Bioactivation and resistance to mitomycin C

Advancements and limitations in traditional anti-cancer therapies: a comprehensive review of surgery, chemotherapy, radiation therapy, and hormonal therapy

Cancer remains a major global health challenge, consistently ranking as the second leading cause of mortality worldwide. Despite significant advancements in research and technology, the need to deepen our understanding of tumor biology and improve therapeutic strategies persists. This review focuses on the progress and challenges of four traditional cancer treatment modalities: surgery, chemotherapy, radiation therapy, and hormonal therapy. Surgery, the primary method for tumor removal, has evolved with the integration of fluorescence-based technology and robotic systems, enhancing precision and minimizing collateral damage. Radiation therapy has progressed with improved focus, intensity control, and 3D technology, refining both diagnosis and treatment. Chemotherapy has advanced from natural extracts to synthesized derivatives with amplified cytotoxicity against cancer cells. Hormonal therapy has emerged as a crucial strategy for hormone-dependent cancers, restraining growth or inducing regression. Despite these advancements, each approach faces ongoing challenges. Surgery struggles with complete tumor removal due to heterogeneity. Chemotherapy contends with drug resistance and side effects. Radiation therapy grapples with precision issues and limited access in some regions. Hormonal therapy faces resistance development and quality of life impacts. This study provides a comprehensive analysis of the evolution of these traditional anti-cancer therapies, offering insights into their progress and highlighting areas for future research. By examining these modalities, we aim to underscore their relevance in the current oncology landscape and identify opportunities for improvement in cancer treatment strategies.

Beneficial properties of Drimia numidica leaf methanolic extract against the cytogenotoxic effects of mitomycin C on human lymphocytes

This study investigated the phytochemical profile of Drimia numidica leaf methanolic extract, as well as its cyto-genotoxic and cyto/genoprotective potential against mitomycin C (MMC) mediated effects on healthy human lymphocytes. Photosynthetic pigments, trace elements, and secondary metabolites were estimated and/or identified in methanolic extract of mature leaves, and the latter was further used for assessing its in vitro biological effects on MMC-free and/or MMC-treated human lymphocytes (at low, non-toxic concentrations of 0.001 and 0.01% v/v). The results showed that D. numidica leaf methanolic extract, being rich in carotenoids, phenolics, flavonoids, organic acids and bufadienolides, could be protective against MMC mediated cyto/genotoxic potential in healthy human lymphocytes. Biomolecules possessing antioxidant and antitumor potential, such as beta-carotene and lutein among others, chlorogenic acid, caffeic acid and their derivatives, minerals such as Si, as well as apigenin- and luteolin-derived glycosides, either individual or in a mixture, could be beneficial rather than harmful, at least at the extract concentrations tested. Although further in vitro and in vivo studies are still needed for elucidating the beneficial (individual and/or additive/synergistic) role of those compounds, the results of the present study are quite promising, thus encouraging new challenges for the appropriate utilization of D. numidica leaf extract.

NRF2-Dependent Bioactivation of Mitomycin C as a Novel Strategy To Target KEAP1-NRF2 Pathway Activation in Human Cancer

Activating mutations in the KEAP1-NRF2 pathway are found in approximately 25% of lung tumors, where the hijacking of NRF2's cytoprotective functions results in aggressive tumor growth, chemoresistance, and a poor prognosis for patients. There are currently no approved drugs which target aberrant NRF2 activation, which means that there is an urgent clinical need to target this orphan oncogenic pathway in human tumors. In this study, we used an isogenic pair of wild-type and Keap1 knockout cells to screen a range of chemotherapeutic and pathway-targeted anticancer drugs in order to identify compounds which display enhanced toxicity toward cells with high levels of Nrf2 activity. Through this approach, complemented by validation across a panel of eight human cancer cell lines from a range of different tissues, we identified the DNA-damaging agent mitomycin C to be significantly more toxic in cells with aberrant Nrf2 activation. Mechanistically, we found that the NRF2 target genes for cytochrome P450 reductase, NQO1, and enzymes in the pentose phosphate pathway are all responsible for the NRF2-dependent enhanced bioactivation of mitomycin C. As mitomycin C is already approved for clinical use, it represents as excellent drug repositioning candidate to target the currently untreatable NRF2 activation in human tumors.

Biological effects of mitomycin C on late corneal haze stromal fibrosis following PRK

This review details the current understanding of the mechanism of action and corneal effects of mitomycin C (MMC) for prophylactic prevention of stromal fibrosis after photorefractive keratectomy (PRK), and includes discussion of available information on dosage and exposure time recommended for MMC during PRK. MMC is an alkylating agent, with DNA-crosslinking activity, that inhibits DNA replication and cellular proliferation. It acts as a pro-drug and requires reduction in the tissue to be converted to an active agent capable of DNA alkylation. Although MMC augments the early keratocyte apoptosis wave in the anterior corneal stroma, its most important effect responsible for inhibition of fibrosis in surface ablation procedures such as PRK is via the inhibition of mitosis of myofibroblast precursor cells during the first few weeks after PRK. MMC use is especially useful when treating eyes with higher levels of myopia (≥approximately 6 D), which have shown higher risk of developing fibrosis (also clinically termed late haze). Studies have supported the use of MMC at a concentration of 0.02%, rather than lower doses (such as 0.01% or 0.002%), for optimal reduction of fibrosis after PRK. Exposure times for 0.02% MMC longer than 40 s may be beneficial for moderate to high myopia (≥6D), but shorter exposures times appear to be equally effective for lower levels of myopia. Although MMC treatment may also be beneficial in preventing fibrosis after PRK treatments for hyperopia and astigmatism, more studies are needed. Thus, despite the clinical use of MMC after PRK for nearly twenty years-with limited evidence of harmful effects in the cornea-many decades of experience will be needed to exclude late long-term effects that could be noted after MMC treatment.

Fanconi anemia and homologous recombination gene variants are associated with functional DNA repair defects in vitro and poor outcome in patients with advanced head and neck squamous cell carcinoma

Mutations in Fanconi Anemia or Homologous Recombination (FA/HR) genes can cause DNA repair defects and could therefore impact cancer treatment response and patient outcome. Their functional impact and clinical relevance in head and neck squamous cell carcinoma (HNSCC) is unknown. We therefore questioned whether functional FA/HR defects occurred in HNSCC and whether they are associated with FA/HR variants. We assayed a panel of 29 patient-derived HNSCC cell lines and found that a considerable fraction is hypersensitive to the crosslinker Mitomycin C and PARP inhibitors, a functional measure of FA/HR defects. DNA sequencing showed that these hypersensitivities are associated with the presence of bi-allelic rare germline and somatic FA/HR gene variants. We next questioned whether such variants are associated with prognosis and treatment response in HNSCC patients. DNA sequencing of 77 advanced stage HNSCC tumors revealed a 19% incidence of such variants. Importantly, these variants were associated with a poor prognosis (p = 0.027; HR = 2.6, 1.1–6.0) but favorable response to high cumulative cisplatin dose. We show how an integrated in vitro functional repair and genomic analysis can improve the prognostic value of genetic biomarkers. We conclude that repair defects are marked and frequent in HNSCC and are associated with clinical outcome.

Solubilization and Stability of Mitomycin C Solutions Prepared for Intravesical Administration

BACKGROUND

Mitomycin C (MMC) is an antitumor agent that is often administered intravesically to treat bladder cancer. Pharmacologically optimized studies have suggested varying methods to optimize delivery, with drug concentration and solution volume being the main drivers. However, these MMC concentrations (e.g. 2.0 mg/mL) supersede its solubility threshold, raising major concerns of inferior drug delivery.

OBJECTIVE

In this study, we seek to confirm that the pharmacologically optimized MMC concentrations are achievable in clinical practice through careful modifications of the solution preparation methods.

METHODS

MMC admixtures (1.0 and 2.0 mg/mL) were prepared in normal saline using conventional and alternative compounding methods. Conventional methodology resulted in poorly soluble solutions, with many visible particulates and crystallates. However, special compounding methods, which included incubation of solutions at 50 °C for 50 min followed by storage at 37 °C, were sufficient to solubilize drug. Chemical degradation of MMC solutions was determined over 6 h using high-performance liquid chromatography (HPLC) analytics, while physical stability was tested in parallel.

RESULTS

Immediately following the 50 min incubation, both MMC solutions exhibited approximately 5-7% drug degradation. Based on the measured concentrations and linear regression of degradation plots, additional storage of these solutions at 37 °C for 5 h retained chemical stability criterion (< 10% overall drug loss). No physical changes were observed in any solutions at any test time points.

CONCLUSION

We recommend that the described alternative preparation methods may improve intravesicular delivery of MMC in this urological setting, and advise that clinicians employing these changes should closely monitor patients for MMC toxicities and pharmacodynamics (change in clinical outcomes) that result from the potential enhancement of MMC exposure in the bladder.

Total synthesis of natural productsviairidium catalysis

An overview of the highlights in total synthesis of natural products using iridium as a catalyst is given.

The small 6C RNA of Corynebacterium glutamicum is involved in the SOS response

The 6C RNA family is a class of small RNAs highly conserved in Actinobacteria, including the genera Mycobacterium, Streptomyces and Corynebacterium whose physiological function has not yet been elucidated. We found that strong transcription of the cgb_03605 gene, which encodes 6C RNA in C. glutamicum, was driven by the SigA- and SigB-dependent promoter Pcgb_03605. 6C RNA was detected at high level during exponential growth phase (180 to 240 molcules per cell) which even increased at the entry of the stationary phase. 6C RNA level did not decrease within 240 min after transcription had been stopped with rifampicin, which suggests high 6C RNA stability. The expression of cgb_03605 further increased approximately twofold in the presence of DNA-damaging mitomycin C (MMC) and nearly threefold in the absence of LexA. Deletion of the 6C RNA gene cgb_03605 resulted in a higher sensitivity of C. glutamicum toward MMC and UV radiation. These results indicate that 6C RNA is involved in the DNA damage response. Both 6C RNA level-dependent pausing of cell growth and branched cell morphology in response to MMC suggest that 6C RNA may also be involved in a control of cell division.

Defects in the Fanconi Anemia Pathway and Chromatid Cohesion in Head and Neck Cancer

Failure to repair DNA damage or defective sister chromatid cohesion, a process essential for correct chromosome segregation, can be causative of chromosomal instability (CIN), which is a hallmark of many types of cancers. We investigated how frequent this occurs in head and neck squamous cell carcinoma (HNSCC) and whether specific mechanisms or genes could be linked to these phenotypes. The genomic instability syndrome Fanconi anemia is caused by mutations in any of at least 16 genes regulating DNA interstrand crosslink (ICL) repair. Since patients with Fanconi anemia have a high risk to develop HNSCC, we investigated whether and to which extent Fanconi anemia pathway inactivation underlies CIN in HNSCC of non-Fanconi anemia individuals. We observed ICL-induced chromosomal breakage in 9 of 17 (53%) HNSCC cell lines derived from patients without Fanconi anemia. In addition, defective sister chromatid cohesion was observed in five HNSCC cell lines. Inactivation of FANCM was responsible for chromosomal breakage in one cell line, whereas in two other cell lines, somatic mutations in PDS5A or STAG2 resulted in inadequate sister chromatid cohesion. In addition, FANCF methylation was found in one cell line by screening an additional panel of 39 HNSCC cell lines. Our data demonstrate that CIN in terms of ICL-induced chromosomal breakage and defective chromatid cohesion is frequently observed in HNSCC. Inactivation of known Fanconi anemia and chromatid cohesion genes does explain CIN in the minority of cases. These findings point to phenotypes that may be highly relevant in treatment response of HNSCC.

A procedure for transforming indoles into indolequinones

A procedure that converts a series of structurally diverse, readily available indole derivatives to their corresponding indolequinones is described. The three-step route commences with an iridium catalyzed C-H borylation to give a 7-borylindole that upon oxidation-hydrolysis affords the 7-hydroxyindole. Subsequent oxidation provides the indolequinone.

Understanding cancer and the anticancer activities of naphthoquinones – a review

Naphthoquinone moieties are present in drugs such as doxorubicin which are used clinically to treat solid cancers.

Role of protein-protein interactions in cytochrome P450-mediated drug metabolism and toxicity

Through their unique oxidative chemistry, cytochrome P450 monooxygenases (CYPs) catalyze the elimination of most drugs and toxins from the human body. Protein–protein interactions play a critical role in this process. Historically, the study of CYP–protein interactions has focused on their electron transfer partners and allosteric mediators, cytochrome P450 reductase and cytochrome b5. However, CYPs can bind other proteins that also affect CYP function. Some examples include the progesterone receptor membrane component 1, damage resistance protein 1, human and bovine serum albumin, and intestinal fatty acid binding protein, in addition to other CYP isoforms. Furthermore, disruption of these interactions can lead to altered paths of metabolism and the production of toxic metabolites. In this review, we summarize the available evidence for CYP protein–protein interactions from the literature and offer a discussion of the potential impact of future studies aimed at characterizing noncanonical protein–protein interactions with CYP enzymes.

Evaluation of bioreductive activation of anticancer drugs idarubicin and mitomycin C by NADH-cytochrome b5 reductase and cytochrome P450 2B4

This study attempted to investigate the ability of microsomal NADH-cytochrome b5 reductase and cytochrome P450 2B4 to reductively activate idarubicin and mitomycin C. In vitro plasmid DNA damage experiments and assays using purified hepatic enzymes were employed to examine their respective roles in the metabolic activation of anticancer drugs. Mitomycin C was found to be not a good substrate for microsomal b5 reductase unlike P450 reductase. It produced low amounts of strand breaks in DNA when incubated with b5 reductase and its one-electron reduction by purified enzyme was found as negligible. Our findings revealed that P450 reductase-mediated metabolism of idarubicin resulted in a large increase in single-strand DNA breaks, whereas, b5 reductase neither catalyzed the reduction of idarubicin nor mediated the formation of DNA damage in the presence of idarubicin. The reconstitution studies, on the other hand, have identified rabbit liver CYP2B4 isozyme as being a potential candidate enzyme for reductive bioactivation of idarubicin and mitomycin C. Thus, the present novel findings strongly suggest that while b5 reductase could not play a key role in the cytotoxic and/or antitumor effects of idarubicin and mitomycin C, CYP2B4 could potentiate their activity in combination with P450 reductase.

COMPARE analysis of the toxicity of an iminoquinone derivative of the imidazo[5,4-f]benzimidazoles with NAD(P)H:quinone oxidoreductase 1 (NQO1) activity and computational docking of quinones as NQO1 substrates

Synthesis and cytotoxicity of imidazo[5,4-f]benzimidazolequinones and iminoquinone derivatives is described, enabling structure-activity relationships to be obtained. The most promising compound (an iminoquinone derivative) has undergone National Cancer Institute (NCI) 60 cell line (single and five dose) screening, and using the NCI COMPARE program, has shown correlation to NQO1 activity and to other NQO1 substrates. Common structural features suggest that the iminoquinone moiety is significant with regard to NQO1 specificity. Computational docking into the active site of NQO1 was performed, and the first comprehensive mitomycin C (MMC)-NQO1 docking study is presented. Small distances for hydride reduction and high binding affinities are characteristic of MMC and of iminoquinones showing correlations with NQO1 via COMPARE analysis. Docking also indicated that the presence of a substituent capable of hydrogen bonding to the His194 residue is important in influencing the orientation of the substrate in the NQO1 active site, leading to more efficient reduction.

Xanthoria elegans (Link) (lichen) extract counteracts DNA damage and oxidative stress of mitomycin C in human lymphocytes

Several lichen species have been used for medicinal purposes throughout the ages, and they are reported to be effective in the treatment of different disorders including ulcer and cancer. It is revealed that lichens may be easily accessible sources of natural drugs and possible food supplements after their safety evaluations. The main objective in this study was to evaluate the roles of aqueous extracts of Xanthoria elegans (at 25, 50 and 100 μg/ml) upon mitomycin C (MMC; at 10(-7) M) induced genotoxic and oxidative damages in cultured human lymphocytes. X. elegans were collected from the Erzurum and Artvin provinces (in Turkey) during August 2010. After the application of MMC and X. elegans extract (XEE), separate and together, human whole blood cultures were assessed by four genotoxicity end-points including chromosomal aberration, micronucleus, sister chromatid exchange (SCE) and 8-oxo-2-deoxyguanosine (8-OH-dG) assays. In addition, biochemical parameters [total antioxidant capacity (TAC) and total oxidative stress (TOS)] were examined to determine oxidative effects. According to our results, the frequencies of cytogenetic endpoints and 8-OH-dG levels were significantly increased by MMC compared with controls in human peripheral lymphocytes. MMC caused oxidative stress by altering TAC and TOS levels. On the contrary, XEE led to increases of TAC level without changing TOS level. XEE had no genotoxic effect. Furthermore, our findings revealed that MMC induced increases in the mean frequencies of four genotoxic indices were diminished by XEE in dose dependent manner, indicating its protective role towards cells from MMC exerted injury. In conclusion, the results obtained in the present study indicate for the first time that XEE is a potential source of natural antigenotoxicants.

First synthesis of an aziridinyl fused pyrrolo[1,2-a]benzimidazole and toxicity evaluation towards normal and breast cancer cell lines

Anionic aromatic ipso-substitution has allowed an aziridine ring to be fused onto pyrrolo[1,2-a]benzimidazole. This diazole analogue of aziridinomitosene, and N-[(aziridinyl)methyl]-1H-benzimidazole are shown to be significantly more cytotoxic towards the human breast cancer cell lines MCF-7 and HCC1937 than towards a human normal fibroblast cell line (GM00637). The aziridinyl fused pyrrolo[1,2-a]benzimidazole is less cytotoxic than the non-ring fused aziridinyl analogue towards all three cell lines. The BRCA1-deficient HCC1937 cells are more sensitive to mitomycin C (MMC) compared to GM00637 and MCF-7 cells. The evidence provided indicates that different pathways may mediate cellular response to benzimidazole-containing aziridine compounds compared to MMC.

Distinct roles of cytochrome P450 reductase in mitomycin C redox cycling and cytotoxicity

Mitomycin c (MMC), a quinone-containing anticancer drug, is known to redox cycle and generate reactive oxygen species. A key enzyme mediating MMC redox cycling is cytochrome P450 reductase, a microsomal NADPH-dependent flavoenzyme. In the present studies, Chinese hamster ovary (CHO) cells overexpressing this enzyme (CHO-OR cells) and corresponding control cells (CHO-WT cells) were used to investigate the role of cytochrome P450 reductase in the actions of MMC. In lysates from both cell types, MMC was found to redox cycle and generate H(2)O(2); this activity was greater in CHO-OR cells (V(max) = 1.2 +/- 0.1 nmol H(2)O(2)/min/mg protein in CHO-WT cells versus 32.4 +/- 3.9 nmol H(2)O(2)/min/mg protein in CHO-OR cells). MMC was also more effective in generating superoxide anion and hydroxyl radicals in CHO-OR cells, relative to CHO-WT cells. Despite these differences in MMC redox cycling, MMC-induced cytotoxicity, as measured by growth inhibition, was similar in the two cell types (IC(50) = 72 +/- 20 nmol/L for CHO-WT and 75 +/- 23 nmol/L for CHO-OR cells), as was its ability to induce G(2)-M and S phase arrest. Additionally, in nine different tumor cell lines, although a strong correlation was observed between MMC-induced H(2)O(2) generation and cytochrome P450 reductase activity, there was no relationship between redox cycling and cytotoxicity. Hypoxia, which stabilizes MMC radicals generated by redox cycling, also had no effect on the sensitivity of tumor cells to MMC-induced cytotoxicity. These data indicate that NADPH cytochrome P450 reductase-mediated MMC redox cycling is not involved in the cytotoxicity of this chemotherapeutic agent.

CNOB/ChrR6, a new prodrug enzyme cancer chemotherapy

We report the discovery of a new prodrug, 6-chloro-9-nitro-5-oxo-5H-benzo(a)phenoxazine (CNOB). This prodrug is efficiently activated by ChrR6, the highly active prodrug activating bacterial enzyme we have previously developed. The CNOB/ChrR6 therapy was effective in killing several cancer cell lines in vitro. It also efficiently treated tumors in mice with up to 40% complete remission. 9-Amino-6-chloro-5H-benzo(a)phenoxazine-5-one (MCHB) was the only product of CNOB reduction by ChrR6. MCHB binds DNA; at nonlethal concentration, it causes cell accumulation in the S phase, and at lethal dose, it induces cell surface Annexin V and caspase-3 and caspase-9 activities. Further, MCHB colocalizes with mitochondria and disrupts their electrochemical potential. Thus, killing by CNOB involves MCHB, which likely induces apoptosis through the mitochondrial pathway. An attractive feature of the CNOB/ChrR6 regimen is that its toxic product, MCHB, is fluorescent. This feature proved helpful in in vitro studies because simple fluorescence measurements provided information on the kinetics of CNOB activation within the cells, MCHB killing mechanism, its generally efficient bystander effect in cells and cell spheroids, and its biodistribution. The emission wavelength of MCHB also permitted its visualization in live animals, allowing noninvasive qualitative imaging of MCHB in mice and the tumor microenvironment. This feature may simplify exploration of barriers to the penetration of MCHB in tumors and their amelioration.

Contributions of DNA interstrand cross-links to aging of cells and organisms

Impaired DNA damage repair, especially deficient transcription-coupled nucleotide excision repair, leads to segmental progeroid syndromes in human patients as well as in rodent models. Furthermore, DNA double-strand break signalling has been pinpointed as a key inducer of cellular senescence. Several recent findings suggest that another DNA repair pathway, interstrand cross-link (ICL) repair, might also contribute to cell and organism aging. Therefore, we summarize and discuss here that (i) systemic administration of anti-cancer chemotherapeutics, in many cases DNA cross-linking drugs, induces premature progeroid frailty in long-term survivors; (ii) that ICL-inducing 8-methoxy-psoralen/UVA phototherapy leads to signs of premature skin aging as prominent long-term side effect and (iii) that mutated factors involved in ICL repair like ERCC1/XPF, the Fanconi anaemia proteins, WRN and SNEV lead to reduced replicative life span in vitro and segmental progeroid syndromes in vivo. However, since ICL-inducing drugs cause damage different from ICL and since all currently known ICL repair factors work in more than one pathway, further work will be needed to dissect the actual contribution of ICL damage to aging.

Natural and synthetic quinones and their reduction by the quinone reductase enzyme NQO1: from synthetic organic chemistry to compounds with anticancer potential

The quinone reductase enzyme NAD(P)H: quinone oxidoreductase 1 (NQO1) is a ubiquitous flavoenzyme that catalyzes the two-electron reduction of quinones. This Perspective briefly reviews the structure and mechanism, physiological role, and upregulation and induction of the enzyme, but focuses on the synthesis of new heterocyclic quinones and their metabolism by recombinant human NQO1. Thus a range of indolequinones, some of which are novel analogues of mitomycin C, benzimidazolequinones, benzothiazolequinones and quinolinequinones have been prepared and evaluated, leading to detailed knowledge of the structural requirements for efficient metabolism by the enzyme. Potent mechanism-based inhibitors (suicide substrates) of NQO1 have also been developed. These indolequinones irreversibly alkylate the protein, preventing its function both in standard enzyme assays and also in cells. Some of these quinones are also potent inhibitors of growth of human pancreatic cancer cells, suggesting a potential role for such compounds as therapeutic agents.

Reduction of mitomycin C is catalysed by human recombinant NRH:quinone oxidoreductase 2 using reduced nicotinamide adenine dinucleotide as an electron donating co-factor

NRH:Quinone Oxidoreductase 2 (NQO2) has been described as having no enzymatic activity with nicotinamide adenine dinucleotide (NADH) or NADPH as electron donating cosubstrates. Mitomycin C (MMC) is both a substrate for and a mechanistic inhibitor of the NQO2 homologue NQO1. NRH:quinone oxidoreductase 2 catalysed the reduction of MMC at pH 5.8 with NADH as a co-factor. This reaction results in species that inhibit the NQO2-mediated metabolism of CB1954. In addition, MMC caused an increase in DNA cross-links in a cell line transfected to overexpress NQO2 to an extent comparable to that observed with an isogenic NQO1-expressing cell line. These data indicate that NQO2 may contribute to the metabolism of MMC to cytotoxic species.

Binding of the anticancer prodrug CB1954 to the activating enzyme NQO2 revealed by the crystal structure of their complex

CB1954 is an attractive prodrug for directed-enzyme prodrug therapy (DEPT) and a conventional prodrug against tumors in which the enzyme NQO2 is highly expressed. We have determined the crystal structure of the NQO2-CB1954 complex to 2.0 A resolution. The binding of the prodrug is governed by hydrophobic forces, while two key electrostatic contacts determine the specific orientation of the ligand. The structure also reveals an unfavorable interaction, therefore suggesting possible avenues for DEPT-tailored engineering studies.

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.