Mutagenic and cytotoxic effects of oxygen free radicals generated by methylviologen (paraquat) on Escherichia coli with different DNA-repair capacities

Coping with Reactive Oxygen Species to Ensure Genome Stability in Escherichia coli

The facultative aerobic bacterium Escherichia coli adjusts its cell cycle to environmental conditions. Because of its lifestyle, the bacterium has to balance the use of oxygen with the potential lethal effects of its poisonous derivatives. Oxidative damages perpetrated by molecules such as hydrogen peroxide and superoxide anions directly incapacitate metabolic activities relying on enzymes co-factored with iron and flavins. Consequently, growth is inhibited when the bacterium faces substantial reactive oxygen insults coming from environmental or cellular sources. Although hydrogen peroxide and superoxide anions do not oxidize DNA directly, these molecules feed directly or indirectly the generation of the highly reactive hydroxyl radical that damages the bacterial chromosome. Oxidized bases are normally excised and the single strand gap repaired by the base excision repair pathway (BER). This process is especially problematic in E. coli because replication forks do not sense the presence of damages or a stalled fork ahead of them. As consequence, single-strand breaks are turned into double-strand breaks (DSB) through replication. Since E. coli tolerates the presence of DSBs poorly, BER can become toxic during oxidative stress. Here we review the repair strategies that E. coli adopts to preserve genome integrity during oxidative stress and their relation to cell cycle control of DNA replication.

AP endonuclease EXO-3 deficiency causes developmental delay and abnormal vulval organogenesis, Pvl, through DNA glycosylase-initiated checkpoint activation in Caenorhabditis elegans

AP endonuclease deficiency causes cell death and embryonic lethality in mammals. However, the physiological roles of AP endonucleases in multicellular organisms remain unclear, especially after embryogenesis. Here, we report novel physiological roles of the AP endonuclease EXO-3 from larval to adult stages in Caenorhabditis elegans, and elucidated the mechanism of the observed phenotypes due to EXO-3 deficiency. The exo-3 mutants exhibited developmental delay, whereas the apn-1 mutants did not. The delay depended on the DNA glycosylase NTH-1 and checkpoint kinase CHK-2. The exo-3 mutants had further developmental delay when treated with AP site-generating agents such as methyl methane sulfonate and sodium bisulfite. The further delay due to sodium bisulfite was dependent on the DNA glycosylase UNG-1. The exo-3 mutants also demonstrated an increase in dut-1 (RNAi)-induced abnormal vulval organogenesis protruding vulva (Pvl), whereas the apn-1 mutants did not. The increase in Pvl was dependent on UNG-1 and CHK-2. Methyl viologen, ndx-1 (RNAi) and ndx-2 (RNAi) enhanced the incidence of Pvl among exo-3 mutants only when combined with dut-1 (RNAi). This further increase in Pvl incidence was independent of NTH-1. These results indicate that EXO-3 prevents developmental delay and Pvl in C. elegans, which are induced via DNA glycosylase-initiated checkpoint activation.

Construction and evaluation of two biosensors based on yeast transcriptional response to genotoxic chemicals

It has been well established that essentially all microbial mutagens are rodent carcinogens, yet current mutagen detection systems are limited by their detection sensitivity. Here we report the construction of a pair of hypersensitive biosensors by optimizing both reporters and the host strain. The resulting RNR3-yEGFP and HUG1-yEGFP reporters and the septuple yeast mutant in combination with the automated protocol not only remarkably enhance the detection sensitivity, but also allow a high throughput screen of environmental genotoxins. This system is deemed much more sensitive than similar yeast and bacterium-based tests for all selected chemicals examined in this study.

Inactivation of YAP1 enhances sensitivity of the yeast RNR3-lacZ genotoxicity testing system to a broad range of DNA-damaging agents

Despite the great advances by using microorganism-based genotoxicity testing systems to assess environmental genotoxic compounds, most of them respond poorly, particularly to oxidative agents. In this study, we systematically examined the RNR3-lacZ reporter gene expression in Saccharomyces cerevisiae mutant strains defective in the protection against reactive oxygen species and found that only YAP1 deletion resulted in a significant enhancement in the detection of oxidative damage. To our surprise, YAP1 deletion also caused an increased cellular sensitivity to a variety of DNA damage. This altered sensitivity appears to be independent of oxidative damage because under conditions in which vitamin C treatment rescued oxidative damage, it failed to reverse the phenotypes caused by other types of DNA damage. Furthermore, although inactivation of cell permeability genes enhanced the RNR3-lacZ detection sensitivity particularly to large molecular weight compounds, their effects on small molecular oxidative agents are minimal. Taken together, this study helps to create a hypersensitive genotoxicity testing system to a broad range of DNA-damaging agents by deleting a single yeast gene.

XRCC1 protects against the lethality of induced oxidative DNA damage in nondividing neural cells

XRCC1 is a critical scaffold protein that orchestrates efficient single-strand break repair (SSBR). Recent data has found an association of XRCC1 with proteins causally linked to human spinocerebellar ataxias—aprataxin and tyrosyl-DNA phosphodiesterase 1—implicating SSBR in protection against neuronal cell loss and neurodegenerative disease. We demonstrate herein that shRNA lentiviral-mediated XRCC1 knockdown in human SH-SY5Y neuroblastoma cells results in a largely selective increase in sensitivity of the nondividing (i.e. terminally differentiated) cell population to the redox-cycling agents, menadione and paraquat; this reduced survival was accompanied by an accumulation of DNA strand breaks. Using hypoxanthine–xanthine oxidase as the oxidizing method, XRCC1 deficiency affected both dividing and nondividing SH-SY5Y cells, with a greater effect on survival seen in the former case, suggesting that the spectrum of oxidative DNA damage created dictates the specific contribution of XRCC1 to cellular resistance. Primary XRCC1 heterozygous mouse cerebellar granule cells exhibit increased strand break accumulation and reduced survival due to increased apoptosis following menadione treatment. Moreover, knockdown of XRCC1 in primary human fetal brain neurons leads to enhanced sensitivity to menadione, as indicated by increased levels of DNA strand breaks relative to control cells. The cumulative results implicate XRCC1, and more broadly SSBR, in the protection of nondividing neuronal cells from the genotoxic consequences of oxidative stress.

Role of lipid peroxidation and DNA damage in paraquat toxicity and the interaction of paraquat with ionizing radiation

Since the introduction of paraquat (PQ) as a herbicide in 1963, there have been many speculations concerning the critical lesion in PQ toxicity. Damage to membrane lipids might be an initial event leading to PQ-induced cell killing. The ability of PQ to induce lipid peroxidation was tested in liver homogenates of the mouse. Lipid peroxidation was indeed induced by PQ and shown to be dose dependent, starting to be significant at 2.5 mM. Subsequently, a possible correlation between lipid peroxidation and PQ-induced cell death was investigated in mouse fibroblasts (LM) and Ehrlich ascites tumour (EAT) cells using a clonogenic assay. It was found that in order to be cytotoxic PQ needs enzymatic activation (incubation at 37 degrees). In both cell lines, PQ-induced cell killing appeared to be dose dependent, starting at a dose of 0.5 mM. Supplementation of LM cells with the antioxidant vitamin E had no effect on PQ-induced cell killing and modification of the membranes of LM cells by incorporation of the polyunsaturated fatty acid 20:4 (arachidonic acid) did not sensitize the cells to PQ toxicity. PQ had no effect on the glutathione (GSH) level in EAT cells and complete GSH depletion by DL-buthionine-(SR)-sulfoximine could not sensitize the cells to PQ toxicity. In LM cells PQ-induced cell killing was enhanced after complete GSH depletion by DEM. This sensitization might, however, be attributed to the binding of DEM to proteins. From these results it seems unlikely that lipid peroxidation is the primary cause for PQ-induced cell killing. Another critical target in PQ toxicity is DNA. This possibility was investigated in EAT cells. PQ was found to induce DNA damage (detected by the alkaline unwinding assay) in the same dose range that caused cell death. A good correlation was obtained for cell killing after PQ treatment and DNA damage measured 2 hr after 37 degrees post-incubation. A proposed possible interaction between PQ and X-rays was also investigated. In EAT cells, X-ray-induced cell death was significantly enhanced by pre-incubation with PQ at doses of 0.5 mM and above. At the level of 10% survival an enhancement factor of 1.6 could be observed by treatment with 1 mM PQ when cell killing by PQ is not taken into account. Induction as well as processing of radiation-induced DNA damage seems to be unaffected by pre-incubation with PQ. The mechanism of radiosensitization by PQ is yet unclear.

Characteristics of mutagenesis by glyoxal in Salmonella typhimurium: contribution of singlet oxygen

The characteristics of mutagenesis by glyoxal in Salmonella tester strains TA100 and TA104, and particularly a possible role of active oxygen species, were investigated. Glyoxal was converted into a non-mutagenic chemical with glutathione (GSH) by glyoxalase I, and the mutagenic activity was enhanced by the depletion of intracellular GSH. Glyoxal caused the reduction of nitro blue tetrazolium, which was suppressed by the addition of 2,5-diphenylfuran, superoxide dismutase (SOD) and catalase (CAT), scavengers of singlet oxygen (1O2), superoxide radical (O2-) and hydrogen peroxide (H2O2), respectively. However, only the 1O2 scavenger almost completely suppressed the mutagenic activity of glyoxal. Mutagenicity assays using strains pretreated with N,N-diethyldithiocarbamate of a SOD inhibitor and strains with low levels of SOD and CAT indicated that the mutagenesis by glyoxal was independent of intracellular levels of SOD and CAT, though glyoxal itself repressed them. Therefore, all the results suggest that 1O2 formed from glyoxal is related to its mutagenesis, but that neither O2- nor H2O2 is intracellularly predominantly related to it. The action of glyoxal against SOD and CAT, and the formation of glyoxal adducts with amino acids as their components are also discussed.

An oxygen dependence in chromium mutagenesis

Evidence is provided that mutagenicity in Salmonella by a chromium(VI) salt and a chromium(III) compound has a differential dependence on the presence of oxygen. The mutagenic chromium(III) compound, cis-dichlorobis(2,2'-bipyridyl)chromium(III), reverted Salmonella strains, TA102 and TA2638, only under aerobic conditions. Potassium dichromate (chromium VI) required the presence of oxygen to revert the Salmonella strain TA102 but induced a moderate reversion frequency in TA2638 under anaerobic conditions. The data also support a role for oxygen radicals in chromium-mediated mutagenesis and suggests at least two pathways by which chromium compounds can induce mutations.

Paraquat poisoning. An overview of the current status

Paraquat is a bipyridyl compound with no known chronic toxicity or teratogenicity. It is poorly absorbed when inhaled, but causes severe illness when ingested orally, death usually occurring within 2 days of ingestion of 50 mg/kg. At lower doses death may be delayed for several weeks. The toxic compound accumulates in lung tissue where free radicals are formed, lipid peroxidation is induced and nicotinamide adenine dinucleotide phosphate (NADPH) is depleted. This produces diffuse alveolitis followed by extensive pulmonary fibrosis. The most important prognostic indicator is the quantity of paraquat absorbed, as shown by the plasma paraquat concentration. While renal failure will develop in the majority of those patients who eventually die, it may not, if present alone, indicate a fatal outcome. The absence of caustic burns in the upper digestive tract indicates a good prognosis. Treatment of paraquat poisoning remains ineffective, but Fuller's earth, activated charcoal and resins may prevent some absorption of the toxin. When tubular necrosis occurs, renal excretion of the compound decreases rapidly. A 3-compartment pharmacokinetic model has been described following ingestion of tracer doses including a 'deep' compartment for active pulmonary accumulation. Haemodialysis, haemoperfusion and forced dialysis have been attempted, with no clear improvement in survival rates. Superoxide dismutase, glutathione peroxidase, N-acetylcysteine and other 'free radical scavengers' have failed to alter the outcome in poisoned patients. Other theoretical treatments, such as deferoxamine, immunotherapy, NADPH repletion and lung transplantation still require clinical validation.

Effects of pH, glucose, and chelating agents on lethality of paraquat to Escherichia coli

Retention of paraquat by Escherichia coli B was greatest after exposure at pH 9.0 and was progressively less after exposure at pH 7.0 and 5.0, respectively. This retained paraquat was capable of persistent growth inhibition. Uptake and retention of paraquat by E. coli B was dependent upon a carbon source, such as glucose. Under comparable conditions E. coli K-12 did not retain paraquat. The lethality of paraquat was seen in TSY medium but not in VB medium. The addition of Soytone, tryptone, or yeast extract, to the VB medium allowed the lethality of paraquat to be seen. A variety of chelating agents, including EDTA, 8-hydroxyquinoline, and o-phenanthroline, prevented the lethal effect of paraquat in TSY medium. Although EDTA protected against the lethality of paraquat, it did not protect against its bacteriostatic effect.

Effects of paraquat on Escherichia coli: differences between B and K-12 strains

Escherichia coli B and K-12 are equally susceptible to the bacteriostatic effects of aerobic paraquat, but they differed strikingly when the lethality of paraquat was evaluated. E. coli B suffered an apparent loss of viability when briefly exposed to paraquat, whereas E. coli K-12 did not. This difference depended on the ability of the B strain, but not the K-12 strain, to retain internalized paraquat; the B strain was killed on aerobic tryptic soy-yeast extract plates during the incubation which preceded the counting of colonies. This difference in retention of paraquat between strains was demonstrated by delayed loss of viability, by growth inhibition, and by cyanide-resistant respiration after brief exposure to paraquat, washing, and testing in fresh medium. This difference was also shown by using [14C]paraquat. This previously unrecognized difference between E. coli B and K-12 has been the cause of apparently contradictory reports and should lead to some reevaluation of the pertinent literature.

The role of hydroxyl radicals in tetrachlorohydroquinone induced DNA strand break formation in PM2 DNA and human fibroblasts

Tetrachlorohydroquinone (TCHQ), which has previously been identified as a metabolite of pentachlorophenol, induces DNA strand breaks in isolated DNA and in human fibroblasts. Strand break formation in PM2 DNA is prevented by the addition of catalase and the hydroxyl radical scavengers DMSO, ethanol and mannitol, whereas addition of SOD reduced SSB only slightly. Oxygen radicals are formed by the autoxidation of TCHQ to the tetrachlorosemiquinone radical. Desferrioxamine (0.2 mM) completely abolished strand break formation, whereas the metal chelator DETAPAC (1 mM) reduced SSB by only 8.5%. The formation of the semiquinone radical at physiological conditions is shown by ESR spectroscopy. Exposure of human fibroblasts to TCHQ also leads to DNA single strand breaks measured by the alkaline elution assay. These were reduced by addition of 5% DMSO. This indicates that at least part of the strand break formation in human cells is also due to the action of hydroxyl radicals.

The nature of oxidants and antioxidant systems in the inhibition of mutation and cancer

We briefly review current concepts with regard to the nature of oxygen-derived oxidants in biological systems. Of these substances, hydroxyl radicals derived from hydrogen peroxide seem most likely to be involved in the various stages of carcinogenesis. Hydrogen peroxide detoxification, primarily through glutathione activity, is essential in preventing hydroxyl-radical formation. Transition metals such as iron play a central role in this latter process. Alterations in cellular macromolecules are most likely to take place if hydroxyl-radical formation is directed toward specific intramolecular sites by appropriately sequestered metals. For this reason, repair and turnover events are apt to be more important protective devices than are the actions of molecules which scavenge hydroxyl radicals. Although many cellular constituents are potential targets in free-radical and oxidant attacks leading to carcinogenesis, nucleic acids have been most extensively studied in this connection. On the basis of these investigations, it is a facile conclusion that oxidants might be involved in the early events of carcinogenesis as well as in transformation or promotion. The literature on antioxidants in chemoprevention in animals is supportive of such a role. However, other biochemical effects of antioxidants should raise a note of caution in the interpretation of animal experiments.