Reactive oxygen and nitrogen species have been implicated in diverse pathophysiological conditions, including inflammation, neurodegenerative diseases and cancer. Accumulating evidence indicates that oxidative damage to biomolecules including lipids, proteins and DNA, contributes to these diseases. Previous studies suggest roles of lipid peroxidation and oxysterols in the development of neurodegenerative diseases and inflammation-related cancer. Our recent studies identifying and characterizing carbonylated proteins reveal oxidative damage to heat shock proteins in neurodegenerative disease models and inflammation-related cancer, suggesting dysfunction in their antioxidative properties. In neurodegenerative diseases, DNA damage may not only play a role in the induction of apoptosis, but also may inhibit cellular division via telomere shortening. Immunohistochemical analyses showed co-localization of oxidative/nitrative DNA lesions and stemness markers in the cells of inflammation-related cancers. Here, we review oxidative stress and its significant roles in neurodegenerative diseases and cancer.

Inflammation can be induced by chronic infection, inflammatory diseases and physicochemical factors. Chronic inflammation is estimated to contribute to approximately 25% of human cancers. Under inflammatory conditions, inflammatory and epithelial cells release reactive oxygen (ROS) and nitrogen species (RNS), which are capable of causing DNA damage, including the formation of 8-oxo-7,8-dihydro-2′-deoxyguanosine and 8-nitroguanine. We reported that 8-nitroguanine was clearly formed at the sites of cancer induced by infectious agents including Helicobacter pylori, inflammatory diseases including Barrett’s esophagus, and physicochemical factors including asbestos. DNA damage can lead to mutations and genomic instability if not properly repaired. Moreover, DNA damage response can also induce high mobility group box 1-generating inflammatory microenvironment, which is characterized by hypoxia. Hypoxia induces hypoxia-inducible factor and inducible nitric oxide synthase (iNOS), which increases the levels of intracellular RNS and ROS, resulting DNA damage in progression with poor prognosis. Furthermore, tumor-producing inflammation can induce nuclear factor-κB, resulting in iNOS-dependent DNA damage. Therefore, crosstalk between DNA damage and inflammation may play important roles in cancer development. A proposed mechanism for the crosstalk may explain why aspirin decreases the long-term risk of cancer mortality.

Dihydrodiol dehydrogenase (DD; EC 1.3.1.20) catalyzes the oxidation of polycyclic aromatic hydrocarbon (PAH) trans-dihydrodiols (proximate carcinogens) to catechols which rapidly autoxidize to yield o-quinones (Smithgall, T. E., Harvey, R. G., and Penning, T. M. (1988) J. Biol. Chem 263, 1814-1820). Although this pathway suppresses the formation of the PAH anti- and syn-diol epoxides (ultimate carcinogens), the process of autoxidation is anticipated to yield reactive oxygen species (ROS). We now show that the NADP+ dependent oxidation of (+/-)-trans-1,2-dihydroxy-1,2-dihydronaphthalene (Npdiol) and (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (Bpdiol) catalyzed by homogeneous DD is accompanied by the consumption of molecular oxygen and the production of H2O2. With both trans-dihydrodiol substrates, oxygen consumption was stoichiometric with H2O2 production consistent with the reaction: QH2 + O2 = H2O2 + Q, where QH2 is the catechol and Q is the o-quinone. Using Npdiol or Bpdiol as substrates, a burst of superoxide anion production is catalyzed by DD which can be detected as the rate of cyt c reduction that is inhibited by superoxide dismutase. Using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as spin-trapping agent, secondary spin adducts corresponding to DMPO-CH3 were formed during the enzymatic oxidation of Npdiol and Bpdiol. The formation of the CH3. radical arises from the OH. attack of DMSO, which was used as cosolvent. These spin adducts were attenuated by superoxide dismutase and catalase, implying that O2-. and H2O2 are obligatory for the formation of DMPO-CH3. It is proposed that O2-. is the radical that propagates autoxidation and that the resultant H2O2 undergoes Fenton chemistry to produce the OH. radical. Identical spin adducts were observed using a superoxide anion generating system (hypoxanthine/xanthine oxidase) and DMPO as spin-trapping agent in the presence of DMSO. The ability of DD to generate ROS during the oxidation of PAH trans-dihydrodiols (proximate carcinogens) may have important implications for tumor initiation and promotion.

Infection and chronic inflammation have been recognized as important factors for carcinogenesis. Under inflammatory conditions, reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated from inflammatory and epithelial cells and result in oxidative and nitrative DNA damage, such as 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) and 8-nitroguanine. The DNA damage can cause mutations and has been implicated in the initiation and/or promotion of inflammation-mediated carcinogenesis. It has been estimated that various infectious agents are carcinogenic to humans (IARC group 1), including parasites (Schistosoma haematobium (SH) and Opisthorchis viverrini (OV)), viruses (hepatitis C virus (HCV), human papillomavirus (HPV), and Epstein-Barr virus (EBV)), and bacterium Helicobacter pylori (HP). SH, OV, HCV, HPV, EBV, and HP are important risk factors for bladder cancer, cholangiocarcinoma, hepatocellular carcinoma, cervical cancer, nasopharyngeal carcinoma, and gastric cancer, respectively. We demonstrated that 8-nitroguanine was strongly formed via inducible nitric oxide synthase (iNOS) expression at these cancer sites of patients. Moreover, 8-nitroguanine was formed in Oct3/4-positive stem cells in SH-associated bladder cancer tissues and in Oct3/4- and CD133-positive stem cells in OV-associated cholangiocarcinoma tissues. Therefore, it is considered that oxidative and nitrative DNA damage in stem cells may play a key role in inflammation-related carcinogenesis.

In previous studies, benzo[a]pyrene-7,8-dione (BPQ), a polycyclic aromatic hydrocarbon (PAH) o-quinone, was found to be 200-fold more potent as a nuclease than (+/-)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9, 10-tetrahydrobenzo[a]pyrene, a suspect human carcinogen. The mechanism of strand scission mediated by naphthalene-1,2-dione (NPQ) and BPQ was further characterized using either phiX174 DNA or poly(dG).poly(dC) as the target DNA. Strand scission was extensive, dependent on the concentration of o-quinone (0-10 microM), and required the presence of NADPH (1 mM) and CuCl2 (10 microM). The production of reactive species, i.e., superoxide anion radical, o-semiquinone anion (SQ) radical, hydrogen peroxide (H2O2), hydroxyl radical (OH.), and Cu(I), was measured in the incubation mixtures. The formation of SQ radicals was measured by EPR spectroscopy under anaerobic conditions in the presence of NADPH. A Cu(II)/Cu(I) redox cycle was found to be critical for DNA cleavage. No strand scission occurred in the absence of Cu(II) or when Cu(I) was substituted, yet Cu(I) was required for OH* production. Both DNA strand scisson and OH. formation were decreased to an equal extent, albeit not completely, by the inclusion of OH. scavengers (mannitol, soduim benzoate, and formic acid) or Cu(I) chelators (bathocuproine and neocuproine). In contrast, although the SQ radical signals of NPQ and BPQ were quenched by DNA, no strand scission was observed. When calf thymus DNA was treated with PAH o-quinones, malondialdehyde (MDA) was released by acid hydrolysis. The formation of MDA was inhibited by OH. scavengers suggesting that OH* cleaved the 2'-deoxyribose moiety in the DNA to produce base propenals. These studies indicate that for PAH o-quinones to act as nucleases, NADPH, Cu(II), Cu(I), H2O2, and OH*, were necessary and that the primary species responsible for DNA fragmentation was OH., generated by a Cu(I)-catalyzed Fenton reaction. The genotoxicity of PAH o-quinones may play a role in the carcinogenicity and mutagenicity of the parent hydrocarbons.