ReviewUrinary 8-OHdG: a marker of oxidative stress to DNA and a risk factor for cancer, atherosclerosis and diabetics
Introduction
In living cells reactive oxygen species (ROS), including superoxide anion radical, hydrogen peroxide(•O2), hydroxyl radical (OH⋅), are formed continuously as a consequence of metabolic reactions. For example, mitochondria (oxidative phosphorylation), leukocytes (oxidative burst), peroxisomes (degradation of fatty acids) and cytochrome p450 system (mixed function oxidation system) can all release ROS. Under normal physiological conditions, there is a balance maintained between endogenous oxidants and antioxidants. When an imbalance occurs, created by the excessive generation of oxidants or a decrease of antioxidants, the abnormal oxidant system then enters what is called oxidative stress [1]. In the presence of oxidative stress, ROS generated in vivo can cause oxidative damage to lipids, proteins and nucleic acids. As a result, the DNA is constantly being damaged and oxidatively modified. It should be noted that DNA oxidative damage can also occur from exogenous ROS, such as cigarette smoking, UV radiation, and ionizing radiation [2].
Various markers of oxidative damage have been identified [1]. In the past, the most popular markers were designed for lipid peroxidation, such as malondialdehye (MDA), oxidized LDL, MDA-modified LDL, auto-antibodies against oxidized LDL and MDA-modified LDL, F2-isoprostane, and conjugated diene. The detection of a new carbonyl group, dityrosine and oxidized histidine has been measured to indicate protein oxidation. Markers for DNA oxidation were few. Only in recent years has 8-hydroxy-2′-deoxyguanosine (8-OHdG, or 8-oxodG) emerged as a marker of oxidative stress [3]. Urinary 8-OHdG, in particular, has been measured most frequently to indicate the extent of oxidative damage because it is noninvasive and technically less involved.
It is well known that the study of oxidative DNA damage is clinically important [1]. Numerous studies have shown that oxidative DNA damage links pathogenically to a variety of aging-associated degenerative diseases such as cancer, coronary heart disease and diabetes. Quantification of urinary 8-OHdG, a specific DNA repair products in the urine, can be made with a simple ELISA [4].
Section snippets
Oxidative DNA lesion
Nuclear and mitochondrial DNA from tissue and blood lymphocyte is usually the site of oxidation damage [5]. Among all purine and pyridine bases, guanine is most prone to oxidation. Upon oxidation a hydroxyl group is added to the 8th position of the guanine molecule and the oxidatively modified product 8-OHdG (Fig. 1) is one of the predominant forms of free radical-induced lesions of DNA. Oxidative modified DNA in the form of 8-OHdG can be quantified to indicate the extent of DNA damage. In
Nomenclature
Names used in the literature for oxidatively oxidized products have not been standardized and have created much confusions during reading. For example:
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The abbreviation for the major oxidized nucleoside mentioned in this review, 8-hydroxy-2′-deoxyguanosine, is 8-OHdG. But 8-oxodG is also being used in the literature.
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8-OHdG is the oxidized product of DNA, whereas 8-OHG is from RNA.
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8-OHGua is the oxidized product of free guanine base, whereas 8-OHG represents 8-hydroxy-2′-guanosine, an oxidized
High-performance liquid chromatography (HPLC)
In the past, an HPLC procedure with electrochemical detection has been used most frequently for the determination of 8-OHdG. HPLC remains the method to quantify 8-OHdG in tissue, lymphocyte and plasma. To analyze 8-OHdG in tissue and lymphocyte, 8-OHdG has to be released from the nuclear DNA into soluble compound with enzymes before it can be quantified by HPLC. HPLC procedure has the advantage of measuring several oxidized products at the same time.
HPLC procedure has also been used to
Impact on carcinogenesis
The oxidative hydroxylation of guanine in the 8-position is the most frequent and most mutagenic lesion in nuclear DNA. Oxidative damage to DNA, reflected in the formation of 8-OHdG, is important in mutagenesis and carcinogenesis. The 8-hydroxylation of guanine (8-OHGua) leads to lack of base pairing specificity and misreading of the modified base and adjacent residues. During the repair of deficient bacterial and yeast cells, it was found that there is an increase of the frequency of
Impact on atherogenesis
It is known that ROS and reactive nitrogen species (such as peroxynitrite) are generated in both atherogenesis and advanced atherosclerosis, particularly by macrophages [31], [32]. Therefore, oxidative DNA damage not only linked to an increased risk of developing cancer but also several degenerative chronic diseases, including coronary artery disease (CAD).
It is well known that the formation of ROS promotes cell proliferation in atheroslerosis, hypertrophy, growth arrest, apoptosis and
Impact on diabetes
Increasing evidence in both experimental and clinical studies suggests that oxidative stress plays a major role in the pathogenesis of both types of diabetes mellitus (I and II). Oxidative stress appears to be the pathogenic factor in underlying diabetic complications. Free radicals are formed disproportionately in diabetes by glucose oxidation, nonenzymatic glycation of proteins, and the subsequent oxidative degradation of glycated proteins. Consequently, the free radicals thus generated
Normal reference range
We now realize that because ROS can be produced endogenously such as from mitochondria, oxidative damage also occurs in normal individuals. Therefore we have made an attempt establishing normal reference range for oxidatively modified DNA and RNA including urinary 8-OHdG with our in-house ELISA [4]. We found that the normal range of urinary oxidatively damaged products for females was 43.9±42.1 ng/mg creatinine (N=486), and 29.6±24.5 ng/mg creatinine for males (N=548). The age distribution of
Monitoring antioxidant supplementation
Urinary 8-OHdG has also been used to assess the effectiveness of dietary supplements with regard to whether they will reduce the oxidative damage. For example, the measurement of the urinary excretion rate of 8OHdG was used to demonstrate the protective effect of dietary supplementation with antioxidants on the prevention of carcinogenesis [50]. The level of 8-OHdG in sperm DNA has also been used to determine the effect of vitamin C intake [52]. Levels of 8-OHdG in mononuclear cell DNA, serum
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