When DNA is attacked by oxidative stress such as ROS, ultraviolet light, or genotoxic agents, guanine is easily oxidized into 8-hydroxydeoxyguanosine (8-OHdG). The existence of this oxidized guanine in genomic DNA can cause transversion mutation such as G-T or G-A binding, accumulation of which can lead to detrimental consequences. Fortunately, mammalian cells have multiple repair systems such as base excision repair enzymes or nucleotide excision repair (NER) enzymes. Consequently, 8-OHdG, a nucleoside form of 8-oxo-Gua, is generated from either damaged oligomer, which contains 8-oxo-Gua by NER, or from cytoplasmic oxidized nucleotides like 8-hydroxy-deoxyguanosine triphosphate (8-hydroxy-dGTP). Fortunately, exogenously administered 8-OHdG cannot reincorporate into genomic DNA because the activity of deoxynucleotide kinase which converts 8-OHdG into 8-hydroxy-dGTP is very low, although wild deoxyguanosine can be actively converted to deoxyguanosine triphosphate which can be used as a substrate of DNA polymerase.
Figure 1. Chemical structures of guanine base and its 8-hydroxylated derivative.
Although many studies have shown increased levels of 8-OHdG in oxidative-stress-associated diseases, the exact biological role of 8-OHdG has not been investigated. Oxidized deoxyguanosine is notorious for inducing mutagenesis, therefore, most researchers have felt that 8-OHdG might have mutagenic or at least harmful effects in cells, and that is why mammalian physiology tries hard to excrete this oxidized guanosine. However, under the innovative hypothesis that the generation of this molecule can be one of the defense mechanisms of cells against oxidative-stress-induced inflammation, we have tried to obtain evidence that oxidized guanosine can interact with the GTPase family, which is broadly involved in cytoskeleton modification, triggering inflammation, regulating apoptosis, and carcinogenesis. Interestingly, genetically modified oxidized GTP, 8-oxo-GTPγS, seems to interact with the small GTPase family such as Ras, Rho, Rac and cdc42. Among these, we have focused on the role of Rac1 in inflammatory cascades because Rac1 activation is crucial for aggregating NADPH oxidase (NOX) complex and subsequent ROS production. As a result, we have concluded that inhibition of Rac1 by exogenous 8-OHdG, which is a transmittable form of oxidized guanosine, can significantly block ROS-mediated inflammation. Compared with other nucleoside products, 8-OHdG has a potent anti-inflammatory effect by inhibiting the activity of Rac1 on lipopolysaccharide (LPS)-stimulated microglial cells, chemokine-activated neutrophils, and inflammatory mediator-stimulated macrophages. Moreover, since endogenously produced 8-OHdG is much lower than exogenously treated concentrations of 8-OHdG, we propose the biological role of the antioxidative and anti-inflammatory actions of 8-OHdG, implying that 8-OHdG formation can be a defense mechanism against oxidative stress, and enrichment with exogenous 8-OHdG can be a strategy to prevent the initiation or progression of inflammatory disease, backed up with additional fact that only pretreatment or earlier administration of 8-OHdH is effective. Intraperitoneal LPS injection to mice causes severe inflammation in lung tissues by inducing tumor necrosis factor (TNF)-α, interleukins, and myeloperoxidase activity and recruiting neutrophils. Simultaneous treatment with 8-OHdG significantly decreases the level of the above markers, and the efficacy of 8-OHdG is even more potent than that of aspirin; a conventional anti-inflammatory agent.
There is extensive experimental evidence that oxidative damage permanently occurs to lipids of cellular membranes, proteins and DNA. In nuclear and mitochondrial DNA, 8-OHdG or 8-oxodG is one of the predominant agents of free-radical-induced oxidative lesions. This is why 8-OHdG has been used widely in many studies as a biomarker for the measurement of endogenous oxidative DNA damage, and as a risk factor for many diseases including cancer, because urinary 8-OHdG is a good biomarker for risk assessment of various cancers and other degenerative diseases. 8-OHdG can be measured with immunohistochemistry and, for example by enzyme-linked immunosorbent assay (ELISA) or high pressure liquid chromatography, with mass spectrometric or electrochemical detection (HPLC-MS/MS) in serum or urine samples. Creative Proteomics can provide feasible methods to our customs and satisfy the needs of academic and industrial study.
With integrated set of separation, characterization, identification and quantification systems featured with excellent robustness & reproducibility, high and ultra-sensitivity, Creative Proteomics provides reliable, rapid and cost-effective 8-Hydroxydeoxyguanosine targeted metabolomics services.
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