A Bacterial DNA Repair Test Evaluating the Genotoxicity of Light Sources

Modulation of light-induced skin tumors by N -acetylcysteine and/or ascorbic acid in hairless mice

The light emitted by halogen quartz bulbs contains a broad spectrum of UV wavelengths, is strongly genotoxic and is a potent inducer of skin tumors in hairless mice. By using a UVC filter, this light mimics solar radiation and induces a variety of genomic and transcriptional alterations in mouse skin. UV-related carcinogenesis involves depletion of antioxidants and glutathione in skin cells. On this basis, we evaluated modulation of carcinogenicity of UVC-filtered halogen lamps in SKH-1 hairless mice by the antioxidants N-acetyl-l-cysteine (NAC) and ascorbic acid (AsA). Both agents were given in the drinking water, either individually or in combination. The earliest skin lesions were detected after 300 days' exposure to light and became confluent in a number of mice after 480 days. NAC administration prolonged the latency time by 90 days. Moreover, NAC considerably and significantly decreased both incidence and multiplicity of light-induced skin tumors, prevented the occurrence of malignant lesions (squamocellular carcinomas) and reduced the tumor size. In contrast, AsA, which may behave as a prooxidant rather than an antioxidant, increased the multiplicity of total skin tumors, carcinomas in situ and squamocellular carcinomas. Co-administration of NAC with AsA significantly attenuated the negative effect of AsA, presumably due to the ability of this thiol to maintain a reduced environment. Therefore, in agreement with our previous in vitro findings, oral NAC is able to attenuate the detrimental effects of AsA.

Genotoxic effects in bacteria of the light emitted by halogen tungsten lamps having treated quartz bulbs

Traditional halogen tungsten lamps, which are extensively used worldwide for the illumination of indoor environments, have a quartz bulb which transmits not only visible light but also ultraviolet (UV) light. Due to the output of far-UV wavelengths, halogen lamps were found in previous studies to be potently genotoxic in bacteria, clastogenic in cultured human cells, and carcinogenic in hairless mice. This discovery prompted the launching of new halogen lamps, known as UV-Stop, UV-Block, or similar trade names, which have the quartz glass treated in such a way to reduce its permeability to UV radiation. Surprisingly, these lamps are advertised for attenuating discolouration of UV-sensitive materials, such as fabrics, paintings, works of art and furniture, whereas protection of the human skin from potential carcinogenic risks is overlooked. We tested forty-seven 12 V-powered lamps with treated quartz bulb, which were made available by five producers as blind-coded samples. After exposure to either 1000 lx for 30 min or 2500 lx for 60 min, the 50 W lamps from two producers were borderline mutagenic in strains TA100 and TA104 of S. typhimurium, and induced an evident and dose-related DNA damage in the E. coli strain CM871 (uvrA- recA- lexA-), as compared to its isogenic, DNA repair-proficient counterpart WP2. The 50 W lamps supplied by the other three producers also induced a significant genotoxic damage, but only after exposure for 60 min at illuminance levels of 2500 lx or higher. In calibration experiments, one of these three lamp brands was found to induce in 60 min a genotoxic damage which was equivalent to the one induced in just 55 s by a traditional halogen lamp. Therefore, the new types of lamps with treated quartz bulbs provide an appreciable step forward in the safety of halogen lamps, but some output of genotoxic UV radiations does still occur. Moreover, the lamps manufactured by different producers are not equally effective to this respect. By comparison, the simple application of a glass cover to a traditional halogen lamp completely prevented genotoxic effects, even after 60 min of exposure at an illuminance of 10,000 lx. Suitable regulations are urgently needed for controlling the biological safety or artificial illumination systems.

Breakthrough of ultraviolet light from various brands of fluorescent lamps: lethal effects on DNA repair-defective bacteria

In a comparative study of 17 pairs of 15 W fluorescent lamps intended for use in homes and purchased in local stores, we detect over 10-fold differences in UVB + UVC emissions between various lamps. This breakthrough of ultraviolet (UV) light is in part correlated with ability of lamps to kill DNA repair-defective recA-uvrB- Salmonella. Relative proficiency of lamps in eliciting photoreactivation of UV-induced DNA lesions also plays a prominent role in the relative rates of bacterial inactivation by emissions from different lamps. Lamps made in Chile, such as Philips brand lamps and one type of General Electric lamp, produce far less UVB + UVC and fail to kill recA-uvrB- bacteria. In contrast, all tested lamps manufactured in the USA, Hungary, and Japan exhibit readily observed deleterious biological effects. When an E. coli recA-uvrB-phr- (photolyase-negative) triple mutant is used for assay, lethal radiations are detected from all lamps, and single-hit exponential inactivation rates rather closely correlate to amount of directly measured UVB + UVC output of each pair of lamps. Although all lamps tested may meet international and United States standards for radiation safety, optimal practices in lamp manufacture are clearly capable of decreasing human exposure to indoor UV light.

Neoplastic transformation of neonatal human fibroblasts exposed in vitro to radiation from a quartz-halogen lamp

The use of unfiltered quartz-halogen lamps exposes human skin to radiation that spans much of the ultraviolet (UV) spectrum. Reports indicate that exposure to quartz-halogen lamps is erythemogenic, mutagenic, and carcinogenic. To compare the carcinogenic potential of quartz-halogen lamps with that of other UV sources, we determined the dose dependence for cytotoxicity and neoplastic transformation in neonatal human fibroblasts exposed in vitro to: a 15 W germicidal lamp (primarily 254 nm radiation), a 15 W Cool White fluorescent lamp, and an unfiltered 20 W quartz-halogen lamp. Fluence-survival relationships were multiphasic with linear dose response below about 40% survival, and all three sources produced fluence-dependent transformation as indicated by induction of anchorage-independent growth. Maximum transformation frequencies were observed at fluences of 5-8 J/m2 for the germicidal lamp, 6.3 kJ/m2 for the fluorescent lamp, and 300 J/m2 for the quartz-halogen lamp. These data confirm the carcinogenic potential of the quartz-halogen lamp.

Detection of p53 and histopathological classification of skin tumours induced by halogen lamps in hairless mice

Skin lesions induced by exposure of three strains of female hairless mice to the light emitted by uncovered halogen quartz lamps were subjected to histopathological analysis. We examined 170 representative specimens out of a total of 597 skin lesions, i.e. 38 out of 74 SKH-1 mice, 110 out of 472 MF-1 mice, and 42 out of 51 C3H mice. The results provided evidence of various types of alterations, including preneoplastic changes, such as epidermal hyperplasia, and benign tumours, such as papillomas, as well as tumours with an increasing degree of malignancy, i.e., keratoacanthoma-like tumours, appendage/basal tumours, actinic keratoses/carcinomas in situ, and squamocellular carcinomas. SKH-1 was the most sensitive strain to the far-ultraviolet wavelengths delivered by halogen lamps, as shown not only by the shortest latency time and the highest multiplicity of skin lesions but also by the highest frequency of malignant tumours. Some areas of atypical melanocyte proliferation were only detected in C3H pigmented mice. Eighty-two of the lesions excised from MF-1 mice were additionally examined for p53 protein by immunohistochemical methods. Formalin-fixed, paraffin-embedded sections and frozen sections were analyzed in parallel by using polyclonal CM-1 antibody and monoclonal PAb240 antibody, respectively. A positive response for p53 was only observed in squamocellular carcinomas, and was related to the size of cancers; in fact, six out of 10 cancers of 10-30 mm in diameter were positive, whereas all 16 cancers of 2-9 mm in diameter were negative. All six positive squamocellular carcinomas were detected by using the CM-1 antibody, which recognizes both wild-type and mutant forms of p53 protein, and five of them were also positive with the PAb 240 antibody, which only recognizes the mutant form. Thus, p53 mutation appears to be a late event in the development of halogen-induced skin tumours in hairless mice, requiring a severe degree of malignancy and an advanced stage of the neoplastic mass growth.

Effects of light on the cytotoxicity and genotoxicity of benzo(a)pyrene and an oil refinery effluent in the newt

The genotoxicity and/or toxicity of benzo(a)pyrene (BaP) were evaluated under different lighting conditions in larvae and embryos of the newt Pleurodeles waltl. Visible light alone (1,220 lx; 2.1 mW/cm2), UVA alone (250 microW/cm2), or BaP alone (500, 50, 25, 12.5 ppb) had no toxic effects on the larvae. Conversely, toxic effects were observed in animals exposed to BaP (500 ppb)+daylight, or BaP (> or = 25 ppb)+UVA. BaP solutions (50 or 12.5 ppb) were irradiated for 24 hr (Irr BaP) prior to being assayed in the dark. The genotoxicity of BaP (50 ppb) as evaluated in the micronucleus test (on day 8) was halved by its previous exposure to UVA, and was abolished at the lowest concentration (12.5 ppb). In other experiments, the larvae were exposed alternatively to BaP or Irr BaP (18 hr in the dark) and UVA (6 hr in water), every day for 8 days. All animals that had accumulated non-irradiated BaP (50 ppb) showed signs of severe toxicity, and 90% died before the end of the test. On the other hand, irradiated BaP (50 ppb) was 4-fold less toxic and half as genotoxic as non-irradiated BaP. In addition, exposure of the animals to UVA alone for 4 days prior to treatment with BaP did not affect the genotoxicity or toxicity of this hydrocarbon. In the dark, the embryotoxicity of BaP was markedly attenuated by the presence of the jelly coats (10% vs. 24%). With dejellied embryos, comparable mortality (24%) was observed after exposure to 25 ppb BaP for 54 or 312 hr. Although UVA alone did not affect growth of the embryos, the toxicity of BaP was enhanced by the combined action of the two agents together or in succession (BaP+UVA or BaP then UVA). Larvae were treated with an oil refinery effluent (EF). At the concentration employed (125 ml/l), EF was not found to be genotoxic in the dark. However, in animals exposed to both EF and UVA, there was a progressive increase in level of micronucleated erythrocytes with increasing duration of daily exposure to UVA (9, 15, 24 hr). Moreover, the genotoxic potential of irradiated EF+UVA was systematically below that of non-irradiated EF+UVA for all durations of exposure to ultraviolet light. Irradiation of this type of effluent might help reduce its harmful effects on aquatic species. Our results also suggest that metabolic activation is not necessary for hydrocarbons to induce toxic effects.

Genotoxicity of volatile and secondary reactive oxygen species generated by photosensitization

Reactive oxygen species were generated in the gas phase by photosensitization involving illumination of Rose Bengal. Depending on whether the chromophore is dry or solubilized, this system produces either energy-transfer reactions leading to generation of singlet oxygen specifically, or a combination of energy-transfer and electron-transfer reactions, providing both singlet oxygen and reduced forms of oxygen, such as superoxide anion and hydrogen peroxide. In neither case were the reactive species mutagenic in strain TA104 of Salmonella typhimurium, which had been previously shown to be reverted by oxygen species generated by the hypoxanthine-xanthine oxidase system in aqueous medium. However, mixed oxygen species induced an increased lethality in a variety of DNA repair-deficient Escherichia coli strains. This genotoxic effect, mainly reparable by the uvrA and recA mechanisms, was efficiently prevented by the thiol N-acetyl-L-cysteine. Singlet oxygen itself failed to exert direct genotoxic effects, although secondary reactants produced by its reaction with cell components enhanced lethality in some repair-deficient bacteria. Distance-dependence analyses provided measurements of the lifetimes of the oxygen species generated in the gas phase.