Mitomycin C-DNA adducts generated by DT-diaphorase. Revised mechanism of the enzymatic reductive activation of mitomycin C

Structural Basis of Substrate Recognition by the Postmitosane Modification Enzyme MitM in Mitomycin Biosynthesis

Mitomycins make up a class of natural molecules produced by Streptomyces with strong antibacterial and antitumor activities. MitM is a key postmitosane modification enzyme involved in mitomycin biosynthesis in Streptomyces caespitosus. This protein was previously suggested to catalyze the aziridinium methylation of mitomycin A and the mitomycin intermediate 9a-demethyl-mitomycin A as an N-methyltransferase. The structural basis for MitM to recognize cofactor S-adenosyl-l-methionine (SAM) and substrate mitomycin A is unknown. Here, we determined the crystal structures of apo-MitM and MitM-mitomycin A-S-adenosylhomocysteine (SAH) ternary complexes with resolutions of 2.23 and 2.80 Å, respectively. We found that MitM adopts a class I SAM-dependent methyltransferase fold and forms a homodimer in solution. Conformational changes in a series of residues involved in the formation of active pockets assist MitM in binding SAH and mitomycin A. In particular, the 28ALGAASLGE36 loop changes most significantly. When mitomycin A binds, the bending direction of this loop is reversed, changing the entrance of the active site from open to closed. This study provides structural insights into MitM's involvement in the postmitosane stage of mitomycin biosynthesis and provides a template for the engineering of methyltransferases.

Cyclopropanation and aziridination catalyzed by non-heme iron and 2-oxoglutarate dependent enzymes

Cyclopropane and azacyclopropane, also known as aziridine, moieties are found in natural products. These moieties serve as pivotal components that lead to a broad spectrum of biological activities. While diverse strategies involving various classes of enzymes are utilized to catalyze formation of these strained three-membered rings, how non-heme iron and 2-oxoglutarate (Fe/2OG) dependent enzymes enable regio- and stereo-selective C-C and C-N ring closure has only been reported very recently. Herein, we present detailed experimental protocols for mechanistically studying Fe/2OG enzymes that catalyze cyclopropanation and aziridination reactions. These protocols include protein purification, in vitro assays, biophysical spectroscopies, and isotope-tracer experiments. We also report how to use in silico approaches to look for Fe/2OG aziridinases. Furthermore, our current mechanistic understanding of three-membered ring formation is discussed. These results not only shed light on the reaction mechanisms of Fe/2OG enzymes-catalyzed cyclopropanation and aziridination, but also open avenues for expanding the reaction repertoire of the Fe/2OG enzyme superfamily.

Three-membered ring formation catalyzed by α-ketoglutarate-dependent nonheme iron enzymes

Epoxides, aziridines, and cyclopropanes are found in various medicinal natural products, including polyketides, terpenes, peptides, and alkaloids. Many classes of biosynthetic enzymes are involved in constructing these ring structures during their biosynthesis. This review summarizes our current knowledge regarding how α-ketoglutarate-dependent nonheme iron enzymes catalyze the formation of epoxides, aziridines, and cyclopropanes in nature, with a focus on enzyme mechanisms.

Insight Into Factors Governing Formation, Synthesis and Stereochemical Configuration of DNA Adducts Formed by Mitomycins

Mitomycin C, (MC), an antitumor drug used in the clinics, is a DNA alkylating agent. Inert in its native form, MC is reduced to reactive mitosenes in cellulo which undergo nucleophilic attack by DNA bases to form monoadducts as well as interstrand crosslinks (ICLs). These properties constitute the molecular basis for the cytotoxic effects of the drug. The mechanism of DNA alkylation by mitomycins has been studied for the past 30 years and, until recently, the consensus was that drugs of the mitomycins family mainly target CpG sequences in DNA. However, that paradigm was recently challenged. Here, we relate the latest research on both MC and dicarbamoylmitomycin C (DMC), a synthetic derivative of MC which has been used to investigate the regioselectivity of mitomycins DNA alkylation as well as the relationship between mitomycins reductive activation pathways and DNA adducts stereochemical configuration. We also review the different synthetic routes to access mitomycins nucleoside adducts and oligonucleotides containing MC/DMC DNA adducts located at a single position. Finally, we briefly describe the DNA structural modifications induced by MC and DMC adducts and how site specifically modified oligonucleotides have been used to elucidate the role each adduct plays in the drugs cytotoxicity.

Tracing of Acyl Carrier Protein-channeled Mitomycin Intermediates in Streptomyces caespitosus Facilitates Characterization of the Biosynthetic Steps for AHBA-GlcN Formation and Processing

Mitomycins are a family of naturally occurring, potent alkylating agents in which the C member has been clinically used for cancer chemotherapy for over 5 decades. In Streptomyces caespitosus, mitomycins are derived from an N-glycoside composed of a 3-amino-5-hydroxybenzoic acid (AHBA) unit and a d-glucosamine (GlcN) unit; however, how this N-glycoside is formed and rearranged to a mitosane, for example, the compact polycyclic ring system of mitomycin C, remains elusive. Benefiting from the development of a method used to trace the mitomycin intermediates that accumulate on an acyl carrier protein (ACP), we here dissect the enzymatic steps for AHBA-GlcN formation and processing to underlie the mitosane structure. Following the N-glycosylation of AHBA with activated N-acetyl-GlcN, deacetylation occurs on ACP to provide AHBA-GlcN. Then, the sugar portion of this N-glycoside is transformed into a linear aminodiol that terminates with an epoxyethane, yielding an ACP-channeled intermediate that is ready for mitosane formation through crosslinking between the AHBA and linearized sugar units. This transformation is unusual and relies on the functional association of a dihydronicotinamide adenine dinucleotide (phosphate)-dependent protein with a radical S-adenosyl-l-methionine protein. Characterization of these ACP-based enzymatic steps for AHBA-GlcN formation and processing sheds light on the poorly understood biosynthetic pathway of mitomycins.

Synthesis of Oligonucleotides Containing Trans Mitomycin C DNA Adducts at N6 of Adenine and N2 of Guanine

Mitomycin C, (MC), an antitumor drug, is a DNA alkylating agent currently used in the clinics. Inert in its native form, MC is reduced to reactive mitosenes, which undergo nucleophilic attack by guanine or adenine bases in DNA to form monoadducts as well as interstrand crosslinks (ICLs). Although ICLs are considered the most cytotoxic lesions, the role of each individual adduct in the drug's cytotoxicity is still not fully understood. Synthetic routes have been developed to access modified oligonucleotides containing dG MC-monoadducts and dG-MC-dG ICL at a single position of their base sequences to investigate the biological effects of these adducts. However, until now, oligonucleotides containing monoadducts formed by MC at the adenine base had not been available, thus preventing the examination of the role played by these lesions in the toxicity of MC. Here, we present a route to access these substrates. Structural proof of the adducted oligonucleotides were provided by enzymatic digestion to nucleosides and high-resolution mass spectral analysis. Additionally, parent oligonucleotides containing a dG monoadduct and a dG-MC-dG ICL were also produced. The stability and physical properties of all substrates were compared via CD spectroscopy and UV melting temperature studies. Finally, virtual models were created to explore the conformational space and structural features of these MC-DNA complexes.

Exerting DNA Damaging Effects of the Ilimaquinones through the Active Hydroquinone Species

Possessing the quinone moiety, ilimaquinone (1), a sponge–derived sesquiterpene quinone, has been hypothesised to express its cytotoxicity through a redox cycling process, yielding active product(s) that can cause DNA damage. To determine the DNA damaging effects of 1 and examine whether a redox transformation may participate in its functions, the DNA damaging properties of 1, the corresponding hydroquinone (2) and hydroquinone triacetates (3) and their 5-epimeric counterparts (4–6) were tested and compared. When incubated directly with plasmid DNA, the hydroquinones were the only active species capable of cleaving the DNA. In cell-based assays, however, the quinones and hydroquinone triacetates were active in the same range as that of the corresponding hydroquinones, and all damaged the cellular DNA in a similar manner. The in situ reduction of 1 and 4 were supported by the decreases in the cytotoxicity when cells were pre-exposed to dicoumarol, an NAD(P)H:quinone oxidoreductase 1 (NQO1) inhibitor. The results confirmed the DNA damaging activities of the ilimaquinones 1 and 4, and indicated the necessity to undergo an in-situ transformation into the active hydroquinones, thereby exerting the DNA damaging properties as parts of the cytotoxic mechanisms.

Identification of a Dps contamination in Mitomycin-C-induced expression of Colicin Ia

Colicins are bacterial toxins targeting Gram-negative bacteria, including E. coli and related Enterobacteriaceae strains. Some colicins form ion-gated pores in the inner membrane of attacked bacteria that are lethal to their target. Colicin Ia was the first pore-forming E. coli toxin, for which a high-resolution structure of the monomeric full-length protein was determined. It is so far also the only colicin, for which a low-resolution structure of its membrane-inserted pore was reported by negative-stain electron microscopy. Resolving this structure at the atomic level would allow an understanding of the mechanism of toxin pore formation. Here, we report an observation that we made during an attempt to determine the Colicin Ia pore structure at atomic resolution. Colicin Ia was natively expressed by mitomycin-C induction under a native SOS promotor and purified following published protocols. The visual appearance in the electron microscope of negatively stained preparations and the lattice parameters of 2D crystals obtained from the material were highly similar to those reported earlier resulting from the same purification protocol. However, a higher-resolution structural analysis revealed that the protein is Dps (DNA-binding protein from starved cells), a dodecameric E. coli protein. This finding suggests that the previously reported low-resolution structure of a "Colicin Ia oligomeric pore" actually shows Dps.

Translesion DNA Synthesis and Reinitiation of DNA Synthesis in Chemotherapy Resistance

Many chemotherapy drugs block tumor cell division by damaging DNA. DNA polymerases eta (Pol η), iota (Pol ι), kappa (Pol κ), REV1 of the Y-family and zeta (Pol ζ) of the B-family efficiently incorporate nucleotides opposite a number of DNA lesions during translesion DNA synthesis. Primase-polymerase PrimPol and the Pol α-primase complex reinitiate DNA synthesis downstream of the damaged sites using their DNA primase activity. These enzymes can decrease the efficacy of chemotherapy drugs, contribute to the survival of tumor cells and to the progression of malignant diseases. DNA polymerases are promising targets for increasing the effectiveness of chemotherapy, and mutations and polymorphisms in some DNA polymerases can serve as additional prognostic markers in a number of oncological disorders.

Interdependent Sequence Selectivity and Diastereoselectivity in the Alkylation of DNA by Decarbamoylmitomycin C

Mitomycin C (MC), an antitumor drug, and decarbamoylmitomycin C (DMC), a derivative of MC, alkylate DNA and form deoxyguanosine monoadducts and interstrand crosslinks (ICLs). Interestingly, in mammalian culture cells, MC forms primarily deoxyguanosine adducts with a 1"-R stereochemistry at the guanine-mitosene bond (1"-α) whereas DMC forms mainly adducts with a 1"-S stereochemistry (1"-β). The molecular basis for the stereochemical configuration exhibited by DMC has been investigated using biomimetic synthesis. Here, we present the results of our studies on the monoalkylation of DNA by DMC. We show that the formation of 1"-β-deoxyguanosine adducts requires bifunctional reductive activation of DMC, and that monofunctional activation only produces 1"-α-adducts. The stereochemistry of the deoxyguanosine adducts formed is also dependent on the regioselectivity of DNA alkylation and on the overall DNA CG content. Additionally, we found that temperature plays a determinant role in the regioselectivity of duplex DNA alkylation by mitomycins: At 0 °C, both deoxyadenosine (dA) and deoxyguanosine (dG) alkylation occur whereas at 37 °C, mitomycins alkylate dG preferentially. The new reaction protocols developed in our laboratory to investigate DMC-DNA alkylation raise the possibility that oligonucleotides containing DMC 1"-β-deoxyguanosine adducts at a specific site may be synthesized by a biomimetic approach.

Isolation and Rationale for the Formation of Isomeric Decarbamoylmitomycin C- N6-deoxyadenosine Adducts in DNA

Mitomycin C (MC) is an anticancer agent that alkylates DNA to form monoadducts and interstrand cross-links. Decarbamoylmitomycin C (DMC) is an analogue of MC lacking the carbamate on C10. The major DNA adducts isolated from treatment of culture cells with MC and DMC are N2-deoxyguanosine (dG) adducts and adopt an opposite stereochemical configuration at the dG-mitosene bond. To elucidate the molecular mechanisms of DMC-DNA alkylation, we have reacted short oligonucleotides, calf thymus, and M. luteus DNA with DMC using biomimetic conditions. These experiments revealed that DMC is able to form two stereoisomeric deoxyadenosine (dA) adducts with DNA under bifuntional reduction conditions and at low temperature. The dA-DMC adducts formed were detected and quantified by HPLC analysis after enzymatic digestion of the alkylated DNA substrates. Results revealed the following rules for DMC dA alkylation: (i) DMC dA adducts are formed at a 48- to 4-fold lower frequency than dG adducts, (ii) the 5'-phosphodiester linkage of the dA adducts is resistant to snake venom diesterase, (iii) end-chain dA residues are more reactive than internal ones in duplex DNA, and (iv) nucleophilic addition by dA occurs on both faces of DMC and the ratio of stereoisomeric dA adducts formed is dependent on the end bases located at the 3' or 5' position. A key finding was to discover that temperature plays a determinant role in the regioselectivity of duplex DNA alkylation by DMC: at 0 °C, both dA and dG alkylation occur, whereas at 37 °C, DMC preferentially alkylates dG residues.

Cellular Proliferation and Migration of Human Pterygium Cells: Mitomycin Versus Small-Molecule Inhibitors

PURPOSE

Nutlin is a drug that has been reported to activate p53 in various cell lines. We aim to study the effects of Nutlin in pterygium and compare the effects of Nutlin and mitomycin C (MMC) in pterygium cell lines.

METHODS

Pterygium samples (n = 3) were collected during surgical excision. Normal conjunctival tissues (n = 3) were collected from another quadrant of the same eye. Cell lines were established, and cells from passages 2 to 5 were used. Pterygium and conjunctival cells were treated with different doses of Nutlin and MMC. Cell proliferation and cell migration were measured.

RESULTS

Cell proliferation was reduced by 39-fold after treatment with 50 μM Nutlin. Cell migration was inhibited with increasing dosages of Nutlin (95% and 28% after treating with 2 and 50 μM Nutlin, respectively). Compared with MMC, Nutlin induced more pterygium cell death and less conjunctival cell death at low doses. At 50% lethal dose for pterygium cells, 95% of conjunctival cells survived after Nutlin treatment, whereas only 63% of conjunctival cells survived after MMC treatment. p21 expression was not detectable in MMC-treated pterygium cells but was detectable after Nutlin treatment.

CONCLUSIONS

In our study, MMC induced cell death in pterygium and conjunctival cell lines, whereas Nutlin had a targeted impact on pterygium cells. Our results implied that MMC inhibited both pterygium cell proliferation and migration through an apoptosis-independent pathway.

Sequence-Dependent Diastereospecific and Diastereodivergent Crosslinking of DNA by Decarbamoylmitomycin C

Mitomycin C (MC), a potent antitumor drug, and decarbamoylmitomycin C (DMC), a derivative lacking the carbamoyl group, form highly cytotoxic DNA interstrand crosslinks. The major interstrand crosslink formed by DMC is the C1'' epimer of the major crosslink formed by MC. The molecular basis for the stereochemical configuration exhibited by DMC was investigated using biomimetic synthesis. The formation of DNA-DNA crosslinks by DMC is diastereospecific and diastereodivergent: Only the 1''S-diastereomer of the initially formed monoadduct can form crosslinks at GpC sequences, and only the 1''R-diastereomer of the monoadduct can form crosslinks at CpG sequences. We also show that CpG and GpC sequences react with divergent diastereoselectivity in the first alkylation step: 1"S stereochemistry is favored at GpC sequences and 1''R stereochemistry is favored at CpG sequences. Therefore, the first alkylation step results, at each sequence, in the selective formation of the diastereomer able to generate an interstrand DNA-DNA crosslink after the "second arm" alkylation. Examination of the known DNA adduct pattern obtained after treatment of cancer cell cultures with DMC indicates that the GpC sequence is the major target for the formation of DNA-DNA crosslinks in vivo by this drug.

Combined molecular modelling and 3D-QSAR study for understanding the inhibition of NQO1 by heterocyclic quinone derivatives

A combination of three-dimensional quantitative structure-activity relationship (3D-QSAR), and molecular modelling methods were used to understand the potent inhibitory NAD(P)H:quinone oxidoreductase 1 (NQO1) activity of a set of 52 heterocyclic quinones. Molecular docking results indicated that some favourable interactions of key amino acid residues at the binding site of NQO1 with these quinones would be responsible for an improvement of the NQO1 activity of these compounds. The main interactions involved are hydrogen bond of the amino group of residue Tyr128, π-stacking interactions with Phe106 and Phe178, and electrostatic interactions with flavin adenine dinucleotide (FADH) cofactor. Three models were prepared by 3D-QSAR analysis. The models derived from Model I and Model III, shown leave-one-out cross-validation correlation coefficients (q²_LOO ) of .75 and .73 as well as conventional correlation coefficients (R² ) of .93 and .95, respectively. In addition, the external predictive abilities of these models were evaluated using a test set, producing the predicted correlation coefficients (r²_pred ) of .76 and .74, respectively. The good concordance between the docking results and 3D-QSAR contour maps provides helpful information about a rational modification of new molecules based in quinone scaffold, in order to design more potent NQO1 inhibitors, which would exhibit highly potent antitumor activity.

Preliminary study of mechanism of action of SN38 derivatives. Physicochemical data, evidence of interaction and alkylation of DNA octamer d(GCGATCGC)2

The synthesis of water-soluble SN38 derivatives is presented, and their stability in solutions used during drug development studies has been investigated. A preliminary study of mechanism of action of 9-aminomethyl SN38 is presented. Using NMR techniques, the interaction of the oligomer d(GCGATCGC)₂ is studied, showing that the terminal GC base pairs are the main site of interaction. Using pulsed field gradient spin echo and mass spectroscopy, evidence of a spontaneous alkylation reaction of the DNA oligomer with SN38 derivatives is presented. A proposed mechanism of reaction is suggested. Copyright © 2016 John Wiley & Sons, Ltd.

Cellular response to DNA interstrand crosslinks: the Fanconi anemia pathway

Interstrand crosslinks (ICLs) are a highly toxic form of DNA damage. ICLs can interfere with vital biological processes requiring separation of the two DNA strands, such as replication and transcription. If ICLs are left unrepaired, it can lead to mutations, chromosome breakage and mitotic catastrophe. The Fanconi anemia (FA) pathway can repair this type of DNA lesion, ensuring genomic stability. In this review, we will provide an overview of the cellular response to ICLs. First, we will discuss the origin of ICLs, comparing various endogenous and exogenous sources. Second, we will describe FA proteins as well as FA-related proteins involved in ICL repair, and the post-translational modifications that regulate these proteins. Finally, we will review the process of how ICLs are repaired by both replication-dependent and replication-independent mechanisms.

Comparative Error-Free and Error-Prone Translesion Synthesis of N(2)-2'-Deoxyguanosine Adducts Formed by Mitomycin C and Its Metabolite, 2,7-Diaminomitosene, in Human Cells

Mitomycin C (MC) is a cytotoxic and mutagenic antitumor agent that alkylates DNA upon reductive activation. 2,7-Diaminomitosene (2,7-DAM) is a major metabolite of MC in tumor cells, which also alkylates DNA. MC forms seven DNA adducts, including monoadducts and inter- and intrastrand cross-links, whereas 2,7-DAM forms two monoadducts. Herein, the biological effects of the dG-N(2) adducts formed by MC and 2,7-DAM have been compared by constructing single-stranded plasmids containing these adducts and replicating them in human embryonic kidney 293T cells. Translesion synthesis (TLS) efficiencies of dG-N(2)-MC and dG-N(2)-2,7-DAM were 38 ± 3 and 27 ± 3%, respectively, compared to that of a control plasmid. This indicates that both adducts block DNA synthesis and that dG-N(2)-2,7-DAM is a stronger replication block than dG-N(2)-MC. TLS of each adducted construct was reduced upon siRNA knockdown of pol η, pol κ, or pol ζ. For both adducts, the most significant reduction occurred with knockdown of pol κ, which suggests that pol κ plays a major role in TLS of these dG-N(2) adducts. Analysis of the progeny showed that both adducts were mutagenic, and the mutation frequencies (MF) of dG-N(2)-MC and dG-N(2)-2,7-DAM were 18 ± 3 and 10 ± 1%, respectively. For both adducts, the major type of mutation was G → T transversions. Knockdown of pol η and pol ζ reduced the MF of dG-N(2)-MC and dG-N(2)-2,7-DAM, whereas knockdown of pol κ increased the MF of these adducts. This suggests that pol κ predominantly carries out error-free TLS, whereas pol η and pol ζ are involved in error-prone TLS. The largest reduction in MF by 78 and 80%, respectively, for dG-N(2)-MC and dG-N(2)-2,7-DAM constructs occurred when pol η, pol ζ, and Rev1 were simultaneously knocked down. This result strongly suggests that, unlike pol κ, these three TLS polymerases cooperatively perform the error-prone TLS of these adducts.

Synthesis of Mitomycin C and Decarbamoylmitomycin C N(2) deoxyguanosine-adducts

Mitomycin C (MC) and Decarbamoylmitomycin C (DMC) - a derivative of MC lacking the carbamate on C10 - are DNA alkylating agents. Their cytotoxicity is attributed to their ability to generate DNA monoadducts as well as intrastrand and interstrand cross-links (ICLs). The major monoadducts generated by MC and DMC in tumor cells have opposite stereochemistry at carbon one of the guanine-mitosene bond: trans (or alpha) for MC and cis (or beta) for DMC. We hypothesize that local disruptions of DNA structure from trans or cis adducts are responsible for the different biochemical responses produced by MC and DMC. Access to DNA substrates bearing cis and trans MC/DMC lesions is essential to verify this hypothesis. Synthetic oligonucleotides bearing trans lesions can be obtained by bio-mimetic methods. However, this approach does not yield cis adducts. This report presents the first chemical synthesis of a cis mitosene DNA adduct. We also examined the stereopreference exhibited by the two drugs at the mononucleotide level by analyzing the formation of cis and trans adducts in the reaction of deoxyguanosine with MC or DMC using a variety of activation conditions. In addition, we performed Density Functional Theory calculations to evaluate the energies of these reactions. Direct alkylation under autocatalytic or bifunctional conditions yielded preferentially alpha adducts with both MC and DMC. DFT calculations showed that under bifunctional activation, the thermodynamically favored adducts are alpha, trans, for MC and beta, cis, for DMC. This suggests that the duplex DNA structure may stabilize/oriente the activated pro-drugs so that, with DMC, formation of the thermodynamically favored beta products are possible in a cellular environment.

Modification of cellular DNA by synthetic aziridinomitosenes

Two synthetic aziridinomitosenes (AZMs), Me-AZM and H-AZM, structurally related to mitomycin C (MC) were evaluated for their anticancer activity against six cancer cell lines (HeLa, Jurkat, T47D, HepG2, HL-60, and HuT-78) and tested for their DNA-modifying abilities in Jurkat cells. Cytotoxicity assays showed that Me-AZM is up to 72-fold and 520-fold more potent than MC and H-AZM, respectively. Me-AZM also demonstrated increased DNA modification over MC and H-AZM in alkaline COMET and Hoechst fluorescence assays that measured crosslinks in cellular DNA. Me-AZM and H-AZM treatment of Jurkat cells was found to sponsor significant DNA-protein crosslinks using a K-SDS assay. The results clearly indicate that the AZM C6/C7 substitution pattern plays an important role in drug activity and supports both DNA-DNA and DNA-protein adduct formation as mechanisms for inducing cytotoxic effects.

Genome-Wide Mutational Signature of the Chemotherapeutic Agent Mitomycin C in Caenorhabditis elegans

Cancer therapy largely depends on chemotherapeutic agents that generate DNA lesions. However, our understanding of the nature of the resulting lesions as well as the mutational profiles of these chemotherapeutic agents is limited. Among these lesions, DNA interstrand crosslinks are among the more toxic types of DNA damage. Here, we have characterized the mutational spectrum of the commonly used DNA interstrand crosslinking agent mitomycin C (MMC). Using a combination of genetic mapping, whole genome sequencing, and genomic analysis, we have identified and confirmed several genomic lesions linked to MMC-induced DNA damage in Caenorhabditis elegans. Our data indicate that MMC predominantly causes deletions, with a 5'-CpG-3' sequence context prevalent in the deleted regions of DNA. Furthermore, we identified microhomology flanking the deletion junctions, indicative of DNA repair via nonhomologous end joining. Based on these results, we propose a general repair mechanism that is likely to be involved in the biological response to this highly toxic agent. In conclusion, the systematic study we have described provides insight into potential sequence specificity of MMC with DNA.

A dip-stick type biosensor using bioluminescent bacteria encapsulated in color-coded alginate microbeads for detection of water toxicity

The use of genetically engineered bioluminescent bacteria, in which bioluminescence is induced by different modes of toxic action, represents an alternative to acute toxicity tests using living aquatic organisms (plants, vertebrates, or invertebrates) in an aqueous environment.

Local chemotherapeutic agents for the treatment of ocular malignancies

We critically analyze available peer-reviewed literature, including clinical trials and case reports, on local ocular cancer treatments. Recent innovations in many areas of ocular oncology have introduced promising new therapies, but, for the most part, the optimal treatment of ocular malignancies remains elusive.

Tandem mass spectrometry for characterization of covalent adducts of DNA with anticancer therapeutics

The chemotherapeutic activities of many anticancer and antibacterial drugs arise from their interactions with nucleic acid substrates. Some of these ligands interact with DNA in a way that causes conformational changes or damage to the nucleic acid targets, ultimately altering recognition by key DNA-specific enzymes, interfering with DNA transcription or prohibiting replication, and terminating cell growth and proliferation. The design and synthesis of ligands that bind to nucleic acids remains a dynamic field in medicinal chemistry and pharmaceutical research. The quest for more selective and efficacious DNA-interactive anticancer chemotherapeutics has likewise catalyzed the need for sensitive analytical methods that can provide structural information about the nature of the resulting DNA adducts and provide insight into the mechanistic pathways of the DNA/drug interactions and the impact on the cellular processes in biological systems. This review focuses on the array of tandem mass spectrometric strategies developed and applied for characterization of covalent adducts formed between DNA and anticancer ligands.

A new mechanism of action for the anticancer drug mitomycin C: mechanism-based inhibition of thioredoxin reductase

Mitomycin C (MMC) is a chemotherapeutic drug that requires an enzymatic bioreduction to exert its biological effects. Upon reduction, MMC is converted into a highly reactive bis-electrophilic intermediate that alkylates cellular nucleophiles. Alkylation of DNA is the most favored mechanism of action for MMC, but other modes of action, such as redox cycling and inhibition of rRNA, may also contribute to the biological action of the drug. In this work, we show that thioredoxin reductase (TrxR) is also a cellular target for MMC. We show that MMC inhibits TrxR in vitro, using purified enzyme, and in vivo, using cancer cell cultures. The inactivation presents distinctive parameters of mechanism-based inhibitors: it is time- and concentration-dependent and irreversible. Additionally, spectroscopic experiments (UV, circular dichroism) show that the inactivated enzyme contains a mitosene chromophore. On the basis of kinetic and spectroscopic data, we propose a chemical mechanism for the inactivation of the enzyme that starts with a reduction of the quinone ring of MMC by the selenolthiol active site of TrxR and a subsequent alkylation of the active site by the activated drug. We also report that MMC inactivates TrxR in cancer cell cultures and that this inhibition correlates directly with the cytotoxicity of the drug, indicating that inhibition of TrxR may play a major role in the biological mode of action of the drug.

Stereoselective synthesis of complex polycyclic aziridines: use of the Brønsted acid-catalyzed aza-Darzens reaction to prepare an orthogonally protected mitomycin C intermediate with maximal convergency

A concise synthesis of a highly functionalized intermediate lacking only C10 of the mitomycin backbone is described. The key to this development is the Brønsted acid-catalyzed aza-Darzens reaction used to forge the cis-aziridine. Additionally an oxidative ketalization fortuitously occurs during the quinone-enamine coupling step, leading to an orthogonally protected hydroquinone.

Resistance of uveal melanoma to the interstrand cross-linking agent mitomycin C is associated with reduced expression of CYP450R

BACKGROUND

Uveal melanoma (UM) is the most common primary intraocular tumour of adults, frequently metastasising to the liver. Hepatic metastases are difficult to treat and are mainly unresponsive to chemotherapy. To investigate why UM are so chemo-resistant we explored the effect of interstrand cross-linking agents mitomycin C (MMC) and cisplatin in comparison with hydroxyurea (HU).

METHODS

Sensitivity to MMC, cisplatin and HU was tested in established UM cell lines using clonogenic assays. The response of UM to MMC was confirmed in MTT assays using short-term cultures of primary UM. The expression of cytochrome P450 reductase (CYP450R) was analysed by western blotting, and DNA cross-linking was assessed using COMET analysis supported by γ-H2AX foci formation.

RESULTS

Both established cell lines and primary cultures of UM were resistant to the cross-linking agent MMC (in each case P<0.001 in Student's t-test compared with controls). In two established UM cell lines, DNA cross-link damage was not induced by MMC (in both cases P<0.05 in Students's t-test compared with damage induced in controls). In all, 6 out of 6 UMs tested displayed reduced expression of the metabolising enzyme CYP450R and transient expression of CYP450R increased MMC sensitivity of UM.

CONCLUSION

We suggest that reduced expression of CYP450R is responsible for MMC resistance of UM, through a lack of bioactivation, which can be reversed by complementing UM cell lines with CYP450R.

The SNM1/Pso2 family of ICL repair nucleases: from yeast to man

Efficient interstrand crosslink (ICL) repair in yeast depends on the Pso2/Snm1 protein. Pso2 is a member of the highly conserved metallo-beta-lactamase structural family of nucleases. Mammalian cells possess three SNM1/Pso2 related proteins, SNM1A, SNM1B/Apollo, and SNM1C/Artemis. Evidence that SNM1A and SNM1B contribute to ICL repair is mounting, whereas Artemis appears to primarily contribute to non-ICL repair pathways, particularly some double-strand break repair events. Yeast Pso2 and all three mammalian SNM1-family proteins have been shown to possess nuclease activity. Here, we review the biochemical, genetic, and cellular evidence for the SNM1 family as DNA repair factors, focusing on ICL repair.

Mitomycins syntheses: a recent update

Mitomycins are a class of very potent antibacterial and anti-cancer compounds having a broad activity against a range of tumours. They have been used in clinics since the 1960's, and the challenges represented by their total synthesis have challenged generations of chemists. Despite these chemical and medicinal features, these compounds, in racemic form, have succumbed to total synthesis only four times over the last 30 years.

Quinone methide chemistry of prekinamycins: 13C-labeling, spectral global fitting and in vitro studies

In this article, we address the presence of the prekinamycin quinone methide using the techniques of spectral global fitting and the 13C-labeling of the reactive centre. Two-electron reduction of a prekinamycin affords a long-lived quinone methide species that was characterised spectrally. A correlation was made between the calculated DeltaE (kcal/mol) values for quinone methide tautomerisation and cytostatic activity to support the postulate that the quinone methide plays a role in prekinamycin biological activity. We also prepared a stable quinone methide of prekinamycin and studied its solution chemistry directly.

Mapping DNA adducts of mitomycin C and decarbamoyl mitomycin C in cell lines using liquid chromatography/ electrospray tandem mass spectrometry

The antitumor antibiotic and cancer chemotherapeutic agent mitomycin C (MC) alkylates and crosslinks DNA, forming six major MC-deoxyguanosine adducts of known structures in vitro and in vivo. Two of these adducts are derived from 2,7-diaminomitosene (2,7-DAM), a nontoxic reductive metabolite of MC formed in cells in situ. Several methods have been used for the analysis of MC-DNA adducts in the past; however, a need exists for a safer, more comprehensive and direct assay of the six-adduct complex. Development of an assay, based on mass spectrometry, is described. DNA from EMT6 mouse mammary tumor cells, Fanconi Anemia-A fibroblasts, normal human fibroblasts, and MCF-7 human breast cancer cells was isolated after MC or 10-decarbamoyl mitomycin C (DMC) treatment of the cells, digested to nucleosides, and submitted to liquid chromatography electrospray-tandem mass spectrometry. Two fragments of each parent ion were monitored ("multiple reaction monitoring"). Identification and quantitative analysis were based on a standard mixture of six adducts, the preparation of which is described here in detail. The lower limit of detection of adducts is estimated as 0.25 pmol. Three initial applications of this method are reported as follows: (i) differential kinetics of adduct repair in EMT6 cells, (ii) analysis of adducts in MC- or DMC-treated Fanconi Anemia cells, and (iii) comparison of the adducts generated by treatment of MCF-7 breast cancer cells with MC and DMC. Notable results are the following: Repair removal of the DNA interstrand cross-link and of the two adducts of 2,7-DAM is relatively slow; both MC and DMC generate DNA interstrand cross-links in human fibroblasts, Fanconi Anemia-A fibroblasts, and MCF-7 cells as well as EMT6 cells; and DMC shows a stereochemical preference of linkage to the guanine-2-amino group opposite from that of MC.

Relationship between DNA lesions, DNA repair and chromosomal damage induced by acetaldehyde

Acetaldehyde (AA) was tested along with two other crosslinking agents: formaldehyde (FA), an inducer of DNA-protein crosslinks (DPCs) and mitomycin C (MMC), an inducer of interstrand crosslinks (ICLs), to find out whether the mechanism of action of AA resembles more MMC or FA. Using a modification of the standard protocol for comet assay we demonstrate that AA induces crosslinks. Using a combination of alkaline comet version and proteinase-K, a clear abrogation of AA-induced reduction in DNA migration, like after FA treatment, was observed demonstrating that both agents induce DPCs, whereas MMC induces predominantly ICLs. A possible correlation between the types of induced crosslink and the induction chromosome damage in different repair deficient mutant Chinese hamster ovary cell lines treated with AA, MMC and FA was investigated. TCR/NER pathways are involved in repairing FA induced DPCs, but less in AA-induced DPCs. Our preliminary data suggest that DPCs are weaker inducers of SCEs in comparison with ICLs.

Involvement of a periplasmic protein kinase in DNA strand break repair and homologous recombination in Escherichia coli

The involvement of signal transduction in the repair of radiation-induced damage to DNA has been known in eukaryotes but remains understudied in bacteria. This article for the first time demonstrates a role for the periplasmic lipoprotein (YfgL) with protein kinase activity transducing a signal for DNA strand break repair in Escherichia coli. Purified YfgL protein showed physical as well as functional interaction with pyrroloquinoline-quinone in solution and the protein kinase activity of YfgL was strongly stimulated in the presence of pyrroloquinoline-quinone. Transgenic E. coli cells producing Deinococcus radiodurans pyrroloquinoline-quinone synthase showed nearly four log cycle improvement in UVC dark survival and 10-fold increases in gamma radiation resistance as compared with untransformed cells. Pyrroloquinoline-quinone enhanced the UV resistance of E. coli through the YfgL protein and required the active recombination repair proteins. The yfgL mutant showed higher sensitivity to UVC, mitomycin C and gamma radiation as compared with wild-type cells and showed a strong impairment in homologous DNA recombination. The mutant expressing an active YfgL in trans recovered the lost phenotypes to nearly wild-type levels. The results strongly suggest that the periplasmic phosphoquinolipoprotein kinase YfgL plays an important role in radiation-induced DNA strand break repair and homologous recombination in E. coli.

Mitomycin-DNA adducts induce p53-dependent and p53-independent cell death pathways

10-Decarbamoyl-mitomycin C (DMC), a mitomycin C (MC) derivative, generates an array of DNA monoadducts and interstrand cross-links stereoisomeric to those that are generated by MC. DMC was previously shown in our laboratory to exceed the cytotoxicity of MC in a human leukemia cell line that lacks a functional p53 pathway (K562). However, the molecular signal transduction pathway activated by DMCDNA adducts has not been investigated. In this study, we have compared molecular targets associated with signaling pathways activated by DMC and MC in several human cancer cell lines. In cell lines lacking wild-type p53, DMC was reproducibly more cytotoxic than MC, but it generated barely detectable signal transduction markers associated with apoptotic death. Strikingly, DMCs increased cytotoxicity was not associated with an increase in DNA double-strand breaks but was associated with early poly(ADP-ribose) polymerase (PARP) activation and Chk1 kinase depletion. Alkylating agents can induce increased PARP activity associated with programmed necrosis, and the biological activity of DMC in p53-null cell lines fits this paradigm. In cell lines with a functional p53 pathway, both MC and DMC induced apoptosis. In the presence of p53, both MC and DMC activate procaspases; however, the spectrum of procaspases involved differs for the two drugs, as does induction of p73. These studies suggest that in the absence of p53, signaling to molecular targets in cell death can shift in response to different DNA adduct structures to induce non-apoptotic cell death.

The Anticancer Drug Ellipticine is an Inducer of Rat NAD(P)H:Quinone Oxidoreductase

The antineoplastic agent ellipticine was investigated for its ability to induce the biotransformation enzyme NAD(P)H:quinone oxidoreductase (DT-diaphorase, EC 1.6.99.2) in male Wistar rats. Using the real-time polymerase chain reaction, the levels of NAD(P)H:quinone oxidoreductase mRNA were determined in livers, kidneys and lungs of rats treated intraperitoneally with ellipticine (40 mg/kg body weight) and of control (untreated) rats. Cytosolic fractions were isolated from the same tissues of control and ellipticine-treated rats and tested for NAD(P)H:quinone oxidoreductase protein expression and its enzymatic activity. The results demonstrate that ellipticine is a potent inducer of NAD(P)H:quinone oxidoreductase in rat livers and kidneys, while no induction of this enzyme was detectable in rat lungs. The increase in levels of NAD(P)H:quinone oxidoreductase mRNA correlates with the increase in expression of its protein and enzymatic activity, measured with menadione and 3-nitrobenzanthrone as substrates. The results, the identification of the potential of ellipticine to induce NAD(P)H:quinone oxidoreductase, suggest that this drug is capable of modulating biological efficiencies of the toxicants and/or drugs that are reductively metabolized by this enzyme.

A newly established GDL1 cell line from gpt delta mice well reflects the in vivo mutation spectra induced by mitomycin C

In order to create a novel in vitro test system for detection of large deletions and point mutations, we developed an immortalized cell line. A SV40 large T antigen expression unit was introduced into fibroblasts derived from gpt delta mouse lung tissue and a selected clone was established as the gpt delta L1 (GDL1) cell line. The novel GDL1 cells were examined for mutant frequencies (MFs) and for molecular characterization of mutations induced by mitomycin C (MMC). The GDL1 cells were treated with MMC at doses of 0.025, 0.05, and 0.1 microg/mL for 24h and mutations were detected by Spi- and 6-thioguanine (6-TG) selections. The MFs of the MMC-treated cells increased up to 3.4-fold with Spi- selection and 3.5-fold with 6-TG selection compared to MFs of untreated cells. In the Spi- mutants, the number of large (up to 76 kilo base pair (kbp)) deletion mutations increased. A majority of the large deletion mutations had 1-4 base pairs (bp) of microhomology in the deletion junctions. A number of the rearranged deletion mutations were accompanied with deletions and insertions of up to 1.1 kbp. In the gpt mutants obtained from 6-TG selection, single base substitutions of G:C to T:A, tandem base substitutions occurring at the 5'-GG-3' or 5'-CG-3' sequence, and deletion mutations larger than 2 bp were increased. We compared the spectrum of MMC-induced mutations observed in vitro to that of in vivo using gpt delta mice, which we reported previously. Although a slight difference was observed in MMC-induced mutation spectra between in vitro and in vivo, the mutations detected in vitro included all of the types of mutations observed in vivo. The present study demonstrates that the newly established GDL1 cell line is a useful tool to detect and analyze various mutations including large deletions in mammalian cells.

RETRACTED: Si RNA inhibition of GRP58 associated with decrease in mitomycin C-induced DNA cross-linking and cytotoxicity

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted at the request of the Office of Integrity of the University of Maryland due to data entered in Fig 3 of the publication that were not supported by raw data, in addition to the fact that the statistical evaluations were adultered.

DNA repair, a novel antibacterial target: Holliday junction-trapping peptides induce DNA damage and chromosome segregation defects

Holliday junction intermediates arise in several central pathways of DNA repair, replication fork restart, and site-specific recombination catalysed by tyrosine recombinases. Previously identified hexapeptide inhibitors of phage lambda integrase-mediated recombination block the resolution of Holliday junction intermediates in vitro and thereby inhibit recombination, but have no DNA cleavage activity themselves. The most potent peptides are specific for the branched DNA structure itself, as opposed to the integrase complex. Based on this activity, the peptides inhibit several unrelated Holliday junction-processing enzymes in vitro, including the RecG helicase and RuvABC junction resolvase complex. We have found that some of these hexapeptides are potent bactericidal antimicrobials, effective against both Gm+ and Gm- bacteria. Using epifluorescence microscopy and flow cytometry, we have characterized extensively the physiology of bacterial cells treated with these peptides. The hexapeptides cause DNA segregation abnormalities, filamentation and DNA damage. Damage caused by the peptides induces the SOS response, and is synergistic with damage caused by UV and mitomycin C. Our results are consistent with the model that the hexapeptides affect DNA targets that arise during recombination-dependent repair. We propose that the peptides trap intermediates in the repair of collapsed replication forks, preventing repair and resulting in bacterial death. Inhibition of DNA repair constitutes a novel target of antibiotic therapy. The peptides affect targets that arise in multiple pathways, and as expected, are quite resistant to the development of spontaneous antibiotic resistance.

Bioreduction activated prodrugs of camptothecin: molecular design, synthesis, activation mechanism and hypoxia selective cytotoxicity

Several water-soluble derivatives (CPT3, CPT3a-d) of camptothecin (CPT) were synthesized, among which CPT3 bearing an N,N'-dimethyl-1-aminoethylcarbamate side-chain was further conjugated with reductively eliminating structural units of indolequinone, 4-nitrobenzyl alcohol and 4-nitrofuryl alcohol to produce novel prodrugs of camptothecin (CPT4-6). All CPT derivatives were of lower cytotoxicity than their parent compound of CPT. In contrast, CPT4 and CPT6 showed higher hypoxia selectivity of cytotoxicity towards tumor cells than CPT. A mechanism by which a representative prodrug CPT4 is activated in the presence of DT-diaphorase to release CPT was also discussed. The bioreduction activated CPT prodrugs including CPT4 and CPT6 are identified to be promising for application to the hypoxia targeting tumor chemotherapy.

Nuclear localization of NADPH:cytochrome c (P450) reductase enhances the cytotoxicity of mitomycin C to Chinese hamster ovary cells

Overexpression of endoplasmic reticulum-localized NADPH: cytochrome c (P450) reductase (NPR) in Chinese hamster ovary cells increases the hypoxic/aerobic differential toxicity of the mitomycins. Because considerable evidence indicates that DNA cross-links are the major cytotoxic lesions generated by the mitomycins, we proposed that bioactivation of the mitomycins in the nucleus close to the DNA target would influence the cytotoxicity of these drugs. The simian virus 40 large T antigen nuclear localization signal was fused to the amino-terminal end of a human NPR protein that lacked its membrane anchor sequence. Immunofluorescent imaging of transfected cell lines expressing the fusion protein confirmed the nuclear location of the enzyme. Regardless of the oxygenation state of the cell, mitomycin C (MC) cytotoxicity was enhanced in cells with overexpressed NPR localized to the nuclear compartment compared with cells overexpressing an endoplasmic reticulum localized enzyme. Enhanced cytotoxicity in cells treated under hypoxic conditions correlated with increases in genomic DNA alkylations, with more MC-DNA adducts being formed when the enzyme was expressed closer to its DNA target. No change was observed in the hypoxic/aerobic differential toxicity as a function of enzyme localization. These findings indicate that drug efficacy is increased when the subcellular site of drug activation corresponds to its site of action.

Enzyme-catalyzed activation of anticancer prodrugs

The rationale fo the development of prodrugs relies upon delivery of higher concentrations of a drug to target cells compared to administration of the drug itself. In the last decades, numerous prodrugs that are enzymatically activated into anti-cancer agents have been developed. This review describes the most important enzymes involved in prodrug activation notably with respect to tissue distribution, up-regulation in tumor cells and turnover rates. The following endogenous enzymes are discussed: aldehyde oxidase, amino acid oxidase, cytochrome P450 reductase, DT-diaphorase, cytochrome P450, tyrosinase, thymidylate synthase, thymidine phosphorylase, glutathione S-transferase, deoxycytidine kinase, carboxylesterase, alkaline phosphatase, beta-glucuronidase and cysteine conjugate beta-lyase. In relation to each of these enzymes, several prodrugs are discussed regarding organ- or tumor-selective activation of clinically relevant prodrugs of 5-fluorouracil, axazaphosphorines (cyclophosphamide, ifosfamide, and trofosfamide), paclitaxel, etoposide, anthracyclines (doxorubicin, daunorubicin, epirubicin), mercaptopurine, thioguanine, cisplatin, melphalan, and other important prodrugs such as menadione, mitomycin C, tirapazamine, 5-(aziridin-1-yl)-2,4-dinitrobenzamide, ganciclovir, irinotecan, dacarbazine, and amifostine. In addition to endogenous enzymes, a number of nonendogenous enzymes, used in antibody-, gene-, and virus-directed enzyme prodrug therapies, are described. It is concluded that the development of prodrugs has been relatively successful; however, all prodrugs lack a complete selectivity. Therefore, more work is needed to explore the differences between tumor and nontumor cells and to develop optimal substrates in terms of substrate affinity and enzyme turnover rates fo prodrug-activating enzymes resulting in more rapid and selective cleavage of the prodrug inside the tumor cells.

Nuclear overexpression of NAD(P)H:quinone oxidoreductase 1 in Chinese hamster ovary cells increases the cytotoxicity of mitomycin C under aerobic and hypoxic conditions

The effects of the subcellular localization of overexpressed bioreductive enzyme NAD(P)H:quinone oxidoreductase 1 (NQO1) on the activity of the antineoplastic agent mitomycin C (MC) under aerobic and hypoxic conditions were examined. Chinese hamster ovary (CHO-K1/dhfr(-)) cells were transfected with NQO1 cDNA to produce cells that overexpressed NQO1 activity in the nucleus (148-fold) or the cytosol (163-fold) over the constitutive level of the enzyme in parental cells. Subcellular localization of the enzyme was confirmed using antibody-assisted immunofluorescence. Nuclear localization of transfected NQO1 activity increased the cytotoxicity of MC over that produced by overexpression in the cytosol under both aerobic and hypoxic conditions, with greater cytotoxicity being produced under hypoxia. The greater cytotoxicity of nuclear localized NQO1 was not attributable to greater metabolic activation of MC but instead was the result of activation of the drug in close proximity to its target, nuclear DNA. A positive relationship existed between the degree of MC-induced cytotoxicity and the number of MC-DNA adducts produced. The findings indicate that activation of MC proximal to nuclear DNA by the nuclear localization of transfected NQO1 increases the cytotoxic effects of MC regardless of the degree of oxygenation and support the concept that the mechanism of action of MC involves alkylation of DNA.

3-Substituted-5-aziridinyl-1-methylindole-4,7-diones as NQO1-directed antitumour agents: mechanism of activation and cytotoxicity in vitro

Indolequinone agents are a unique class of bioreductive cytotoxins that can function as dual substrates for both one- and two-electron reductases. This endows them with the potential to be either hypoxia-selective cytotoxins or NAD(P)H:quinone oxidoreductase 1 (NQO1)-directed prodrugs, respectively. We have studied the structure-activity relationships of four novel indolequinone analogues with regard to one- and/or two-electron activation. Single-electron metabolism was achieved by exposing the human carcinoma cell line T47D to each agent under hypoxic conditions, whilst concerted two-electron metabolism was assessed by stably expressing the cDNA for human NQO1 in a cloned cell line of T47D. The C-3 and C-5 positions of the indolequinone nucleus were modified to manipulate reactivity of the reduction products and the four prodrugs were identified as NQO1 substrates of varying specificity. Two of the four prodrugs, in which both C-3 and C-5 groups remained functional, proved to be NQO1-directed cytotoxins with selectivity ratios of 60- to 80-fold in the T47D (WT) versus the NQO1 overexpressing T47D cells. They also retained selectivity as hypoxic cytotoxins with oxic/hypoxic ratios of 20- to 22-fold. Replacement of the C-3 hydroxymethyl leaving group with an aldehyde group ablated all selectivity in air and hypoxia in both cell lines. Addition of a 2-methyl group on the C-5 aziridinyl group to introduce steric hinderance reduced but did not abolish NQO1-dependent metabolism. However, it enhanced single-electron metabolism-dependent DNA cross-linking in a manner that was independent of cytotoxicity. These data demonstrate that subtle structure-activity relationship exists for different cellular reductases and under certain circumstances distinct forms of DNA damage can arise, the cytotoxic consequences of which can vary. This study identifies a candidate indolequinone analogue for further development as a dual hypoxia and NQO1-directed prodrug.