Use of the Comet assay to investigate the role of superoxide in glutathione-induced DNA damage

Abnormal increases in reactive oxygen species in dying insects infected with nematodes

Stress enhances the concentration of reactive oxygen species (ROS) in animal plasma. Increased ROS alter various physiological functions, such as development and the immune response, but excessive increases could be harmful. In this study, we tested the hypothesis that abnormally increased plasma ROS levels are associated with animal death. Injection of the nematode Caenorhabditis elegans into insect larvae caused high mortality in Galleria mellonella, and the plasma ROS concentration was four times higher than M9 buffer-injected larvae. There was no difference in plasma antioxidant activity after nematode injection. However, coinjecting nematodes with an antioxidant (ascorbic acid or N-acetylcysteine) suppressed increases in ROS concentrations by the nematodes and increases in the number of nematodes in the larvae, which increased G. mellonella survival. These results suggest that the abnormal elevation of ROS associated with the stress caused by nematode propagation is lethal for G. mellonella.

Oxidative stress and DNA damage in the cord blood of preterm infants

Preterm birth infants are more susceptible to oxidative stress and aftermaths unwanted outcomes such as DNA damage due to hyperoxic stress. In this study, we compared the DNA strand breaks as one of the results of DNA oxidation in white blood cells, malondialdehyde (oxidative stress marker), catalase and superoxide dismutase activity, and total antioxidant capacity (markers of antioxidant defense) in a cord blood plasma of a group of preterm (n=25) and full term births (n=25). The primary DNA damage and plasma oxidative stress markers were significantly higher in a preterm group (p<0.05). Cord plasma activity of superoxide dismutase was significantly lower in preterm infants (p≤0.001). However, there were no significant differences in the cord blood total antioxidant capacity, catalase activity and malondialdehyde in preterm and term infants. Among the oxidative stress markers, the malondialdehyde concentration showed the strongest effect size (1.54; 95%CI: 0.9-2.17). For comet parameters, the most powerful effect size was observed for tail length (5.24; 95% CI: 4.05-6.42). However, tail DNA percent and tail moment were also significantly higher in cases compared to controls. Significant negative correlation was observed between comet assay parameters and birth weight and gestational age when all cases and controls entered into the analysis. There was no significant association between the levels of oxidative stress markers and early DNA damage in cord blood plasma with future nutritional tolerance in preterm infants. In the present study, the primary DNA damage and plasma oxidative stress markers significantly were increased in a preterm group. Preterm babies are more prone to the outcomes related to the early DNA damage. Tail DNA percent does not depend on experimental conditions as other parameters (tail length and thus also tail moment) and can be used for comparison with other studies.

Bacillus Calmette-Guérin induces cellular reactive oxygen species and lipid peroxidation in cancer cells

OBJECTIVE

To determine whether Bacillus Calmette-Guérin (BCG) and/or BCG-soluble factors could modulate cellular reactive oxygen species (ROS) in human bladder cancer cells and the impact this could have on response to therapy.

METHODS

The expression of α5β1 integrins on human bladder cancer cell lines and their ability to internalize BCG were determined. The effect of live and lyophilized BCG on cellular ROS, lipid peroxidation, and DNA damage was determined using H(2)DCF-DA, TBARS, and comet assays. The cytotoxic effects of live and lyophilized BCG on cancer cells were determined after 24 hours. ROS modulation by Antigen 85B and mycobacterial protein tyrosine phosphatases was monitored.

RESULTS

Live and lyophilized BCG were internalized to a similar extent, but live BCG increased cellular ROS, whereas lyophilized BCG reduced ROS. High ROS levels correlated with increased lipid peroxidation. The cytotoxic effect of BCG was independent of cellular ROS but dependent on internalization. Lyophilized BCG was more cytotoxic to bladder cancer cells than live BCG. BCG soluble factors such as Antigen85B could increase cellular ROS. Internalization of lyophilized BCG abrogated the ROS, and lipid peroxidation increase induced by BCG soluble factors. Both live and lyophilized BCG induced DNA damage but to different extents.

CONCLUSION

The end products of ROS, such as lipid peroxides and superoxide, could induce DNA damage, which could lead to mutations in cancer cells that select for their survival. Reducing BCG instillations may reduce the risk of mutational changes occurring in remnant cancer cells.

Acellular comet assay: a tool for assessing variables influencing the alkaline comet assay

In this study, an acellular modification to the alkaline comet assay to further evaluate key variables within the assay that may influence the outcome of genotoxicity studies is described. This acellular comet assay can detect differences of 0.2 Gy of (60)Co gamma-ray radiation between 0 and 1 Gy and differences of 1 Gy between 0 and 8 Gy; thus, this assay is applicable for a wide range of DNA damage levels. It is also shown that DNA damage from different radiation energies was not significantly different from (60)Co gamma-ray. This assay displayed a statistical increase in DNA damage due to uncontrolled exposure to natural light; however, the slope of the dose-response curve for light-exposed samples was similar to that for samples protected from light. A comparison of the alkaline comet assay with the acellular comet assay allowed for the intrinsic repair capacity of the alkaline comet assay to be quantified.

α-Lipoic acid and glutathione protect against the prooxidant activity of SOD/catalase mimetic manganese salen derivatives

Manganese(III) N,N'-ethylenebis(salicylideneiminato) chloride (Mn-salen chloride) and manganese(III) N,N'-ethylenebis(3-methoxysalicylideneiminato) chloride (Mn-(3,3'-MeO)salen chloride) are in vitro superoxide dismutase and catalase mimetics. They protect against free radical-related disease in animals, but Mn-salen can also be a potent prooxidant, damaging free DNA. Mn-salen protects human fibroblast DNA against hydrogen peroxide damage, however, damage to free DNA was confirmed by the comet assay. The DNA-damaging activity was dramatically reduced by co-administration with glutathione with the combination being less damaging to free DNA than either molecule alone. alpha-Lipoic acid, an antioxidant disulfide commonly used as a dietary supplement, also prevented Mn-salen prooxidant activity. Mn-(3,3'-MeO)salen protected fibroblasts against hydrogen peroxide as efficiently as Mn-salen and showed little damaging activity against free DNA. Protection was invested by both complexes in the presence and in the absence of EDTA, a potential competing chelator. Stabilities of the complexes with respect to decomposition and inactivation were studied by spectroscopic and electrochemical techniques. The complexes' binding to, and cleavage of, DNA was measured using a quartz crystal resonant sensor. Mn-salen was shown to bind strongly to DNA, prior to cleaving it; Mn-(3,3'-MeO)salen bound weakly and left DNA intact. Co-administration of either glutathione or alpha-lipoic acid appears to inhibit binding by Mn-salen thus preventing DNA-cleavage.

Role of endogenous oxidative DNA damage in carcinogenesis: what can we learn from repair-deficient mice?

Basal steady-state levels of oxidative DNA base modifications such as 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxoG) are observed in all types of cells, most probably due to a continuous generation of reactive oxygen species (ROS) in the cellular oxygen metabolism, and it has long been suspected that they might play an important role in the initiation of carcinogenesis. Experimental evidence for this assumption can be obtained by studying the effects of a modulation of the steady-state levels, either by in- or decreasing the generation of oxidative DNA damage, on spontaneous mutation rates and cancer incidence. However, clear answers have not yet been obtained by these strategies. It is still doubtful whether an efficient reduction of the in vivo steady-state levels can be achieved by application of antioxidants, and effects observed under oxidative stress conditions (i.e. increased oxidative DNA damage) are inconclusive due to the pronounced epigenetic effects of ROS on signal transduction and gene expression (tumor promotion). In addition, the reliable quantification of the basal levels of oxidative DNA modifications is still a major problem. Recently, the generation of mice deficient in the repair 8-oxoG (ogg1-/- mice) has opened the door for an alternative approach. Results obtained so far indicate that an increase by less than five 8-oxoG residues per 106 bp in the liver of the knockout animals is associated with a two- to threefold higher spontaneous mutation frequency in transgenic genes. However, the increase in the ogg1-/- mice of the steady-state level of 8-oxoG and the spontaneous mutation frequency was only observed in the liver and apparently too small to enhance the spontaneous cancer incidence significantly. The limited effect seems to be due to a back-up repair system for 8-oxoG in the ogg1-/- mice, and it can be expected that the inactivation of this pathway in double-knockout mice will lead to higher effects and a better assessment of the risk associated with endogenous oxidative DNA damage.

Single cell gel electrophoresis assay: methodology and applications

The single cell gel electrophoresis or Comet assay is a sensitive, reliable, and rapid method for DNA double- and single-strand breaks, alkali-labile sites and delayed repair site detection, in eukaryotic individual cells. Given its overall characteristics, this method has been widely used over the past few years in several different areas. In this paper we review the studies published to date about the principles, the basic methodology with currently used variations. We also explore the applications of this assay in: genotoxicology, clinical area, DNA repair studies, environmental biomonitoring and human monitoring.

Factors affecting the DNA damaging activity of superoxide and nitric oxide

Nitric oxide and superoxide are formed endogenously and can react with each other and with other molecules to form a range of secondary and tertiary products. Some of these (e.g., peroxynitrite) are potent DNA-damaging agents and others (e.g., S-nitrosoglutathione) can act as reservoirs of the reactive species. Although the chemistry of these processes is now becoming understood, the question of which products are significant in vivo is not necessarily clear. To investigate these processes we have developed a cell-free version of the Comet assay, where the DNA from isolated nuclei is treated in agar on a microscope slide, following lysis. This offers an exceptionally sensitive assay for strand breakage in free DNA. Despite being present as a scavenger in the cell at millimolar levels, glutathione can act as a DNA-damaging pro-oxidant. Under appropriate conditions, glutathione-mediated damage is suppressed by superoxide dismutase and we suggest that superoxide may be a direct damaging agent, whose activity can be masked because of the involvement of superoxide in indirect mediation of damage or because of concomitant presence of hydroxyl radical.