Mechanistic Study of Macranthoside B Effects on Apoptotic Cell Death in Human Cervical Adenocarcinoma Cells.

Macranthoside B (MB) is a triterpenoid saponin extracted from Lonicera macranthoides, a traditional Chinese medicine. In the current study, we investigated the anticancer potential of MB in various cancer cells and elucidated its underlying mechanisms. MB exposure inhibited cell proliferation, induced mitochondrial membrane potential (MMP) loss, increased sub-G1 accumulation, and resulted in cleavage of caspase-3 and PARP, which are reflective of apoptosis. In HeLa cells, MB induced down-regulation of SOD2 and GPx1, phosphorylation of Akt and PDK1, and thus promoted ROS-mediated apoptosis. This was further supported by the protection of sub-G1 accumulation, MMP loss, cleavage of caspase-3 and PARP in the presence of N-acetylcysteine (NAC). Additionally, MB induced cell death via down-regulation of ubiquitin-like with PHD and ringfinger domains 1 (UHRF1) and Bcl-xL. Taken together, this study provides a new insight into the apoptosis- inducing potential of MB, and its molecular mechanisms are associated with an increase in oxidative stress and inhibition of the PDK1/Akt pathway.

Proteomic identification of peroxiredoxin 6 for host defence against Opisthorchis viverrini infection.

Opisthorchis viverrini infection causes opisthorchiasis and is a risk factor for cholangiocarcinoma via chronic inflammation. To investigate the mechanism of O. viverrini -induced liver disease, we applied a proteomic approach to examine alterations in hepatic protein levels in O. viverrini -infected hamsters. Two-dimensional gel electrophoresis (2DE) revealed that O. viverrini infection induced upregulation (1.5- to 4.3-fold) of 25 proteins and downregulation (1.5 to 2.5-fold) of 24 proteins compared with uninfected animals. Expression of proteins related to stress response, DNA replication and repair, and cell structure was significantly increased, whereas that of proteins associated with normal liver function, such as metabolism, blood volume maintenance and fatty acid cycle was decreased. Among the upregulated proteins, a 2.7-fold increase in peroxiredoxin 6 (Prdx6), an antioxidant protein, was confirmed by 2DE and immunoblot analysis, Western blot and quantitative PCR. Immunohistochemical analysis showed that Prdx6 expression was observed mainly in the cytoplasm of inflammatory cells. These results suggest that Prdx6 is important for host defence against O. viverrini infection. This study provides basic information for Prdx6 as a potential biomarker and therapeutic target for opisthorchiasis.

Sequence-specific DNA damage induced by carcinogenic danthron and anthraquinone in the presence of Cu(II), cytochrome P450 reductase and NADPH.

The mechanism of metal-mediated DNA damage by carcinogenic danthron (1,8-dihydroxyanthraquinone) and anthraquinone was investigated by the DNA sequencing technique using 32P-labeled human DNA fragments obtained from the human c-Ha-ras-1 protooncogene and the p53 tumor suppressor gene. Danthron caused DNA damage particularly at guanines in the 5'-GG-3', 5'-GGGG-3', 5'-GGGGG-3' sequences (damaged bases are underlined) in the presence of Cu(II), cytochrome P450 reductase and the NADPH-generating system. The DNA damage was inhibited by catalase and bathocuproine, suggesting the involvement of H2O2 and Cu(I). The formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine increased with increasing concentration of danthron. On the other hand, carcinogenic anthraquinone induced less oxidative DNA damage than danthron. Electron spin resonance study showed that the semiquinone radical could be produced by P450 reductase plus NADPH-mediated reduction of danthron, while little signal was observed with anthraquinone. These results suggest that danthron is much more likely to be reduced by P450 reductase and generate reactive oxygen species through the redox cycle, leading to more extensive Cu(II)-mediated DNA damage than anthraquinone. In the case of anthraquinone, its hydroxylated metabolites with similar reactivity to danthron may participate in DNA damage in vivo. We conclude that oxidative DNA damage by danthron and anthraquinone seems to be relevant for the expression of their carcinogenicity.

Protective effect of phytic acid on oxidative DNA damage with reference to cancer chemoprevention.

Phytic acid (myo-inositol hexaphosphate) is one of the most promising cancer chemopreventive agents. We investigated the mechanism by which phytic acid expresses preventive action to cancer. Phytic acid inhibited the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine in cultured cells treated with an H2O2-generating system, although it did not scavenge H2O2. Site-specific DNA damage by H2O2 and Cu(II) at GG and GGG sequences was inhibited by phytic acid, but not by myo-inositol. Phytic acid alone did not cause DNA damage and thus, it should not act as a prooxidant. We conclude that phytic acid acts as an antioxidant to inhibit the generation of reactive oxygen species from H2O2 by chelating metals, resulting in chemoprevention of cancer.

Mechanism of metal-mediated DNA damage induced by metabolites of carcinogenic 2-nitropropane.

2-Nitropropane (2-NP), a widely used industrial solvent, is carcinogenic to rats. To clarify the mechanism of carcinogenesis by 2-NP, we investigated DNA damage by 2-NP metabolites, N-isopropylhydroxylamine (IPHA) and hydroxylamine-O-sulfonic acid (HAS), using 32P-5'-end-labelled DNA fragments obtained from genes that are relevant to human cancer. In the presence of Fe(III) EDTA, both IPHA and HAS caused DNA damage at every nucleotide position without marked site preference. The damage was inhibited by free hydroxyl radical (-*OH) scavengers, catalase and deferoxamine mesilate, an iron chelating agent. These results suggest that the DNA damage was caused by -*OH generated via H(2)O(2) by both IPHA and HAS. In contrast, in the presence of Cu(II), IPHA frequently caused DNA damage at thymine. The Cu(II)-mediated DNA damage caused by IPHA was inhibited by catalase, methional and bathocuproine, a Cu(I)-specific chelator, suggesting the involvement of H(2)O(2) and Cu(I). These results suggest that the DNA damage induced by IPHA in the presence of Cu(II) was caused by a reactive oxygen species like the Cu(I)-hydroperoxo complex. On the other hand, HAS most frequently induced DNA damage at 5'-TG-3', 5'-GG-3' and 5'-GGG-3' sequences. Catalase and methional only partly inhibited the Cu(II)-mediated DNA damage caused by HAS, suggesting that the reactive oxygen species and another reactive species participate in this process. Formation of 8-oxodG by IPHA or HAS increased in the presence of metal ions. This study suggests that metal-mediated DNA damage caused by 2-NP metabolites plays an important role in the mutagenicity and the carcinogenicity of 2-NP.

Catechol estrogens induce oxidative DNA damage and estradiol enhances cell proliferation.

Estrogen-induced carcinogenesis involves enhanced cell proliferation (promotion) and genotoxic effects (initiation). To investigate the contribution of estrogens and their metabolites to tumor initiation, we examined DNA damage induced by estradiol and its metabolites, the catechol estrogens 2-hydroxyestradiol (2-OHE(2)) and 4-hydroxyestradiol (4-OHE(2)). In the presence of Cu(II), catechol estrogens formed piperidine-labile sites at thymine and cytosine residues in (32)P 5'-end-labeled DNA fragments and induced the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine. NADH markedly enhanced Cu(II)-dependent DNA damage mediated by nanomolar concentrations of catechol estrogens. Catalase and bathocuproine inhibited the DNA damage, suggesting the involvement of H(2)O(2) and Cu(I). These results suggest that H(2)O(2), generated during Cu(II)-catalyzed autoxidation of catechol estrogens, reacts with Cu(I) to form the Cu(I)-peroxide complex, leading to oxidative DNA damage, and that NADH enhanced DNA damage through the formation of redox cycle. To investigate the role of estrogens and their metabolites in tumor promotion, we examined their effects on proliferation of estrogen-dependent MCF-7 cells. Estradiol enhanced the proliferation of MCF-7 cells at much lower concentrations than catechol estrogens. These findings indicate that catechol estrogens play a role in tumor initiation through oxidative DNA damage, whereas estrogens themselves induce tumor promotion and/or progression by enhancing cell proliferation in estrogen-induced carcinogenesis.

Amplification of bleomycin-induced DNA cleavage at cytosine residues 3' to GGG sequences by pyrrole triamide.

We investigated the amplification of bleomycin-induced DNA cleavage by synthetic triamides containing N-methylpyrrole (Py) and/or N-methylimidazole (Im), PyPyPy, PyPyIm, PyImPy, and PyImIm, using 32P-labeled DNA fragments obtained from the human c-Ha-ras-1 and p53 genes. Peplomycin, a bleomycin analog, plus Fe(II) caused DNA cleavage at the 5'-GC-3' and 5'-GT-3' sequences (damaged bases are underlined). The addition of PyPyPy dramatically enhanced the cleavage, particularly at cytosine residues 3' to consecutive guanines. Alteration in the site specificity was not observed with other triamides (PyPyIm, PyImPy, and PyImIm). DNase I footprinting revealed that PyPyPy bound to the sites adjacent to the sites where DNA cleavage was enhanced by PyPyPy, and that PyPyPy enhanced DNase I-induced cleavage in GC-rich regions. These findings suggest that binding of PyPyPy to the DNA minor groove changes the DNA conformation to allow peplomycin to cleave DNA more efficiently at GC-rich sequences, resulting in intensive site-specific DNA cleavage particularly at cytosines at the 3'-side of polyguanines. The present study on amplifiers of antitumor drugs would appear to offer a novel approach to the establishment of more effective chemotherapy.

Mechanism of guanine-specific DNA damage by oxidative stress and its role in carcinogenesis and aging.

Reactive species generated by chemicals and UV radiation can cause sequence-specific DNA damage and play important roles in mutagenesis, carcinogenesis and aging. We have investigated sequence specificity of oxidative stress-mediated DNA damage by using 32P-labeled DNA fragments obtained from the human c-Ha-ras-1 and p53 genes. Free hydroxyl radical causes DNA damage with no marked site specificity. Reactive nitrogen species, sulfate radicals, nitrogen-centered radicals, benzoyloxyl radical and alkoxyl radical show different sequence specificity. Benzoyloxyl radical specifically causes damage to the 5'-G in GG sequence. UVA radiation also causes DNA damage at this site through electron transfer in the presence of certain photosensitizers. The 5'-G in GG sequence is easily oxidized because a large part of the highest occupied molecular orbital is distributed on this site. On the basis of these findings, the sequence specificity of DNA damage is presumably determined by (a) redox potential of reactive species; (b) ionization potential of DNA bases; and (c) site-specific binding of metal ion to DNA. Here we discuss the mechanisms of sequence-specific DNA damage in relation to carcinogenesis and aging.

Distinct mechanisms of guanine-specific DNA photodamage induced by nalidixic acid and fluoroquinolone antibacterials.

Fluoroquinolone antibacterials, which have been used for the treatment of a variety of infectious diseases, are reported to be photocarcinogenic. We investigated the mechanisms of DNA damage by UVA radiation (365 nm) plus fluoroquinolone antibacterials using 32P-labeled DNA fragments obtained from the human c-Ha-ras-1 proto-oncogene and the p53 tumor suppressor gene. Photocarcinogenic nalidixic acid (NA), which is an old member of synthetic quinolone antibacterials, caused DNA damage specifically at 5'-GG-3' sequences, whereas lomefloxacin (LFLX) did not exhibit the site preference for consecutive guanines. LFLX-induced DNA photodamage was inhibited by sodium azide and enhanced in D2O, suggesting that singlet oxygen plays the key role in the DNA damage. LFLX plus UVA induced the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) depending on LFLX concentrations, and 8-oxodG formation was enhanced in single-stranded DNA. In contrast, NA induced larger amounts of 8-oxodG in double-stranded DNA. ESR spin destruction method revealed that NA induced DNA photodamage through electron transfer but LFLX did not. These findings indicate that DNA damage induced by photoactivated LFLX and NA plays an important role in expression of their photocarcinogenicity.

Copper-dependent DNA damage induced by hydrazobenzene, an azobenzene metabolite.

Hydrazobenzene is carcinogenic to rats and mice and azobenzene is carcinogenic to rats. Hydrazobenzene is a metabolic intermediate of azobenzene. To clarify the mechanism of carcinogenesis by azobenzene and hydrazobenzene, we investigated DNA damage induced by hydrazobenzene, using 32P-5'-end-labeled DNA fragments obtained from the c-Ha-ras-1 protooncogene and the p53 tumor suppressor gene. Hydrazobenzene caused DNA damage in the presence of Cu(II). Piperidine treatment enhanced the DNA damage greatly, suggesting that hydrazobenzene caused base modification and liberation. However, azobenzene did not cause DNA damage even in the presence of Cu(II). Hydrazobenzene plus Cu(II) caused DNA damage frequently at thymine residues. Catalase and a Cu(I)-specific chelator inhibited Cu(II)-mediated DNA damage by hydrazobenzene. Typical *OH scavengers did not inhibit the DNA damage. The main active species is probably a metal oxygen complex, such as Cu(I)-OOH. Formation of 8-oxo-7, 8-dihydro-2'-deoxyguanosine was increased by hydrazobenzene in the presence of Cu(II). Oxygen consumption and UV-Visible spectroscopic measurements have shown that hydrazobenzene is autoxidized to azobenzene with H2O2 formation. It is considered that the metal-mediated DNA damage by hydrazobenzene through H2O2 generation may be relevant for the expression of carcinogenicity of azobenzene and hydrazobenzene.

Distinct mechanisms of oxidative DNA damage by two metabolites of carcinogenic o-toluidine.

Mechanisms of DNA damage by metabolites of carcinogenic o-toluidine in the presence of metals were investigated by the DNA sequencing technique using (32)P-labeled human DNA fragments. 4-Amino-3-methylphenol, a major metabolite, caused DNA damage in the presence of Cu(II). Predominant cleavage sites were thymine and cytosine residues. o-Nitrosotoluene, a minor metabolite, did not induce DNA damage even in the presence of Cu(II), but addition of NADH induced DNA damage very efficiently. The DNA cleavage pattern was similar to that in the case of 4-amino-3-methylphenol. Bathocuproine and catalase inhibited DNA damage by these o-toluidine metabolites, indicating the participation of Cu(I) and H(2)O(2) in the DNA damage. Typical free hydroxyl radical scavengers showed no inhibitory effects on the DNA damage. o-Toluidine metabolites increased the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine in calf thymus DNA in the presence of Cu(II). UV-visible and ESR spectroscopic studies have demonstrated that 4-amino-3-methylphenol is autoxidized to form the aminomethylphenoxyl radical and o-nitrosotoluene is reduced by NADH to the o-toluolhydronitroxide radical in the presence and absence of Cu(II). Consequently, it is considered that these radicals react with O(2) to form O(-)(2) and subsequently H(2)O(2), and that the reactive species generated by the reaction of H(2)O(2) with Cu(I) participate in the DNA damage. Metal-mediated DNA damage by o-toluidine metabolites through H(2)O(2) seems to be relevant for the expression of the carcinogenicity of o-toluidine.

Involvement of oxidative DNA damage and apoptosis in antitumor actions of aminosugars.

We investigated the mechanisms of apoptosis and DNA damage induced by aminosugars in relation to their antitumor actions. The order of cytotoxic effects of aminosugars was D-mannosamine (ManN) >> D-galactosamine (GalN) > D-glucosamine (GlcN). A comparison of the frequency of apoptotic cells showed the same order. DNA ladders were formed by only ManN and the formation of DNA ladders was inhibited by a caspase inhibitor. Pulsed-field gel electrophoresis showed that ManN caused cellular DNA cleavage at a lower concentration than those causing apoptosis. Cellular DNA cleavage was inhibited by catalase and enhanced by a catalase inhibitor. Flow cytometry showed that ManN enhanced the production of intracellular peroxides. These results suggest that ManN-induced apoptosis is preceded by H2O2-mediated DNA damage. The order of the extent of damage to 32P-labeled DNA fragments by aminosugars plus Cu(II) was ManN >> GalN > GlcN. The DNA damage was inhibited by catalase and bathocuproine, suggesting that H2O2 reacts with Cu(I) to form the metal-peroxide complex capable of causing DNA damage. Two mechanisms of H2O2 generation from aminosugars were proposed: one is the major pathway to form a dioxo compound and NH4+; the other is the minor pathway to form a pyrazine derivative through the condensation of two molecules of an aminosugar. The order of reactivity to generate these products was ManN >> GalN > GlcN. On the basis of these results, it is concluded that aminosugars, especially ManN, produce H2O2 to cause DNA damage, which mediates apoptosis resulting in tumor growth inhibition.

Oxidative DNA damage induced by aminoacetone, an amino acid metabolite.

We investigated DNA damage induced by aminoacetone, a metabolite of threonine and glycine. Pulsed-field gel electrophoresis revealed that aminoacetone caused cellular DNA cleavage. Aminoacetone increased the amount of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in human cultured cells in a dose-dependent manner. The formation of 8-oxodG in calf thymus DNA increased due to aminoacetone only in the presence of Cu(II). DNA ladder formation was observed at higher concentrations of aminoacetone than those causing DNA cleavage. Flow cytometry showed that aminoacetone enhanced the generation of hydrogen peroxide (H2O2) in cultured cells. Aminoacetone caused damage to 32P-5'-end-labeled DNA fragments, obtained from the human c-Ha-ras-1 and p53 genes, at cytosine and thymine residues in the presence of Cu(II). Catalase and bathocuproine inhibited DNA damage, suggesting that H2O2 and Cu(I) were involved. Analysis of the products generated from aminoacetone revealed that aminoacetone underwent Cu(II)-mediated autoxidation in two different pathways: the major pathway in which methylglyoxal and NH+4 are generated and the minor pathway in which 2,5-dimethylpyrazine is formed through condensation of two molecules of aminoacetone. These findings suggest that H2O2 generated by the autoxidation of aminoacetone reacts with Cu(I) to form reactive species capable of causing oxidative DNA damage.

DNA damage induced by Salmonella test-negative carcinogens through the formation of oxygen and nitrogen-derived reactive species (review).

High levels of active oxygen species generated by carcinogenic chemicals can cause DNA damage, which may lead to carcinogenesis. We have investigated the characteristics and mechanisms of DNA damage induced by some carcinogens. Here we show our experimental results and propose the possible mechanisms of DNA damage through a) NADH-dependent and b) manganese-dependent formation of reactive oxygen species. We also discuss the mechanism of c) DNA damage induced by nitric oxide and peroxynitrite.

Amplification of bleomycin-induced DNA cleavage by pyrrole triamide.

We investigated the amplification of bleomycin-induced DNA cleavage by synthetic pyrrole triamide (PyPyPy) using 32P-labeled DNA fragments obtained from human genes. Peplomycin, a kind of bleomycins, plus Fe(II) caused DNA cleavage at the 5'-GC-3' and 5'-GT-3' sequences (damaged bases are underlined). The addition of PyPyPy enhanced the cleavage at cytosine and thymine residues 3' to consecutive guanines, particularly at the 5'-GGGGC-3' and 5'-GGGGT-3' sequences. These results suggest that PyPyPy binds to DNA to induce its conformational change, resulting in alteration of the site specificity and amplification of DNA cleavage. The present study on amplifiers of antitumor drugs would show a novel approach to the establishment of more effective chemotherapy.

DNA damage induced by m-phenylenediamine and its derivative in the presence of copper ion.

To clarify the mechanism of carcinogenesis by hair dyes, we compared the extent of DNA damage induced by mutagenic m-phenylenediamine and 4-methoxy-m-phenylenediamine, using 32P-5'-end-labeled DNA fragments obtained from the human c-Ha-ras-1 protooncogene and the p53 tumor suppressor gene. Carcinogenic 4-methoxy-m-phenylenediamine caused DNA damage at thymine and cytosine residues in the presence of Cu(II). Catalase and bathocuproine, a Cu(I)-specific chelator, inhibited 4-methoxy-m-phenylenediamine-induced DNA damage, suggesting the involvement of H2O2 and Cu(I). Superoxide dismutase (SOD) enhanced the DNA damage. Formation of 8-hydroxy-2'-deoxyguanosine (8-OH-dG) was induced by 4-methoxy-m-phenylenediamine in the presence of Cu(II). UV-visible spectroscopic studies have shown that Cu(II) mediated autoxidation of 4-methoxy-m-phenylenediamine and SOD accelerated the autoxidation. On the other hand, non-carcinogenic m-phenylenediamine did not cause clear DNA damage and significant autoxidation even in the presence of Cu(II). These results suggest that carcinogenicity of m-phenylenediamines is associated with ability to cause oxidative DNA damage rather than bacterial mutagenicity.

Site-specific hydroxylation at polyguanosine in double-stranded DNA by UVA radiation with nalidixic acid.

Nalidixic acid (NA) has been used for urinary tract infections and has been reported to be photocarcinogenic. We examined the mechanism of damage to 32P-labeled DNA fragments obtained from the human c-Ha-ras-1 proto-oncogene and the p53 tumor suppressor gene exposed to 365-nm UVA light in the presence of NA. NA plus UVA light caused damage to the double-stranded DNA fragment at consecutive guanine residues, whereas damage to the single-stranded DNA fragment was caused at single guanines and thymines. The formation of 8-oxo-7, 8-dihydro-2'-deoxyguanosine in native DNA exceeded that in denatured DNA at high NA concentrations. The ESR spin destruction method suggested that DNA damage was caused through electron transfer from guanine residues to photoexcited NA. On the basis of these findings, it is concluded that NA can cause skin cancer through DNA damage mediated by its photoactivation.

Oxidative DNA damage induced by homogentisic acid, a tyrosine metabolite.

We examined the mechanism of DNA damage induced by a mutagenic tyrosine metabolite, homogentisic acid (HGA), using 32P-5'-end-labeled DNA fragments obtained from the human p53 tumor suppressor gene. HGA caused DNA damage in the presence of Cu(II), particularly at thymine and cytosine residues. Catalase and bathocuproine inhibited the DNA damage, suggesting the involvement of H2O2 and Cu(I). The formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine by HGA increased depending on HGA concentration in the presence of Cu(II). It is concluded that H2O2 is generated during Cu(II)-catalyzed HGA autoxidation and reacts with Cu(I) to form the Cu(I)-peroxide complex, capable of causing oxidative DNA damage.

Metal-mediated oxidative DNA damage induced by nitro-2-aminophenols.

Two hair dye components, carcinogenic 4-nitro-2-aminophenol and 5-nitro-2-aminophenol, induced Cu(II)-dependent DNA cleavage frequently at thymine and guanine residues in DNA fragments obtained from the c-Ha-ras-1 protooncogene. When the p53 tumor suppressor gene was used, 4-nitro-2-aminophenol caused Cu(II)-dependent piperidine-labile sites at poly G sequences. In the presence of Cu(II), both components increased 8-oxo-7,8-dihydro-2'-deoxyguanosine formation in DNA. The inhibitory effects of catalase and bathocuproine on DNA damage suggest the involvement of H2O2 and Cu(I). It is speculated that nitro-2-aminophenols undergo Cu(II)-mediated autoxidation to generate active oxygen species causing DNA damage which leads to their carcinogenesis.