DNA adducts formed from 4-hydroxytamoxifen are more mutagenic than those formed by alpha-acetoxytamoxifen in a shuttle vector target gene replicated in human Ad293 cells

Reactivity vs. selectivity of quinone methides: synthesis of pharmaceutically important molecules, toxicity and biological applications

Quinone methides (QMs) are considered to be highly reactive intermediates because of their aromatization both in chemical and biological systems. Being highly accessible, quinone methides (QMs) have been widely exploited and their concurrent use has been manifested for the synthesis of tertiary and quaternary carbon centers of bioactives, drugs and drug-like molecules. In this feature article, the synthetic routes, structure-reactivity relationships and synthetic applications of quinone methides are discussed. Formation of the intermediates during bioactivation of different chemical entities and possible chemical manifestations leading to their toxicity in biological systems are also covered.

Comparison of cytotoxicity and genotoxicity of 4-hydroxytamoxifen in combination with Tualang honey in MCF-7 and MCF-10A cells

Background

The Malaysian Tualang honey (TH) is not only cytotoxic to human breast cancer cell lines but it has recently been reported to promote the anticancer activity induced by tamoxifen in MCF-7 and MDA-MB-231 cells suggesting its potential as an adjuvant for the chemotherapeutic agent. However, tamoxifen produces adverse effects that could be due to its ability to induce cellular DNA damage. Therefore, the study is undertaken to determine the possible modulation of the activity of 4-hydroxytamoxifen (OHT), an active metabolite of tamoxifen, by TH in non-cancerous epithelial cell line, MCF-10A, in comparison with MCF-7 cells.

Methods

MCF-7 and MCF-10A cells were treated with TH, OHT or the combination of both and cytotoxicity and antiproliferative activity were determined using LDH and MTT assays, respectively. The effect on cellular DNA integrity was analysed by comet assay and the expression of DNA repair enzymes was determined by Western blotting.

Results

OHT exposure was cytotoxic to both cell lines whereas TH was cytotoxic to MCF-7 cells only. TH also significantly decreased the cytotoxic effect of OHT in MCF-10A but not in MCF-7 cells. TH induced proliferation of MCF10A cells but OHT caused growth inhibition that was abrogated by the concomitant treatment with TH. While TH enhanced the OHT-induced DNA damage in the cancer cells, it dampened the genotoxic effect of OHT in the non-cancerous cells. This was supported by the increased expression of DNA repair proteins, Ku70 and Ku80, in MCF-10A cells by TH.

Conclusion

The findings indicate that TH could afford protection of non-cancerous cells from the toxic effects of tamoxifen by increasing the efficiency of DNA repair mechanism in these cells.

Quinone Methide Bioactivation Pathway: Contribution to Toxicity and/or Cytoprotection?

The formation of quinone methides (QMs) from either direct 2-electron oxidation of 2- or 4-alkylphenols, isomerization of o-quinones, or elimination of a good leaving group could explain the cytotoxic/cytoprotective effects of several drugs, natural products, as well as endogenous compounds. For example, the antiretroviral drug nevirapine and the antidiabetic agent troglitazone both induce idiosyncratic hepatotoxicity through mechanisms involving quinone methide formation. The anesthetic phencyclidine induces psychological side effects potentially through quinone methide mediated covalent modification of crucial macromolecules in the brain. Selective estrogen receptor modulators (SERMs) such as tamoxifen, toremifene, and raloxifene are metabolized to quinone methides which could potentially contribute to endometrial carcinogenic properties and/or induce detoxification enzymes and enhance the chemopreventive effects of these SERMs. Endogenous estrogens and/or estrogens present in estrogen replacement formulations are also metabolized to catechols and further oxidized to o-quinones which can isomerize to quinone methides. Both estrogen quinoids could cause DNA damage which could enhance hormone dependent cancer risk. Natural products such as the food and flavor agent eugenol can be directly oxidized to a quinone methide which may explain the toxic effects of this natural compound. Oral toxicities associated with chewing areca quid could be the result of exposure to hydroxychavicol through initial oxidation to an o-quinone which isomerizes to a p-quinone methide. Similar o-quinone to p-quinone methide isomerization reactions have been reported for the ubiquitous flavonoid quercetin which needs to be taken into consideration when evaluating risk-benefit assessments of these natural products. The resulting reaction of these quinone methides with proteins, DNA, and/or resulting modulation of gene expression may explain the toxic and/or beneficial effects of the parent compounds.

Is tamoxifen a genotoxic carcinogen in women?

The anti-oestrogen tamoxifen, which is widely used in the treatment of breast cancer and is also approved for the prevention of this disease, causes an increased incidence of endometrial cancer in women. The ability of tamoxifen to induce endometrial tumours and the underlying carcinogenic mechanisms have been a subject of intense interest over the last approximately 20 years. They are central to the assessment of risks versus benefits for the drug, especially in a chemopreventive context. This review outlines the clinical justification for using tamoxifen as a chemopreventive agent and describes the genotoxic mechanisms considered responsible for tamoxifen-induced tumours in rat liver and how these might relate to women. In rat hepatic tissue, tamoxifen is metabolically activated via alpha-hydroxylation and sulphate conjugation to give a reactive species that binds to DNA predominantly at the N(2)-position of guanine, producing pro-mutagenic lesions. Whether tamoxifen-DNA adducts contribute similarly to the development of cancers in women depends on whether they can be formed in human tissues and the type of specific molecular and cellular responses they induce, if present. This review discusses the current data relating to these issues and highlights areas where further research is needed.

Mutagenicity of DNA adducts derived from ethylene oxide exposure in the pSP189 shuttle vector replicated in human Ad293 cells

Ethylene oxide (EO) is a widely used chemical intermediate also formed endogenously from ethylene metabolism. Despite conflicting epidemiological evidence, EO is classified by the IARC as a human carcinogen. The mutagenicity and carcinogenicity of EO is attributed to direct reaction with DNA and formation of multiple 2-hydroxyethyl (HE) DNA adducts. However, the actual lesions responsible for the reported mutagenicity of EO have not been established. This study used the supF mutation assay to investigate the biological relevance of low levels of EO-induced DNA adducts in human Ad293 cells, with respect to the type and level of each HE adduct present. Initial experiments were conducted using pSP189 plasmid containing up to 290 N7-HEGuanine (N7-HEG) adducts/10(6) nucleotides, which far exceeds that typically detected in human DNA. No other HE-lesions were detectable using our validated LC-MS/MS assay. Replication in cells failed to produce a statistically significant increase in relative mutation frequency, above background rates in the solvent control. Furthermore, the mutation spectrum compiled for EO-treated plasmid (10-2000muM) did not differ significantly from the spontaneous distribution, suggesting EO is not strongly mutagenic in this system. Under refined reaction conditions using higher EO concentrations capable of inducing detectable levels of N1-HEdA, O(6)-HEdG and N3-HEdU along with N7-HEG, there was a significant dose-related increase in relative mutation frequency above background (3.76- and 5.30-fold at 10 and 30mM, respectively). EO treatment appeared associated with an elevated frequency of GC-->CG mutations and the occurrence of substitutions at AT base pairs. Additionally, there was a distinct GC-->TA mutational hotspot in the 10mM EO spectrum. Overall, the results suggest a certain level of promutagenic adducts must be attained before mutations become detectable above background, indicating that N7-HEG is not a promutagenic lesion, and support a role for the minor products of DNA hydroxyethylation in the generation of base substitutions by EO.

Light-Induced Toxic Effects of Tamoxifen: A Chemotherapeutic and Chemopreventive Agent

Tamoxifen is a powerful drug used to treat breast cancer patients, and more than 500,000 women in the U. S. are being treated with this drug. In our study, tamoxifen is found to be photomutagenic in Salmonella typhimurium TA102 at concentrations as low as 0.08 muM and reaches maximum photomutagenicity at 0.4 muM under a light dose equivalent to 20 min sunlight. These concentrations are comparable to the plasma tamoxifen concentration of 0.4 to 3 muM for patients undergoing tamoxifen therapy. The toxicity seems to be the result of DNA damage and/or lipid peroxidation caused by light irradiation of tamoxifen. The DNA damage caused by irradiation of PhiX174 DNA in the presence of tamoxifen appears to be formation of DNA-tamoxifen covalent adducts, not single strand/double strand cleavages, and there is no oxygen involvement. This is confirmed by EPR experiments that carbon-centerd radicals are formed by light irradiation of tamoxifen and there is no singlet oxygen formation. Although superoxide radical is formed, it is not involved in DNA damage.

Mutagenicity of tamoxifen DNA adducts in human endometrial cells and in silico prediction of p53 mutation hotspots

Tamoxifen elevates the risk of endometrial tumours in women and alpha-(N(2)-deoxyguanosinyl)-tamoxifen adducts are reportedly present in endometrial tissue of patients undergoing therapy. Given the widespread use of tamoxifen there is considerable interest in elucidating the mechanisms underlying treatment-associated cancer. Using a combined experimental and multivariate statistical approach we have examined the mutagenicity and potential consequences of adduct formation by reactive intermediates in target uterine cells. pSP189 plasmid containing the supF gene was incubated with alpha-acetoxytamoxifen or 4-hydroxytamoxifen quinone methide (4-OHtamQM) to generate dG-N(2)-tamoxifen and dG-N(2)-4-hydroxytamoxifen, respectively. Plasmids were replicated in Ishikawa cells then screened in Escherichia coli. Treatment with both alpha-acetoxytamoxifen and 4-OHtamQM caused a dose-related increase in adduct levels, resulting in a damage-dependent increase in mutation frequency for alpha-acetoxytamoxifen; 4-OHtamQM had no apparent effect. Only alpha-acetoxytamoxifen generated statistically different supF mutation spectra relative to the spontaneous pattern, with most mutations being GC-->TA transversions. Application of the LwPy53 algorithm to the alpha-acetoxytamoxifen spectrum predicted strong GC-->TA hotspots at codons 244 and 273. These signature alterations do not correlate with current reports of the mutations observed in endometrial carcinomas from treated women, suggesting that dG-N(2)-tam adduct formation in the p53 gene is not a prerequisite for endometrial cancer initiation in women.

The mutagenicity of urban particulate matter in an enzyme free system is associated with the generation of reactive oxygen species

Urban particulate matter (UPM) contributes to lung cancer incidence. UPM has been shown to be genotoxic to mammalian cells and to induce mutations in the Ames assay. Here, we have studied the induction of mutations generated by direct acting mutagenic components of UPM, using the supF forward mutation assay. Plasmid pSP189 was exposed to UPM in aqueous solution in the presence of sucrose buffer, to reduce strand breaks. The mutation frequency induced by 1 microg/microl UPM was 4.99 mutants per 10(4) colonies. This was reduced to 0.84 and 1.48 mutants per 10(4) colonies by addition of mannitol (1 mM) or EDTA (1 mM), respectively. A large percentage of mutant plasmids contained frameshift mutations (57%), and 31% of mutant plasmids contained multiple mutations. Of the base substitution mutations, 88% were at GC pairs, with twice as many transversions as transitions. The types of mutations induced, the reduction of mutagenicity by the inclusion of the free radical scavenger, mannitol, or the metal chelator, EDTA, and the sequence context of the induced mutations all support the conclusion that the majority of mutations were induced by reactive oxygen species generated by metal ions present in the UPM. Most mutation studies with UPM have focused on organic carcinogens present on UPM. Our results highlight the potential contribution of metal ions to the mutagenicity of UPM.

Development of a novel site-specific mutagenesis assay using MALDI-ToF MS (SSMA-MS)

We have developed and validated a novel site-specific mutagenesis assay, termed SSMA-MS, which incorporates MALDI-ToF mass spectrometry (MALDI-MS) analysis as a means of determining the mutations induced by a single DNA adduct. The assay involves ligating an adducted deoxyoligonucleotide into supF containing pSP189 plasmid. The plasmid is transfected into human Ad293 kidney cells allowing replication and therefore repair or a mutagenic event to occur. Escherichia coli indicator bacteria are transformed with recovered plasmid and plasmids containing the insert are identified colormetrically, as they behave as frameshift mutations. The plasmid is then amplified and digested using a restriction cocktail of Mbo11 and Mnl1 to yield 12 bp deoxyoligonucleotides, which are characterized by MALDI-MS. MALDI-MS takes advantage of the difference in molecular weight between bases to identify any induced mutations. This analysis method therefore provides qualitative and quantitative information regarding the type and frequency of mutations induced. This assay was developed and validated using an O⁶-methyl-2′-deoxyguanosine adduct, which induced the expected GC→AT substitutions, when replicated in human or bacterial cells. This approach can be applied to the study of any DNA adduct in any biologically relevant gene sequence (e.g. p53) in human cells and would be particularly amenable to high-throughput analysis.

Characterization of the Human Cytochrome P450 Forms Involved in Metabolism of Tamoxifen to Its α-Hydroxy and α,4-Dihydroxy Derivatives

Tamoxifen is a known hepatocarcinogen in rats and is associated with an increased incidence of endometrial cancer in patients. One mechanism for these actions is via bioactivation, where reactive metabolites are generated that are capable of binding to DNA or protein. Several metabolites of tamoxifen have been identified that appear to predispose to adduct formation. These include alpha-hydroxytamoxifen, alpha,4-dihydroxytamoxifen, and alpha-hydroxy-N-desmethyltamoxifen. Previous studies have shown that cytochrome P450 (P450) enzymes play an important role in the biotransformation of tamoxifen. The aim of our work was to determine which P450 enzymes were capable of producing alpha-hydroxylated metabolites from tamoxifen. When tamoxifen (18 or 250 microM) was used as the substrate, P450 3A4, and to a lesser extent, P450 2D6, P450 2B6, P450 3A5, P450 2C9, and P450 2C19 all produced a metabolite with the same HPLC retention time as alpha-hydroxytamoxifen at either substrate concentration tested. This peak was well-separated from 4-hydroxy-N-desmethyltamoxifen, which eluted substantially later under the chromatographic conditions used. No alpha,4-dihydroxytamoxifen was detected in incubations with any of the forms with tamoxifen as substrate. However, when 4-hydroxytamoxifen (100 microM) was used as the substrate, P450 2B6, P450 3A4, P450 3A5, P450 1B1, P450 1A1, and P450 2D6 all produced detectable concentrations of alpha,4-dihydroxytamoxifen. These studies demonstrate that multiple human P450s, including forms found in the endometrium, may generate reactive metabolites in women undergoing tamoxifen therapy, which could subsequently play a role in the development of endometrial cancer.

Mutation Spectra Induced by α-Acetoxytamoxifen−DNA Adducts in Human DNA Repair Proficient and Deficient (Xeroderma Pigmentosum Complementation Group A) Cells

Tamoxifen, a breast cancer drug, has recently been approved for the chemoprevention of this disease. However, tamoxifen causes hepatic carcinomas in rats through a genotoxic mechanism and increases the risk of endometrial tumors in women. DNA adducts have been detected at low levels in human endometrium, and there is much interest in determining whether DNA damage plays a role in tamoxifen-induced endometrial carcinogenesis. This study investigates the mutagenicity of tamoxifen DNA adducts formed by alpha-acetoxytamoxifen, a reactive ester producing the major DNA adduct formed in livers of tamoxifen-treated rats. pSP189 plasmid DNA containing the supF gene was treated with alpha-acetoxytamoxifen and adduct levels (0.5-8.0 adducts per plasmid) determined by (32)P-postlabeling. Adducted plasmids were transfected into nucleotide excision repair proficient (GM00637) or deficient (GM04429, XPA) human fibroblasts. After replication, plasmids were recovered and screened in indicator bacteria. Relative mutation frequencies increased with the adduct level, with 1.3-3.6-fold higher numbers of mutations in the XP cells compared to the GM00637 cells, indicating that NER plays a significant role in the removal of these particular tamoxifen DNA adducts. The majority of sequence alterations (91-96%) occurred at GC base pairs, as did mutation hotspots, although the type and position of mutations was cell-specific. In both cell lines, as the adduct level increased, the proportion of GC --> AT transitions decreased and GC --> TA transversions, mutations known to arise from the major tamoxifen adducts, increased. Given the high mutagenicity of dG-N(2)-tamoxifen adducts, if not excised, they may potentially contribute to the initiation of endometrial cancer in women.

Comparison of the mutagenic activity of the benzene metabolites, hydroquinone and para-benzoquinone in the supF forward mutation assay: a role for minor DNA adducts formed from hydroquinone in benzene mutagenicity

Benzene, a ubiquitous environmental pollutant and occupational hazardous chemical, is a recognised human leukaemogen and rodent carcinogen. The mechanism by which benzene exerts its carcinogenic effects is to date unknown but it is considered that mutations induced by benzene-DNA adducts may play a role. The benzene metabolite, para-benzoquinone (p-BQ) following reaction in vitro with DNA, forms four major adducts, which include two adducts on 2'-deoxyguanosine 3'-monophosphate (dGp). Reaction of DNA with the benzene metabolite hydroquinone (HQ) results in only one major DNA adduct, which corresponds to one of the dGp adducts formed following reaction with p-BQ. The mutagenicity of the adducts formed from these two benzene metabolites was investigated using the supF forward mutation assay. Metabolite-treated plasmid (pSP189) containing the supF gene was replicated in human Ad293 cells before being screened in indicator bacteria. Treatment with 5-20 mM p-BQ gave a 12 to 40-fold increase in mutation rate compared to 5-20 mM HQ treatment, a result reflected in the level of DNA modification observed (8 to 26-fold increase compared to HQ treatment). Treatment with p-BQ gave equal numbers of GC --> TA transversions and GC --> AT transitions, whereas treatment with HQ gave predominantly GC-->AT transitions. The spectra of mutations achieved for the two individual treatments were shown to be significantly different (P = 0.004). A combination of both treatments also resulted in a high level of GC --> AT transitions and a synergistic increase in the number of multiple mutations, which again predominated as GC --> AT transitions. Sites of mutational hotspots were observed for both individual treatments and one mutational hotspot was observed in the multiple mutations for the combined treatment. These results suggest that the dGp adducts formed from benzene metabolite treatment may play an important role in the mutagenicity and myelotoxicity of benzene.

Genotoxic Mechanism of Tamoxifen in Developing Endometrial Cancer

Increased risk of developing endometrial cancers has been observed in women treated with tamoxifen (TAM), a widely used drug for breast cancer therapy and chemoprevention. The carcinogenic effect may be due to genotoxic DNA damage induced by TAM. In fact, TAM-DNA adducts were detected in the endometrium of women treated with this drug. TAM is alpha-hydroxylated by cytochrome P450 3A4 followed by O-sulfonation by hydroxysteroid sulfotransferase, and reacts with guanine residues in DNA, resulting in the formation of alpha-(N2-deoxyguanosinyl)tamoxifen adducts. During this metabolic process, short-lived carbocations are produced at the ethyl moiety of TAM as reactive intermediates. TAM-DNA adducts promote primarily G -->T transversions in mammalian cells. The same mutations have been frequently detected at codon 12 of the K-ras gene in the endometrial tissue of women treated with this drug. TAM-DNA adducts, if not readily repaired, may act as initiators, leading to development of endometrial cancers. The reactivity of TAM metabolites with DNA is inhibited in toremifene, where the hydrogen atom has been replaced by a chlorine atom at the ethyl moiety. Therefore, toremifene may be a safer alternative to TAM. This article describes an overview of the mechanism of TAM-DNA adduct formation, mutagenic events of this adduct, and detection of TAM-DNA adducts in the endometrium of women treated with TAM.

K-ras mutation in tamoxifen-related endometrial polyps

BACKGROUND

K-ras mutation is thought to occur at an early stage of neoplastic progression in the endometrium. The authors investigated mutations in codon 12 of K-ras in tamoxifen (TAM)-related endometrial polyps.

METHODS

DNA was extracted from 11 frozen endometrial polyps from TAM-treated patients with breast carcinoma. Mutations were detected using the mutant allele-specific amplification method. The results subsequently were analyzed for correlations with immunohistochemical data that were obtained using antibodies against estrogen receptors (ERs; alpha and beta forms), progesterone receptors (PRs; A and B forms), and Ki-67.

RESULTS

Mutations in codon 12 of K-ras were observed in 7 of 11 TAM-related endometrial polyps. Expression levels of ER-alpha and PR-B were high in the glandular epithelium and low in the stroma. PR-A expression was high in both the glandular epithelium and the stroma. In the glandular epithelium, expression of ER-beta appeared to be lower than expression of ER-alpha. The Ki-67 index in the glandular epithelium ranged from 2 to 38, whereas the index ranged from 0 to 4 in the stroma (P < 0.01).

CONCLUSIONS

The incidence of mutations in codon 12 of K-ras in TAM-related endometrial polyps (64%) was greater than the incidence of these same mutations in sporadic endometrial hyperplasias (4.5-23%). High expression levels of ER-alpha, PR-A, and PR-B in the glandular epithelium were observed in all polyps, regardless of K-ras codon 12 mutation status and Ki-67 index. The authors' findings may support the hypothesis that the polyp-carcinoma sequence partly indicates the development of endometrial carcinoma in postmenopausal women who have been treated with TAM.