In situ hydrogen peroxide production may account for a portion of NUV (300-400 nm) inactivation of stationary phase Escherichia coli

Lethal and mutagenic action of hydrogen peroxide on Haemophilus influenzae

The lethal and mutagenic effects of H2O2 on wild-type Haemophilus influenzae Rd and on uvr1, uvr2, rec1, and rec2 mutant strains were studied. The first two mutants are sensitive to UV, and the second two are defective in recombination. Rd, urv1, and rec1 strains were more sensitive to the killing effect of H2O2 treatment than were uvr2 and rec2 strains. There were peaks of mutagenesis at two H2O2 concentrations over a range of 30 to 275 mM. Our results suggest a specific repair of H2O2 damage that is independent of the Uvr2 and Rec2 gene products. Sensitivity to the killing effect of H2O2 and to the lethal action of near-UV light were similar for Rd and uvr1 strains. This finding suggests that the mechanisms of killing by and repair of H2O2 damage may have some overlap with those of near-UV radiation.

Molecular mechanisms of ultraviolet radiation carcinogenesis

UV radiation is a potent DNA damaging agent and a known inducer of skin cancer in experimental animals. There is excellent scientific evidence to indicate that most non-melanoma human skin cancers are induced by repeated exposure to sunlight. UV radiation is unique in that it induces DNA damage that differs from the lesions induced by any other carcinogen. The prevalence of skin cancer on sun-exposed body sites in individuals with the inherited disorder XP suggests that defective repair of UV-induced DNA damage can lead to cancer induction. Carcinogenesis in the skin, as elsewhere, is a multistep process in which a series of genetic and epigenetic events leads to the emergence of a clone of cells that have escaped normal growth control mechanisms. The principal candidates that are involved in these events are oncogenes and tumor suppressor genes. Oncogenes display a positive effect on transformation, whereas tumor suppressor genes have an essentially negative effect, blocking transformation. Activated ras oncogenes have been identified in human skin cancers. In most cases, the mutations in the ras oncogenes have been localized to pyrimidine-rich sequences, which indicates that these sites are probably the targets for UV-induced DNA damage and subsequent mutation and transformation. The finding that activation of ras oncogenes in benign and self-regressing keratoacanthomas in both humans and in animals indicates that they play a role in the early stages of carcinogenesis (Corominas et al., 1989; Kumar et al., 1990). Since cancers do not arise immediately after exposure to physical or chemical carcinogens, ras oncogenes must remain latent for long periods of time. Tumor growth and progression into the more malignant stages may require additional events involving activation of other oncogenes or deletion of growth suppressor genes. In addition, amplification of proto-oncogenes or other genes may also be involved in tumor induction or progression. In contrast to the few studies that implicate the involvement of oncogenes in UV carcinogenesis, the role of tumor suppressor genes in UV carcinogenesis is unknown. Since cancer-prone individuals, particularly XP patients, lack one or more repair pathways, one can speculate that DNA repair enzymes would confer susceptibility to both spontaneous and environmentally induced cancers. Another potential candidate that can function as a tumor suppressor gene is the normal c-Ha-ras gene. Spandidos and Wilkie (1988) have shown that the normal c-Ha-ras gene can suppress transformation induced by the mutated ras gene.(ABSTRACT TRUNCATED AT 400 WORDS)

Light-dependent cytotoxic reactions of anthracene

Anthracene is a photodynamic compound in vitro. In the presence of oxygen, it is known to generate singlet oxygen and participate in Type II reactions. In aqueous solution, it also participates in Type I reactions, such as in the photoreduction of cytochrome c, which can be suppressed by superoxide dismutase. In argon, direct photoreduction of cytochrome c also takes place. Anthracene induces the photodynamic hemolysis of human erythrocytes and inactivates Escherichia coli cells photodynamically. By using a series of E. coli strains differing in DNA repair capabilities and catalase proficiency, sensitivity to inactivation by anthracene plus NUV was correlated with catalase deficiency rather than with particular repair deficiencies. The fact that carotenoid genes cloned and expressed in E. coli offered partial protection suggests that the membrane may be one possible target for inactivation by anthracene plus NUV. Anthracene plus NUV inactivated Haemophilus influenzae transforming DNA and led to nicking of supercoiled pBR322 DNA in vitro. In vivo, therefore, anthracene is a phototoxic molecule whose cytotoxicity could be the result of damage to more than one target.

Exonuclease III and the catalase hydroperoxidase II in Escherichia coli are both regulated by the katF gene product

The levels of both exonuclease III (exo III, product of xthA) and hydroperoxidase II (HP-II, product of katE) activity in Escherichia coli were influenced by a functional katF gene. The katF gene product is also necessary for synthesis of HP-II. Mutations in either katF or xthA, but not katE, result in sensitivity to H2O2 and near-UV (300-400 nm) radiation. Exo III, encoded by the xthA locus, recognizes and removes nucleoside 5'-monophosphates near apurinic and apyrimidinic sites in damaged DNA. Extracts of katF mutant strains had little detectable exo III activity. When a katF+ plasmid was introduced into the katF mutant, exo III activity exceeded wild-type levels. We propose that the katF gene is a trans-acting positive regulator of exo III and HP-II enzymes, both of which are involved in cellular recovery from oxidative damage.

Singlet oxygen involvement in the inactivation of cultured human fibroblasts by UVA (334 nm, 365 nm) and near-visible (405 nm) radiations

The UVA (320-380 nm) radiation inactivation of mammalian cells is dependent upon the presence of oxygen. In order to examine the intermediates involved, we have irradiated cells in the presence of chemical probes which are able to modify the activity of various oxygen species. We have also examined the possibility that UVA inactivates cultured human fibroblasts via generation of intracellular hydrogen peroxide. An iron scavenger (desferrioxamine) and a hydroxyl radical scavenger (dimethylsulfoxide) protect the cells against hydrogen peroxide. Diethyldithiocarbamate (a superoxide dismutase inhibitor) and aminotriazole (a catalase inhibitor) sensitize the cells to this oxidizing agent. These data support previous reports that hydrogen peroxide inactivates as a result of the iron-catalyzed generation of hydroxyl radical. None of these agents significantly alter the fluence-dependent inactivation of cell populations by radiation at 365 nm. In contrast, the cells are sensitized to radiation at 334, 365 and 405 nm in the presence of deuterium (an enhancer of singlet oxygen lifetime) and are protected against radiation at 365 nm by sodium azide (a quencher of singlet oxygen). These results are consistent with the conclusion that the generation of singlet oxygen, but not hydrogen peroxide or hydroxyl radical, plays an important role in the inactivation of cultured human cells by UVA and near-visible radiations.

Bacterial genes involved in response to near-ultraviolet radiation

A model of the possible pathways of activities following NUV treatment was presented in Section I and in Fig. 1. Some of the components are firmly established, some are speculative, and many are difficult to evaluate because of insufficient experimental information. Perhaps the most relevant experiments, especially concerning ozone depletion, would be to determine the mutational specificity of NUV. By selecting lacI mutants after exposing cells to NUV, and sequencing the bases of this gene, this is now feasible. There are some problems, however. The mutation frequency is normally so low that it might be difficult to distinguish NUV mutants from spontaneous mutants. However, by irradiating cells having a uvrA or uvrB mutation, the frequency of mutation above background can be increased considerably. There remains the problem as to what fraction of the observed mutations results from oxidative damage. Some of this could be clarified by comparing mutation spectra of cells treated with NUV and cells subjected to excess oxidative damage and determining what fraction results from other avenues of lesion formation in DNA. Different species of reactive oxygen could cause different kinds of DNA lesions, and, fortunately, use of appropriate mutants should allow us to sort out any differences in specificity of lesions. Also, by appropriate manipulation of quantities of endogenous photosensitizers, it might be possible to sort out the specific mutations that are caused by photodynamic action. Another avenue of research is to explore the pathways by which NUV lesions are repaired, and whether such repair is error prone or error free. Again, the use of mutants such as xthA, uvr, and polA should assist in our understanding of the specificity of the mutational events. There are now a number of examples of global control mechanisms whereby cells abruptly shift their protein synthesis pattern under environmental stress. It is important to understand whether NUV stress results in induction of one or more of the known regulatory genes, or whether another regulon might be involved. One particular aspect of regulation that remains unsolved is the role of the katF gene, which is known to regulate the xthA and katE, but it may also regulate other genes as well. A number of striking physiological events occur even at very low fluences of NUV irradiation of cells. In part, this may be related to regulon induction. However, some of these events are in need of special exploration, such as changes at the membrane level.(ABSTRACT TRUNCATED AT 400 WORDS)

Catalase has only a minor role in protection against near-ultraviolet radiation damage in bacteria

In bacterial cells near-ultraviolet radiation (NUV) generates H2O2 which can be decomposed by endogenous catalase to H2O and O2. To assess the roles of H2O2 and catalase in NUV lethality, we manipulated the amount of intracellular catalase (a) by the use of mutant and plasmid strains with altered endogenous catalase, (b) physiologically, by the addition of glucose, and (c) by induction of catalase synthesis with oxidizing agents. Not only was there no direct correlation between NUV-resistance and catalase activity, but in some cases the correlation was inverse. Also, while there was correlation between NUV and H2O2 sensitivity for most strains tested, there were a number of exceptions which indicates that the modes of killing were different for the two agents.

Mutagenic and lethal effects of near-ultraviolet radiation (290-400 nm) on bacteria and phage

Despite decades of study of the effect of near-ultraviolet radiation (NUV) on bacterial cells, insights into mechanisms of deleterious alterations and subsequent recovery are just now emerging. These insights are based on observations that 1) damage by NUV may be caused by a reactive oxygen molecule, since H2O2 may be a photoproduct of NUV; 2) some, but not all, of the effects of NUV and H2O2 are interchangeable; 3) there is an inducible regulon (oxyR) that responds to oxidative stress and is involved in protection against NUV; 4) a number of NUV-sensitive mutants are defective either in the capacity to detoxify reactive oxygen molecules or to repair DNA damage caused by NUV; and 5) recovery from NUV damage may not directly involve induction of the SOS response. Since several distinctly different photoreceptors and targets are involved, it is unknown whether NUV lethality and mutagenesis result from an accumulation of damages or whether there is a particularly critical photoeffect. To fully understand the mechanisms involved, it is important to identify the chromophore(s) of NUV, the mechanism of toxic oxygen species generation, the role of the oxidative defense regulon (oxyR), the specific lesions in the DNA, and the enzymatic events of subsequent repair.

Control of sensitivity to inactivation by H2O2 and broad-spectrum near-UV radiation by the Escherichia coli katF locus

Mutations in the Escherichia coli katF gene (hydroperoxidase II) result in sensitivity to inactivation by H2O2 and broad-spectrum near-UV (NUV; 300 to 400 nm) radiation. Another mutation, nur, originally described as conferring sensitivity to inactivation by broad-spectrum and monochromatic NUV, also confers sensitivity to inactivation by H2O2. Genetic analysis via transduction suggests that the nur mutation allele of the katF locus. As previously reported for broad-spectrum and monochromatic NUV wavelengths, the sensitivity of a particular strain to H2O2 inactivation is also independent of the recA and uvrA alleles. Extracts of nur and katF strains lack catalase (hydroperoxidase II) as revealed by polyacrylamide gels stained for such activity, which is consistent with the genetic results.