The role of extracellular medium components and specific amino acids in the cytotoxic response of Escherichia coli and Chinese hamster ovary cells to hydrogen peroxide

Escherichia coli K-12 Lacks a High-Affinity Assimilatory Cysteine Importer

The most direct route by which microbes might assimilate sulfur would be by importing cysteine. However, alone among the amino acids, cysteine does not have well-characterized importers. We determined that Escherichia coli can rapidly import cysteine, but in our experiments, it did so primarily through the LIV ATP-driven system that is dedicated to branched-chain amino acids. The affinity of this system for cysteine is far lower than for Leu, Ile, and Val, and so in their presence, cysteine is excluded. Thus, this transport is unlikely to be relevant in natural environments. Growth studies, transcriptomics, and transport assays failed to detect any high-affinity importer that is dedicated to cysteine assimilation. Enteric bacteria do not contain the putative cysteine importer that was identified in Campylobacter jejuni This situation is surprising, because E. coli deploys ion- and/or ATP-driven transporters that import cystine, the oxidized form of cysteine, with high affinity and specificity. We conjecture that in oxic environments, molecular oxygen oxidizes environmental cysteine to cystine, which E. coli imports. In anoxic environments where cysteine is stable, the cell chooses to assimilate hydrogen sulfide instead. Calculations suggest that this alternative is almost as economical, and it avoids the toxic effects that can result when excess cysteine enters the cell.IMPORTANCE This investigation discovered that Escherichia coli lacks a transporter dedicated to the assimilation of cysteine, an outcome that is in striking contrast to the many transporters devoted to the other 19 amino acids. We ascribe the lack of a high-affinity cysteine importer to two considerations. First, the chemical reactivity of this amino acid is unique, and its poorly controlled import can have adverse consequences for the cell. Second, our analysis suggests that the economics of biosynthesis depend sharply upon whether the cell is respiring or fermenting. In the anoxic habitats in which cysteine might be found, the value of import versus biosynthesis is strongly reduced compared to that in oxic habitats. These studies may explain why bacteria choose to synthesize rather than to import other useful biomolecules as well.

Low levels of hydrogen peroxide and L-histidine induce DNA double-strand breakage and apoptosis

The results presented in this study demonstrate that L-histidine triggers a lethal response in U937 cells exposed to nontoxic, albeit growth-inhibitory, levels of H2O2. Treatment for 1 h with the cocktail H2O2/L-histidine promotes the formation of a low level of DNA double-strand breaks that are rapidly rejoined, and this process is followed by secondary DNA fragmentation at about 7 h of post-treatment incubation, at which time cells are still viable. The appearance of oligonucleosomal DNA fragments associated with the detection of morphological changes typical of apoptosis strongly suggests that a portion of the cells was undergoing an apoptotic process. The relative level of cells with fragmented chromatin never exceeded 15-20% throughout the 20 h post-treatment incubation. Treatment with high concentrations of H2O2 in the presence of L-histidine was found to trigger necrotic cell death. The results presented in this paper provide further experimental evidence in support of the notion that DNA double-strand breaks mediate the lethal effects of the cocktail H2O2/L-histidine and suggest that this type of DNA lesion can promote both apoptotic and necrotic cell death, depending on the concentration of the oxidant.

The L-histidine-mediated enhancement of hydrogen peroxide-induced DNA double strand breakage and cytotoxicity does not involve metabolic processes

The cytotoxic response of Chinese hamster ovary (CHO) cells to challenge with hydrogen peroxide was highly dependent upon the temperature of exposure, being markedly higher at 37 degrees than at 4 degrees C. Increasing intracellular levels of L-histidine prior to challenge with hydrogen peroxide increased the toxicity elicited by the oxidant at both physiologic and ice-bath temperatures. The effect of the amino acid, however, was more pronounced under conditions at 4 degrees C, as compared to 37 degrees C. Indeed, at 4 degrees C the oxidant was nontoxic at submillimolar levels and pre-exposure to L-histidine restored cytotoxicity to levels slightly higher than those observed after treatment at 37 degrees C (in the micromolar range). Pre-exposure to the amino acid increased the production of DNA double-strand breaks (DSBs) elicited by treatment with the oxidant both at 37 degrees and 4 degrees C. A remarkable correlation was found when the level of this lesion was plotted against the cytotoxic response observed using different concentrations of L-histidine or hydrogen peroxide, or treating the cells with the oxidant either at 37 degrees or 4 degrees C, thus suggesting the existence of a cause-effect relationship. The overlapping correlation curves obtained with cells challenged with the oxidant at 4 degrees or 37 degrees C also suggest that similar molecular mechanisms mediate the formation of DNA DSBs under both experimental conditions. Two lines of evidence provide experimental support for this inference: (1) the kinetics of repair of DNA DSBs generated at 37 degrees or 4 degrees C were virtually superimposable; this would suggest that the same repair pathway(s) is/are responsible for the removal of DNA DSBs generated at the two temperatures; and (2) the size distribution of double-stranded DNA fragments produced under the two treatment conditions, resulting in a similar cytotoxic response, was basically identical. This is indicative of remarkable similarities in the topology of chromosomal domains where DSBs are generated. Overall, the results presented in this paper provide further experimental evidence supporting the notion that DNA DSBs are responsible for the L-histidine-mediated enhancement of hydrogen peroxide-induced cytotoxicity, and demonstrate that the mechanism whereby the amino acid enhances the ability of hydrogen peroxide to produce DNA double strand breakage and cell killing does not depend on cellular metabolism and/or energy-dependent reactions.

The effect of hydrogen peroxide/L-histidine-induced DNA single- vs. double-strand breaks on poly(ADP-ribose)polymerase

L-Histidine markedly increases the ability of hydrogen peroxide to induce DNA cleavage and this effect is associated with a 3-aminobenzamide-inhibitable decline in NAD+ levels, an event which very likely reflects an enhanced stimulation of the enzyme poly(ADP-ribose)polymerase. 3-Aminobenzamide slowed down the removal of alkaline elution-detected strand breaks induced by either H2O2 alone (producing only DNA single-strand breaks) or associated with L-histidine (resulting in the formation of both single-strand breaks and DNA double-strand breaks), and the extent of inhibition was similar under the two experimental conditions. 3-Aminobenzamide did not affect the rate of rejoining of DNA double-strand breaks generated by the cocktail H2O2/L-histidine. The above results suggest that these double-strand breaks have hardly any effect on the induction of poly(ADP-ribose)polymerase activity, a conclusion that is consistent with the observation that the activity of this enzyme appears to be basically identical under conditions that abolish the formation of DNA double-strand breaks, in the absence of measurable variations in the level of induction of DNA single-strand breaks (e.g. in the presence of an excess of L-glutamine, a competitive inhibitor of L-histidine uptake). Finally, 3-aminobenzamide did not affect the toxicity of the oxidant, both in the absence and presence of L-histidine.

Action of cystine in the cytotoxic response of Escherichia coli cells exposed to hydrogen peroxide

Cystine markedly enhanced the cytotoxic response of Escherichia coli cells to concentrations of hydrogen peroxide resulting in mode one killing, but displayed little effect in mode two killed cells. The effect of cystine was concentration-dependent over a range of 5-50 microM and did not further increase at higher levels. Cystine had similar effects in other bacterial systems. In order to sensitize the cells to the oxidative injury, the amino acid must be present during exposure to the oxidant since no enhancement of the cytotoxic response can be observed in cystine pre-loaded cells. In addition, no further enhancement of cytotoxicity could be detected when cystine was added before and left during challenge with the oxidant. The enhancing effect of cystine on oxidative injury of E. coli cells appears to be directly mediated by the amino acid and in fact cysteic acid, the most likely oxidation product, had no effect on the killing of bacterial cells elicited by hydrogen peroxide. Other disulfide compounds such as oxidized glutathione, cystamine and dithionitrobenzoic acid only slightly increased the susceptibility of bacteria to the oxidant. The effect of the disulfides was not concentration-dependent over a range of 200-800 microM and was statistically significant only for cystamine. Taken together, these results indicate that cystine markedly increases the cytotoxic response of bacteria to hydrogen peroxide and suggest that the amino acid might impair the cellular defence machinery against hydrogen peroxide. This effect may involve a thiol-disulfide exchange reaction at the cell membrane level.

The L-histidine-mediated enhancement of hydrogen peroxide-induced cytotoxicity is a general response in cultured mammalian cell lines and is always associated with the formation of DNA double strand breaks

Micromolar concentrations of L-histidine increase the cytotoxicity of hydrogen peroxide in a number of cell lines including CHO (hamster), EAHY, McCoy's, U937 and CCRF-CEM (human), Vero (monkey) and SC-1 (mouse). Importantly, these cell lines displayed different degrees of sensitivity to H2O2 alone and the extent of enhancement elicited by the amino acid was more pronounced in resistant cell lines. The increased cytotoxicity was invariably associated with the formation of DNA DSBs and a remarkable correlation was found by plotting the level of DNA DSBs against the cytotoxic response. These results strongly support the hypothesis that the mechanism whereby L-histidine increases the toxicity elicited by H2O2 involves the formation of DNA DSBs and are consistent with the possibility that the amino acid might participate in the regulation of the physio-pathological response to oxidative stress in mammals.

tert.-butyl hydroperoxide-mediated DNA base damage in cultured mammalian cells

tert.-Butyl hydroperoxide has been utilized to study the effect of oxidative stress on living cells; however, its effect on DNA bases in cells has not been characterized. In the present work, we have investigated DNA base damage in mammalian cells exposed to this organic hydroperoxide. SP2/0 derived murine hybridoma cells were treated with 4 concentrations of tert.-butyl hydroperoxide for varying periods of time. Chromatin was isolated from treated and control cells and subsequently analyzed by gas chromatography-mass spectrometry with selected-ion monitoring for DNA base damage. Quantification of damaged DNA bases was achieved by isotope-dilution mass spectrometry. The amounts of 8 products were significantly higher than control levels in cells treated with tert.-butyl hydroperoxide at a concentration range of 0.01-0.1 mM. At concentrations from 1.0 to 10 mM, product formation was inhibited and the amounts of products were similar to those in control cells. The bimodal nature of the dose-response may be qualitatively analogous to previous reports of bimodal killing of E. coli bacteria by hydrogen peroxide. The nature of the identified DNA base lesions suggests the involvement of the hydroxyl radical in their formation. tert.-Butyl hydroperoxide is known to produce the tert.-butoxyl radical in reactions with metal ions. However, it is unlikely that the tert.-butoxyl radical produces these DNA lesions. It is suggested that DNA base damage arises from tert.-butyl hydroperoxide-mediated oxidative stress in cells, resulting in formation of hydroxyl radicals in close proximity to DNA. The inhibition of product formation at high concentrations of tert.-butyl hydroperoxide may be explained by the scavenging of tert.-butoxyl radical by tert.-butyl hydroperoxide resulting in inhibition of oxidative stress. The plausibility of the scavenging mechanism was evaluated with a mathematical simulation of the dose-response for DNA damage in solutions containing hydrogen peroxide. The simulation model predicted a bimodal dose-response which agreed qualitatively with the results in this study and with other in vivo and in vitro studies reported in the literature.

L-histidine-mediated enhancement of hydrogen peroxide-induced cytotoxicity: relationships between DNA single/double strand breakage and cell killing

Results presented in this study demonstrate an association between the L-Histidine-mediated enhancement of H2O2-induced cytotoxicity and the formation of DNA double strand breakage (DSB), whereas no relationship exists between the increased cytotoxic response and DNA single strand breakage (SSB). Indeed, the higher lethality and the production of DNA DSB occurred in oxidatively-injured cells regardless of whether the exposure to L-Histidine was performed before or during challenge with the oxidant. In fact, the increased level of DNA SSB detected in cells simultaneously exposed to the oxidant and the amino acid was not observed in cells pre-treated with L-Histidine and then challenged with hydrogen peroxide. Further experiments have demonstrated an association between the kinetics of DNA DSB formation and the enhancement of the cytotoxic response. In conclusion, intracellular L-Histidine seems to mediate the formation of DNA DSB and the increased growth-inhibitory response elicited by the oxidant. In addition, these results suggest that the enhancement of DNA SSB is produced by the extracellular/plasma membrane fraction of the amino acid and not causally related to the L-Histidine-mediated increase of the growth-inhibitory response to H2O2-treated cells.

Differential effects of histidine on hydrogen peroxide-induced bacterial killing and DNA nicking in vitro

The hydrogen peroxide dose-response curves for Escherichia coli killing and DNA nicking in vitro display remarkably similar bimodal patterns. The concentrations of the oxidant resulting in maximum mode one killing, however, exceeds by two orders of magnitude those resulting in the mode one DNA nicking response. Addition of histidine differentially affects the experimental curves describing the dose-dependency for bacterial killing and DNA damage in vitro. Indeed, the lethal effect elicited by the oxidant in the presence of the amino acid is strictly concentration-dependent and thus the inactivation curve loses its bimodal character. In marked contrast, histidine abolishes DNA damage generated by low concentrations of hydrogen peroxide (< 100 microM) in the in vitro system (the mode one DNA nicking response) but greatly increases DNA damage produced by concentrations of the oxidant higher than 1 mM (the mode two DNA nicking response). Experimental results also suggest that treatment of covalently closed circular double-stranded super-coiled DNA with hydrogen peroxide, in the presence of both histidine and iron, may result in the formation of DNA double strand breakage, a type of lesion which is not efficiently produced by the oxidant in the absence of the amino acid. Taken together, the above results indicate that histidine differentially affects the in vitro DNA cleavage and E. coli lethality induced by hydrogen peroxide and suggest that different molecular events mediate mode one DNA nicking in vitro and mode one killing of bacterial cells.

Differential effect of L-histidine in human lymphocytes damaged by different oxygen radical producing systems

The effect of histidine on damage induced by oxygen radicals was studied in peripheral blood lymphocytes treated with free oxygen radical-inducing agents: hydrogen peroxide, xanthine oxidase plus hypoxanthine, bleomycin and gamma-rays. L-Histidine, at a concentration of 1 mM, was found to potentiate both cell killing and inhibition of PHA-stimulated cell division brought about by hydrogen peroxide or xanthine oxidase plus hypoxanthine. In contrast, L-histidine did not affect gamma-ray- or bleomycin-induced cell killing and inhibition of PHA-stimulated cell division. We suggest that L-histidine potentiation of cell damage is mainly mediated by interaction of the amino acid with hydrogen peroxide and/or iron rather than with other reactive oxygen species. In addition, these results also indicate that hydrogen peroxide produced by gamma-radiation- or bleomycin-treated cells plays no role in the toxic effects elicited by these agents.

Modification of the in vitro cytotoxicity of hydrogen peroxide by iron complexes

The effect of a range of iron chelates on the cytotoxicity of H2O2 was studied on a mammalian epithelial cell line. Iron complexes which were internalised enhanced the cytotoxicity of H2O2 measured by delayed thymidine incorporation. Iron complexed to 8-hydroxyquinoline (Fe/8-HQ) potentiated the cytotoxicity of 50 microM by 38% and Fe/dextran by 23%. Pre-exposure of cells to Fe/dextran at 4 degrees C did not result in any potentiation of H2O2-induced cytotoxicity which we ascribe to failure of the Fe/dextran to be endocytosed at low temperature. Iron complexes which are slowly taken up or remain extracellular protected the cells from H2O2-induced cytotoxicity. Thus, Fe/EDTA inhibited the cytotoxicity of 50 microM H2O2 by 33%; Fe/ADP by 80% and Fe/ATP by 88%, suggesting mutual extracellular detoxification.