Nitrative and oxidative DNA damage in infection-related carcinogenesis in relation to cancer stem cells
DNA damage in inflammation-related carcinogenesis
Under inflammatory conditions, reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated from inflammatory and epithelial cells. ROS and RNS are capable of causing damage to various cellular constituents, such as nucleic acids, proteins and lipids. ROS are generated from multiple sources, including inflammatory cells, carcinogenic chemicals and their metabolites, and the electron transport chain in mitochondria [2, 3]. ROS can induce the formation of oxidative DNA lesion products, including 8-oxo-7,8-dihydro-2’-deoxyguanosine (8-oxodG), which is considered to be mutagenic [8].
Nitric oxide (NO) is synthesized by NO synthases. There are three isoforms, neuronal NO synthase (nNOS, also known as NOS1), inducible NO synthase (iNOS or NOS2) and endothelial NO synthase (eNOS or NOS3) [9, 10]. iNOS is activated to drastically generate NO in inflammatory and epithelial cells under inflammatory conditions, while eNOS and nNOS are constitutively expressed and produce relatively small amounts of NO. iNOS can be also up-regulated by transcription factors such as NF-kB, HIF-1?, STAT, tumor necrosis factor-? (TNF-?). NF-kB plays a central role in inflammation through its ability to induce transcription of proinflammatory genes, including iNOS, and functions as a tumor promoter in inflammation-associated cancer [11].
Figure 1 shows 8-nitroguanine formation under inflammatory conditions and resulting mutation. NO reacts with superoxide (O2?) to form peroxynitrite (ONOO?), a highly reactive species causing 8-oxodG and 8-nitroguanine [12]. The reaction of guanine with ONOO? forms 8-nitroguanine as the major compound, while adenine nitration is minor compared to its C8-oxidation [13]. The glycosidic bond between 8-nitroguanine and deoxyribose is chemically unstable, and this DNA lesion can be spontaneously released, resulting in the formation of an apurinic site [14]. The apurinic site can form a pair with adenine during DNA synthesis, leading to G:C to T:A transversions [15]. In addition, translesion DNA polymerases were discovered and their role in the mutagenesis has been investigated [16]. Cells deficient in Rev1 and Rev3, subunits of DNA polymerase ?, were hypersensitive to nitrative stress, and translesion DNA synthesis past apurinic sites mediated by this polymerase might contribute to extensive point mutations [17]. It has been reported that adenine is preferentially incorporated opposite 8-nitroguanine during DNA synthesis catalyzed by polymerase ? and a truncated form of polymerase kin a cell-free system, suggesting that G:C to T:A transversions can occur [18].
Proposed mechanism of mutation mediated by 8-nitroguanine formation
8-Nitroguanine is considered to be not only a marker of inflammation, but also a potential mutagenic DNA lesion involved in carcinogenesis [19]. We have investigated the formation of 8-nitroguanine and 8-oxodG in various clinical specimens and animal models in relation to inflammation-related carcinogenesis, as summarized in Table 1. When specimens or cultured cells were pretreated with RNase, 8-nitroguanine was more clearly observed in the nuclei of cells by immunostaining. It suggests that 8-nitroguanine is formed mainly in genomic DNA. It is noteworthy that nitrative and oxidative DNA lesions were specifically induced at cancer sites under chronic infection and various inflammatory conditions, as reviewed previously [2, 3, 20]. We demonstrated that 8-nitroguanine was strongly formed via iNOS expression at related cancer sites of H. pylori, HBV, HCV, HPV, EBV and SH, OV [2, 3, 21, 22]. The IARC classification of CS has been recently updated from 2A to 1, so we have not yet collected enough data for 8-nitroguanine.
Nitrative and oxidative stresses cause DNA damage, contributing to the accumulation of mutations in tissues throughout the carcinogenic process. Particularly, 8-nitroguanine formation may participate in inflammation-related carcinogenesis as a common mechanism. Therefore, 8-nitroguanine could be used as a potential biomarker of inflammation-related carcinogenesis.