Free radicals and DNA damage -
The free radical mechanism to treat certain types of cancers extends beyond enediynes. Tirapazamine generates a free radical under anoxic conditions instead of the trigger mechanism of an enediyne. The free radical then continues on to cleave DNA in a similar manner to 1,4-didehydrobenzene in order to treat cancerous cells.
It is currently in Phase III trials. Meiosis is a central feature of sexual reproduction in eukaryotes. The need to repair oxidative DNA damage caused by oxidative free radicals has been hypothesized to be a major driving force in the evolution of meiosis [16] [17]. Contents move to sidebar hide. Article Talk.
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Damage to DNA as a result of exposure to ionizing radiation or radiomimetic compounds. See also: DNA damage naturally occurring and DNA repair. Ecological Chemistry and Engineering. Advances in Physical Organic Chemistry.
doi : ISBN PMC PMID DNA damage produced by ionizing radiation in mammalian cells: identities, mechanisms of formation, and reparability. Progress in Nucleic Acid Research and Molecular Biology. The Journal of Biological Chemistry.
S2CID Chemical Reviews. Proceedings of the National Academy of Sciences of the United States of America. Bibcode : PNAS A new enediyne warhead". Antitumor activity". The Journal of Antibiotics. Bibcode : Sci On the irrelevancy of hydroxyl radical to DNA damage from oxidative stress and implications for epigenetics.
Aaron M. Fleming a and Cynthia J. This article is part of the themed collection: Nucleic Acids. You have access to this article.
Please wait while we load your content Something went wrong. Try again? Cited by. Download options Please wait Article type Viewpoint. Submitted 21 May First published 12 Aug Download Citation. Other exocyclic DNA adducts that arise from lipid peroxidation have been detected in DNA from healthy human beings.
The precise pathway of their formation in DNA is uncertain but the adducts can be generated in vitro by exposure of DNA to peroxidizing lipid The epoxide of HNE reacts with dA to form etheno-dA so it is a potential intermediate leading to etheno adducts Figure 4 The biological activity of etheno adducts has been extensively investigated by site-specific mutagenesis experiments.
Loeb and co-workers 90 reported that N 2 ,3-etheno-dG induces transitions to A, and Langouet et al. Basu et al. Likewise, Moriya et al. However, when the same adducted shuttle vector is transfected into monkey kidney COS-7 cells, both adducts are highly mutagenic.
Etheno-dA induces mainly transitions to G whereas etheno-dC induces transversions to A and transitions to T The differentials in mutagenic potency between bacterial and mammalian cells are striking and highlight the importance of the cell system used to evaluate the mutagenic activity of a given lesion.
Investigation of the repair of etheno adducts has focused on the base excision repair pathway. Several laboratories have demonstrated that etheno-dA is removed by the action of 3-methyladenine glycosylase and its mammalian homolog AAG also called ANPG 95 — Recently, Saparbaev and Laval reported that etheno-dC is a substrate for the G:T mismatch glycosylase from both E.
coli and human cells Despite the fact that etheno adducts are good substrates for removal by glycosylases, other repair pathways should be considered. For example, the mutagenicity of 1, N 2 -etheno-dG increases when vectors containing it are transfected into E.
coli strains deficient in nucleotide excision repair As mentioned above, the structurally-related adduct, M 1 G, also is removed by the nucleotide excision repair pathway Chung and colleagues have demonstrated that hydroxypropanodeoxyguanosines HO-PdGs are present in human and rodent liver DNA Figure 5 99 , These adducts appear to be derived by reaction of DNA with acrolein and crotonaldehyde generated by lipid peroxidation.
Acrolein and crotonaldehyde are mutagenic in bacteria and mammalian cells but the mutagenic potency of HO-PdGs has not been evaluated by site-specific approaches , This relates to the instability of these adducts, which renders their incorporation into oligonucleotides or shuttle vectors challenging.
A novel, post-oligomerization strategy for the synthesis of oligonucleotides containing the acrolein-derived HO-PdG was reported recently, which should make it possible to construct the requisite adducted vectors The unsubstituted adduct, PdG, has been evaluated by site-specific approaches.
PdG is not a naturally occurring adduct but has been used as a model for several unstable exocyclic adducts, including the HO-PdGs. PdG induces base pair substitution mutations in E. coli with high efficiency 93 , , Interestingly, transformation of PdG-adducted shuttle vectors into COS-7 cells reveals that PdG is much less mutagenic than in E.
coli This is opposite to the effects described above with etheno-dA and etheno-dC. Little is known about the repair of HO-PdGs. PdG is a substrate for the nucleotide excision repair complex of E.
coli as well as mammalian cells and it is also recognized and repaired by the mismatch repair system 81 , The possibility that PdG or HO-PdGs are substrates for base excision repair enzymes need to be evaluated in a comprehensive fashion. Numerous studies have been conducted on the levels of oxidized bases or exocyclic adducts in tumor tissue or in chronic inflammatory states.
A variety of assays have been employed so it is instructive to compare them. Malins and Haimanot first reported an association of oxidized DNA bases with breast cancer, demonstrating a 9-fold elevation in the levels of 8-oxo-G, 8-oxo-A and a formamidopyrimidine in tumor tissue compared with surrounding normal tissue Subsequently, Shimoda et al.
reported that the levels of 8-oxo-dG in liver tissue from individuals with chronic inflammatory diseases hepatitis, cirrhosis are elevated compared with control liver Similar findings were reported in a transgenic mouse model that expressed the hepatitis B virus large envelope protein in the liver Elevated 8-oxo-dG was detected early in life and increased progressively with the progression of disease.
The levels of 8-oxo-dG in the DNA from gastric tissue of patients infected with Helicobacter pylori are elevated relative to the levels in uninfected individuals The levels are highest in patients with chronic atrophic gastritis and reduced in patients with chronic non-atrophic gastritis and patients with gastric cancer.
Individuals with hyperplastic diseases of the stomach exhibit higher levels of 8-oxo-dG than healthy controls Several analyses have been conducted of the levels of 8-oxo-G or 8-oxo-dG excreted in urine. Oxidized base and deoxynucleoside adducts are presumed to represent the products of repair of oxidized DNA but there is some concern that they may reflect damage to the nucleoside pool or dietary oxidized bases.
Surprisingly, no obvious association appears between 8-oxo-dG levels in urine and the incidence of liver inflammation or gastric inflammation triggered by Helicobacter infection Likewise, in clinical studies, no effect on the level of 8-oxo-dG is observed with age or the presence of plasma antioxidants vitamin E, vitamin C, β-carotene, lycopene, coenzyme Q or glutathione transferase Only a relatively modest effect of cigarette smoking has been reported in one study and no effect in another , In contrast, urinary 5- hydroxymethyl uracil levels are increased in smokers compared with non-smokers and in patients treated with adriamycin , This may be a better urinary biomarker than 8-oxo-dG.
The inability to detect increases in urinary 8-oxo-dG with inflammation and oxidative stress may be due to continual whole body oxidation of the guanine nucleotide pool that produces a significant background level of urinary 8-oxo-dG.
This background may make it difficult to detect increases associated with repair of 8-oxo-G or 8-oxo-dG from genomic DNA in a particular tissue. Continual oxidation of the polyunsaturated fatty acid pool occurs as reflected in the levels of plasma and urinary isoprostanes detected in all human beings so it is likely that the nucleotide pool also undergoes continual oxidation However, it should be noted that despite the continual spontaneous production of isoprostanes, an ~3-fold increase in their urinary levels is detectable in cigarette smokers and is reversible on smoking cessation This increase is considerably larger than the modest increases in urinary 8-oxo-dG reported in smokers.
The levels of exocyclic adducts have been investigated in clinical disorders believed to be associated with enhanced oxidative stress.
Wilson's disease is a copper storage disease that leads to abnormal copper accumulation and oxidative stress in the liver. The levels of etheno-A and etheno-C in liver DNA are elevated ~3-fold in patients with Wilson's disease An increase of similar magnitude is detected in individuals with primary hemochromatosis, an iron-storage disease that also causes oxidative stress in the liver HO-PdGs are detectable in oral tissue DNA of humans and their levels are significantly higher in smokers than in non-smokers An interesting dietary study has been reported by Nair et al.
Men and women administered a diet high in ω-6 fatty acids exhibit an ~fold increase in M 1 G levels in their white blood cells, relative to individuals fed a diet high in ω-3 fatty acids.
Interestingly, an even larger increase is detected in the levels of etheno-A and etheno-C ~fold in white cell DNA but only in women; the levels in men are not elevated.
This finding underscores the potential for dietary modulation of DNA damage from endogenous sources but illustrates that variations will be observed in the levels of individual adducts.
This and other studies indicate that one cannot assume that trends in the production of all oxyradical-derived DNA adducts will correlate Animal models of chronic inflammatory diseases also have been probed for the levels of exocyclic adducts.
The levels of both etheno adducts and HO-PdGs are significantly increased in the livers of Long—Evans Cinnamon rats relative to Long—Evans Agouti rats , Long—Evans Cinnamon rats have an abnormality in copper metabolism that renders them highly sensitive to copper-induced oxidative stress Thus, they are a model for Wilson's disease and there appears to be an excellent correlation between the increases in the levels of etheno adducts in the livers of patients with Wilson's disease and in the livers of Long—Evans Cinnamon rats.
Etheno adducts also have been demonstrated to increase in the spleens of SJL mice stimulated to produce high levels of nitric oxide Finally, Nath et al. have demonstrated that depletion of glutathione by administration of buthionine sulfoximine to F rats leads to a significant increase in the levels of the HO-PdGs in the livers of treated animals There is compelling evidence that oxidative metabolism generates a diverse range of adducts in DNA that are efficient premutagenic lesions and that repair systems have evolved to remove some of them.
Thus, it seems reasonable to presume that this background DNA damage contributes to the origin of human genetic diseases such as cancer. However, providing a smoking gun to conclusively establish this is not easy. All of the evidence relating adduct levels to malignancy or treatments associated with malignancy are correlative in nature.
Although some of these correlations are impressive, it is difficult to make the leap from adduct levels to mutations in critical genes causative for cancer. In fact, this is a problem that plagues carcinogenesis research in general, regardless of whether the DNA damage originates from exogenous or endogenous sources.
Attempts have been made to use mutation spectra to perform retroanalyses of the adducts that lead to mutations. There is a complication of too many mutagens and too few mutations for this approach to be generally applied.
However, this double mutation has only been seen in a few tumors of internal organs, at least as judged from the mutation spectra of p53 genes from human cancers It may be possible to use second-order effects e.
sequence-context effects to implicate individual DNA adducts. coli lacI gene and the human p53 and factor IX genes This approach may provide a useful strategy for probing the involvement of other adducts.
Our understanding of the proteins involved in DNA replication and repair is expanding rapidly. Concomitant with this expansion of knowledge is an increasing availability of mice bearing targeted deletions in the genes that code for these proteins.
These animals will be extremely useful for defining which repair enzymes are primarily responsible for the removal of individual DNA adducts. Most DNA adducts derived from oxygen radicals are small and relatively polar and there appears to be overlap in their removal by various repair systems.
For example, M 1 G is removed by both nucleotide excision repair and mismatch repair. It will be interesting to measure the endogenous level of M 1 G in knockout mice that are deficient in genes that code for proteins in this pathway. It also will be interesting to assess whether animals with deletions in DNA repair enzymes, for example coding for specific glycosylases, are more sensitive to mutations and cancer.
Of course, mice deficient in DNA repair genes may not exhibit increased cancer incidence if mutation is not rate-limiting for the development of cancers at particular sites.
Abundant evidence indicates that cell proliferation is a critical component of carcinogenesis and can be rate-limiting even in the presence of a significant adduct load in a given tissue , Thus, it is possible that steady-state adduct levels could be increased by an order of magnitude or more in a particular tissue without any detectable increase in tumor incidence if proliferation or some other event is critical for development of the tumor.
This leads into the final disclaimer of this article. The entire focus of this presentation has been on DNA damage and mutagenesis but oxygen radicals, like all electrophiles, react with many molecules in a cell and can induce a range of responses that are not necessarily dependent on DNA damage Oxygen radicals are known to affect various signal transduction pathways and transcription factors e.
NFκB and to alter cell cycle kinetics Some of these effects may be the result of DNA damage-induced signaling but others may not. For example, HNE, an important product of lipid peroxidation, inhibits signalling by NFκB The target for the effects of HNE are not in the nucleus but rather the upstream events that lead to activation of IκB kinases.
Inhibition of IκB kinase activation prevents dissociation of IκB from NFκB in the cytosol so it cannot diffuse to the nucleus to initiate gene expression. The development of microarrays that allow large-scale and simultaneous profiling of gene expression should provide a facile mechanism for evaluating the epigenetic effects of oxygen radicals and their endogenous products.
This approach should be especially powerful when it employs cell lines from genetically defined animals e. from knockout mice with DNA repair defects. Thus, one may be able to separate events initiated by reactions in the nucleus from those initiated by reactions elsewhere. In roughly 20 years, the study of oxygen radical-dependent damage to DNA has become a major thrust of carcinogenesis research.
To some extent, this reflects a natural shift in emphasis for researchers as new opportunities open up. But it also reflects the more basic consideration that oxygen radical generation is an inevitable consequence of aerobic life.
It is the price we pay for extracting an extra 12 ATP molecules from every molecule of acetyl CoA that we combust. Oxygen radicals conduct an unremitting assault on our genomes that can be augmented or reduced by environmental, nutritional or hormonal influences.
Our bodies construct Star Wars-like defenses to scavenge incoming oxidants and Civil Defense teams to repair the damage. But these defenses are not perfect and some damage persists until the genome needs to be copied.
Then mutations arise that can contribute to the development of cancer. We do not yet know the precise role that oxygen radical damage plays in carcinogenesis and how it synergizes with other forms of genetic and epigenetic events to accelerate cell transformation and malignant progression.
Research in the last 20 years has identified the molecular players, the defenses against them and the consequences of their attack on the genome.
The challenge for the next 20 years is to integrate that knowledge with expanding road maps of intracellular and intercellular signal transduction to define the key steps in the carcinogenic process triggered by oxygen radicals.
Ring-opening of M 1 G in duplex DNA when positioned opposite C but not when opposite T. Reaction of 2,3-epoxyhydroxynonenal with DNA bases to make etheno adducts. Email: marnett toxicology. Work in the author's laboratory is supported by a research grant from the National Institutes of Health CA and Evans,J.
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Aside from impaired energy production, damage fadicals the mitochondria leads also Ffee increased production ad toxic molecules called Rdaicals radicals. Compounds called antioxidants Swift fat breakdown as free radical scavengers by initiating Free radicals and DNA damage that make damate radicals non-toxic to cells. Raxicals indicates that damage by free radicals is a contributing factor to the pathology of HD. Consequently, compounds with antioxidant properties are being studied to see if they can serve as possible treatments for HD. Free radicals are atoms or molecules that are highly reactive with other cellular structures because they contain unpaired electrons. Free radicals are natural by-products of ongoing biochemical reactions in the body, including ordinary metabolic processes and immune system responses. Free radical-generating substances can be found in the food we eat, the drugs and medicines we take, the air we breathe, and the water we drink.
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