ethanolic extracts in Salmonella typhimurium strain TA 104 but did not influence the mutagenicity of agaritine. It was concluded that agaritine is not the principal mutagenic component in the mushroom. The cytosol-induced mutagenicity of the mushroom extracts required NADPH, and was inhibited by dicoumarol and menadione. Moreover, the mutagenic response in the presence of cytosolic fractions was inhibited by superoxide dismutase, catalase, glutathione and dimethyl sulfoxide, thus implicating reactive oxygen species. Finally, tyrosinase, the enzyme converting mushroom phenols to quinones, increased the mutagenicity of the mushroom extracts. Collectively, the above results indicate that phenolic and quinonoid compounds, presumably through the generation of reactive oxygen species, may play a significant role in the mutagenicity of mushroom extracts.

DT-diaphorase in activation and detoxification of quinones. Bioreductive activation of mitomycin C

A role of DTD in the bioreductive activation of mitomycin C was supported by indirect evidence utilizing enzyme inhibitors in cellular systems. Using a cell-free system, we have confirmed that DTD can bioactivate mitomycin C using both purified rat and human DTD. Metabolism and bioactivation of mitomycin C by DTD is pH-dependent. At pH 7.8 alkylation of DTD leading to enzyme inhibition and DTD crosslinking occurs whereas at pH values of 7.4 and below metabolite formation, preservation of catalytic activity of DTD and sequence-selective DNA crosslinking occurs. Bioactivation of mitomycin C by DTD and the cytotoxicity of this drug in DTD-rich cell lines is oxygen-independent. Mitomycin C induces greater DNA crosslinking, even after chemical reduction, at lower pH values. This suggests that if mitomycin C is used in tumors with elevated DTD activity, greater therapeutic activity may be obtained by lowering intratumoral pH. Human NSCLC has elevated DTD activity relative to SCLC and normal lung and may be a target for the development of drugs which can be efficiently bioactivated by DTD. Because of the pH-dependent inactivation of DTD by mitomycin C, however, other drugs which are efficiently metabolized and bioactivated by DTD may be better candidates for the therapy of tumors high in DTD such as NSCLC.

Two structurally related diaziridinylbenzoquinones preferentially cross-link DNA at different sites upon reduction with DT-diaphorase

The nucleotide sequence preferences for the formation of interstrand cross-links induced in DNA by 2,5-diaziridinyl-1,4-benzoquinone (DZQ) and 3,6-dimethyl-2,5-diaziridinyl-1,4-benzoquinone (MeDZQ) were studied using synthetic duplex oligonucleotides and denaturing polyacrylamide gel electrophoresis (PAGE). Reaction of these bifunctional alkylating agents with a DNA duplex containing several potential cross-linking sites resulted in the formation of cross-linked DNAs with different electrophoretic mobilities. Analysis of the principal cross-linked products by piperidine fragmentation revealed that the preferential site of cross-linking was altered from a 5'-GNC to a 5'-GC sequence upon reduction of DZQ to the hydroquinone form by the enzyme DT-diaphorase. In contrast, the reduced form of MeDZQ was found to preferentially cross-link at 5'-GNC sites within the same sequence. These preferences were confirmed in duplex oligonucleotides containing single potential cross-linking sites. Additional minor cross-linked products were characterized and revealed that DZQ and MeDZQ are both capable of cross-linking across four base pairs in a 5'-GNNC sequence.

Reductive metabolism of diaziquone (AZQ) in the S9 fraction of MCF-7 cells. II. Enhancement of the alkylating activity of AZQ by NAD(P)H: quinone-acceptor oxidoreductase (DT-diaphorase)

The alkylating activity of reduced diaziquone was studied by the nitrobenzylpyridine (NBP) assay and was compared to those of the parent compound and aziridine-containing N,N',N"-triethylenethiophosphoramide (Thio-TEPA). Diaziquone (AZQ) was reduced enzymatically by 2e- using S9 cell fraction from MCF-7 cells which is rich in NAA(P)H:quinone-acceptor oxidoreductase (DT-diaphorase) (QAO) activity. One electron enzymatic reduction was performed with NADPH-cytochrome c reductase. The alkylating activity of AZQ increased 3-fold when reduced by 2e-. This increase was inhibited by dicumarol, an inhibitor of QAO. In contrast, the alkylating activity of AZQ did not increase beyond that of the parent compound when reduced by 1e- using purified NADPH-cytochrome c reductase. Similar results were obtained when AZQ was reduced chemically with borohydride (2e-) and with NADPH (1e-). Anaerobic incubations of AZQ with the S9 fraction of MCF-7 cells (2e- reduction) resulted in an increase in NBP alkylation over its aerobic counterpart (1.8-fold) while maintaining the near 3-fold increase in alkylation over untreated AZQ. In contrast, AZQ incubations with NADPH-cytochrome c reductase (1e- reduction) under the same conditions did not result in an NBP alkylation increase over untreated AZQ. These results indicate that AZQ hydroquinone is most likely the responsible species for the observed alkylation of this antitumor agent to DNA and other nucleophiles. The results also suggest that NAD(P)H:quinone-acceptor oxidoreductase is a very important enzyme in the bioactivation of AZQ.

Bioreductive activation of mitomycin C by DT-diaphorase

The role of DT-diaphorase (DTD, EC 1.6.99.2) in the bioreductive activation of mitomycin C was examined using purified rat hepatic DTD. The formation of adducts with reduced glutathione (GSH), binding of [3H]mitomycin C to DNA, and mitomycin C-induced DNA interstrand cross-linking were used as indicators of bioactivation. Mitomycin C was metabolized by DTD in a pH-dependent manner with increasing amounts of metabolism observed as the pH was decreased from 7.8 to 5.8. The major metabolite observed during DTD-mediated reduction of mitomycin C was 2,7-diaminomitosene. GSH adduct formation, binding of [3H]mitomycin C and mitomycin C-induced DNA interstrand cross-linking were observed during DTD-mediated metabolism. In agreement with the pH dependence of metabolism, increased bioactivation was observed at lower pH values. Temporal studies and experiments using authentic material showed that 2,7-diaminomitosene could be further metabolized by DTD resulting in the formation of mitosene adducts with GSH. DNA cross-linking during either chemical (sodium borohydride) or enzymatic (DTD) mediated reduction of mitomycin C could be observed at pH 7.4, but it increased as the pH was decreased to 5.8, showing the critical role of pH in the cross-linking process. These data provide unequivocal evidence that the obligate two-electron reductase DTD can bioactivate mitomycin C to reactive species which can form adducts with GSH and DNA and induce DNA cross-linking. The use of mitomycin C may be a viable approach to the therapy of tumors high in DTD activity, particularly when combined with strategies to lower tumor pH.