Phototoxicity Testing of Polycyclic Aromatic Hydrocarbons (PAH) in Mammalian Cellsin vitro

Phototoxicity and environmental transformation of polycyclic aromatic hydrocarbons (PAHs)-light-induced reactive oxygen species, lipid peroxidation, and DNA damage

Polycyclic aromatic hydrocarbons (PAHs) are a class of mutagenic and tumorigenic environmental contaminants. Although the mechanisms by which PAHs induce cancer in experimental animals have been extensively studied and the metabolic activation pathways have been determined, the environmental fate of PAHs and the phototoxicity exerted by PAHs, as well as their photoreaction products formed in the environment, have received much less attention. In this review, the formation of oxygenated PAHs, PAH quinones, nitro-PAHs, and halogenated PAHs from photoreaction of environmental PAHs are addressed. Upon light irradiation, PAHs and all PAH photoreaction products can absorb light energy to reach photo-excited states, which react with molecular oxygen, medium, and coexisting chemicals to produce reactive oxygen species (ROS) and other reactive intermediates, such as oxygenated PAHs and free radicals. These intermediates, including ROS, induce lipid peroxidation, and DNA damage including DNA strand breakage, oxidation to 8-oxo-2'-deoxyguanosine, and DNA-adducts. Since these toxicological endpoints are associated with age-related diseases, including cancer, environmental PAHs concomitantly exposed to sunlight may potentially promote human skin damage, leading to ageing and skin cancers. Thus, we suggest that (i) in addition to the widely recognized metabolic pathways, more attention must be paid to photoreaction as an important activation pathway for PAHs, (ii) risk assessment of environmental PAHs should take into consideration the complex photochemical reactions leading to mixtures of products that are also phototoxic; and (iii) the study of structure-toxicity relationships should be expanded to cover the complex photoreactions and extrinsic factors that affect phototoxicity endpoints.

Activated toxicity of diesel particulate extract by ultraviolet a radiation in mammalian cells: role of singlet oxygen

BACKGROUND

Diesel exhaust [diesel exhaust particles (DEPs) and their extracts (DPE)] and ultraviolet A radiation (UVA) are two ubiquitous environmental factors that have been identified as essential risk factors for various benign or malignant human diseases, either alone or in combination with other agents.

OBJECTIVES

We aimed to investigate the synergistic effects of DPE and UVA at low-dose exposures in human-hamster hybrid (AL) cells and their underlying mechanisms.

METHODS

We exposed exponentially growing AL cells to DPE and/or UVA radiation with or without reactive oxygen species (ROS) quenchers and then assayed the cells for survival, mutation induction, apoptosis, and micronucleus generation. In addition, using a singlet oxygen (1O2) trapping probe, 2,2,6,6-tetramethyl-4-piperidone, coupled with electron paramagnetic resonance spectroscopy, we determined the production of 1O2.

RESULTS

Treatment of AL cells with DPE+UVA induced significant cytotoxic and genotoxic damage. In contrast, we found no significant damage in cells treated with either UVA or DPE alone at the same doses. Mutation spectra of CD59- mutants showed that treatment with DPE+UVA easily induces multilocus deletions. Sodium azide significantly inhibited both cellular and DNA damage induced by DPE+UVA treatment, whereas other ROS inhibitors had little protecting effect. Furthermore, we found a significant increase of 1O2 in the cells that received DPE+UVA treatment.

CONCLUSION

These findings suggest that UVA activated the genotoxicity and cytotoxicity of DPE in mammalian cells and that 1O2 played an important role in these processes.

A Search for the Plant Ingredients that Protect Cells from Air Pollutants and Benz[a]pyrene Phototoxicity

A diversity of antioxidants and plant ingredients were examined for their protective effect in cultured Balb/c 3T3 cells against ultraviolet A (UVA)-induced cytotoxicities of extracted air pollutants and benz[a]pyrene (B[a]P) in an effort to find effective protectors against the phototoxicity of air pollutants and B[a]P. As has been observed for B[a]P phototoxicity, air pollutants themselves and those previously exposed to UVA light in the absence of cells exhibited faintly weak cytotoxicity, but the toxicity was markedly elevated when they were exposed to UVA light concomitantly with cells. The B[a]P phototoxicity was not eliminated by well-known antioxidants but was markedly diminished by diversity of plant ingredients. Among the plant ingredients tested in the current study, morin, naringin, and quercetin were found to be desirable protectors against B[a]P phototoxicity.

Light-induced cytotoxicity of 16 polycyclic aromatic hydrocarbons on the US EPA priority pollutant list in human skin HaCaT keratinocytes: relationship between phototoxicity and excited state properties

The photocytotoxicity of 16 polycyclic aromatic hydrocarbons (PAHs) on the priority pollutant list of the United States Environmental Protection Agency (US EPA) were tested in human skin HaCaT keratinocytes. A selected PAH was mixed with HaCaT cells and irradiated with a solar simulator lamp for a dose equivalent to 5 min of outdoor sunlight and the cell viability was determined immediately and also after 24 h of incubation. For the cells without incubation after the treatments, it is found that all PAHs with three rings or less, except anthracene, are not photocytotoxic, while the four or five-ring PAHs (except chrysene), benz[a]anthracene, dibenzo[a,h]anthracene, benzo[ghi]perylene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, benzo[b]fluorenthene, fluorenthene, and pyrene, are photocytotoxic to the human skin HaCaT keratinocytes. If the cells were incubated for 24 h after the treatments, the photocytotoxic effect of the PAHs was greatly amplified in comparison to the nonincubated cells. For the 24 h incubated cells, all PAHs except naphthalene exhibit photocytotoxicity to some extent. Exposure to 5 microM of the 4- and 5-ring PAHs (except chrysene) and 3-ring anthracene more than 80% of the cells lose viability. The photocytotoxicity of the PAHs correlates well with several of their excited state properties: light absorption, excited singlet-state energy, excited triplet-state energy, and HOMO-LUMO energy gap. All the photocytotoxic PAHs absorb light at >300 nm, in the solar UVB and UVA region. There is a threshold for each of the three excited state descriptors of a photocytotoxic PAH: singlet energy <355 kJ/mol (corresponding to 337 nm light), triplet energy <230 kJ/mol (corresponding to 520 nm light), HOMO-LUMO gap <3.6 eV (corresponding to 344 nm light) obtained at the Density Functional Theory B3LYP/6-31G(d) level.

Coexposure to benzo[a]pyrene and UVA induces DNA damage: first proof of double-strand breaks in a cell-free system

DNA damage induced by solar ultraviolet (UV) radiation plays an important role in the induction of skin cancer. Although UVA constitutes the majority of solar UV radiation, it is less damaging to DNA than UVB. The DNA damage produced by UVA radiation, however, can be augmented in the presence of a photosensitizer. We previously used benzo[a]pyrene (BaP), an environmental carcinogenic polycyclic aromatic hydrocarbon, as an exogenous photosensitizer, and demonstrated that combined exposure to BaP and UVA resulted in DNA double-strand breaks (DSBs) in cultured Chinese hamster ovary (CHO-K1) cells. In this study, we investigated whether coexposure to BaP and UVA induces DSBs in a cell-free system and whether reactive oxygen species (ROS) were involved in the generation of the DSBs. DSBs were induced by the coexposure both in the cell-free system (in vitro) and in CHO-K1 cells (in vivo), but not by treatment with BaP or UVA alone. DSB induction in vitro required higher doses of UVA and BaP than were required in vivo, suggesting that the mechanism of DSB induction differed. A similar difference in efficiency also was observed in the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) by coexposure to BaP and UVA in vitro and in vivo. A singlet oxygen ((1)O2) scavenger (NaN3) effectively inhibited the production of DSBs and 8-oxodG, suggesting that (1)O2 is a principal ROS generated by BaP and UVA both in vitro and in vivo. Furthermore, repair-deficient xrs-5 cells were more sensitive to coexposure with BaP and UVA than were CHO-K1 cells, but the two cell lines were equally sensitive to the combined treatment in the presence of NaN3. This result suggested that the cell death produced by coexposure to BaP and UVA was at least partly due to the DSBs generated by (1)O2. Our findings indicate that coexposure to BaP and UVA effectively induced DNA damage, especially DSBs, which results in phototoxicity and possibly photocarcinogenesis.

Coexposure to benzo[a]pyrene and UVA induces phosphorylation of histone H2AX

Phosphorylation of histone H2AX (termed gamma-H2AX) was recently identified as an early event after induction of DNA double strand breaks (DSBs). We have previously shown that co-exposure to benzo[a]pyrene (BaP), a wide-spread environmental carcinogen, and ultraviolet A (UVA), a major component of solar UV radiation, induced DSBs in mammalian cells. In the present study, we examined whether co-exposure to BaP and UVA generates gamma-H2AX in CHO-K1 cells. Single treatment with BaP (10(-9)-10(-7)M) or UVA ( approximately 2.4 J/cm(2)) did not result in gamma-H2AX, however, co-exposure drastically induced foci of gamma-H2AX in a dose-dependent manner. gamma-H2AX could be detected even at very low concentration of BaP (10(-9)M) plus UVA (0.6J/cm(2)), which did not change cell survival rates. NaN(3) effectively inhibited the formation of gamma-H2AX induced by co-exposure, indicating the contribution of singlet oxygen. This is the first evidence that co-exposure to BaP and UVA induced DSBs, involving gamma-H2AX.

Coexposure to benzo[a]pyrene plus UVA induced DNA double strand breaks: visualization of Ku assembly in the nucleus having DNA lesions

Benzo[a]pyrene (BaP) is a ubiquitous environmental pollutant with potential carcinogenicity. It has been shown that BaP, upon UVA irradiation, synergistically induced oxidative DNA damage, but other DNA damage was not confirmed. In this study, we examined whether coexposure to BaP plus UVA induces double strand breaks (DSBs) using xrs-5 cells, deficient in the repair of DSBs (Ku80 mutant), and whether Ku translocates involving the formation of DSBs. BaP plus UVA had a significant cytotoxic effect on CHO-K1 cells and an even more drastic effect on Ku80-deficient, xrs-5 cells, suggesting that the DSBs were generated by coexposure to BaP plus UVA. The DSBs were repaired in CHO-K1 cells within 30 min, but not in xrs-5 cells, indicating the involvement of a non-homologous end joining, which needs Ku proteins. Furthermore, we succeeded in visualizing that Ku80 rapidly assembled to the exposed region, in which DSBs might be generated, and clarified that the presence of both Ku70 and Ku80 was important for their accumulation.

Phototoxicity and DNA damage induced by the cosmetic ingredient chemical azulene in human Jurkat T-cells

Previous study showed that the cosmetic ingredient chemical azulene and its derivative gauiazulene exhibited photomutagenicity four- to five-fold higher than spontaneous mutation in Salmonella typhimurium TA102. In this study, phototoxicity including photogenotoxicity of azulene in human Jurkat T-cells is reported. When the cell suspensions are irradiated by light (UVA plus visible light) in the presence of azulene, an azulene dose-dependent cellular DNA damage is observed. At the highest azulene concentration of 50 microM, the average DNA fragmentation is 33 +/- 10%, determined by single cell gel electrophoresis (Comet assay). Cell viability assay using fluorescein diacetate indicates that the cells could endure the damage and remain viable. Further study revealed that the combination of light and azulene can cause single-strand cleavage on pure PhiX174 plasmid DNA in solution. Studies using scavengers reveal that singlet oxygen and free radicals are involved in causing DNA cleavage. This suggests that the photomutagenicity of azulene in S. typhimurium TA102 could be due to DNA fragmentation caused by the concurrent exposure to azulene and light.

Photomutagenicity of 16 polycyclic aromatic hydrocarbons from the US EPA priority pollutant list

The photomutagenicity of 16 polycyclic aromatic hydrocarbons (PAHs), all on the United States Environmental Protection Agency (US EPA) priority pollutant list, was studied. Concomitant exposing the Salmonella typhimurium bacteria strain TA102 to one of the PAHs and light (1.1 J/cm2 UVA+2.1 J/cm2 visible) without the activation enzyme S9, strong photomutagenic response is observed for anthracene, benz[a]anthracene, benzo[ghi]perylene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, and pyrene. Under the same conditions, acenaphthene, acenaphthylene, benzo[k]fluoranthene, chrysene, and fluorene are weakly photomutagenic. Benzo[b]fluoranthene, fluoranthene, naphthalene, phenanthrene, and dibenz[a,h]anthracene are not photomutagenic. These results indicate that PAHs can be activated by light and become mutagenic in Salmonella TA102 bacteria. At the same time, the mutagenicity for all the 16 PAHs was examined with the standard mutagenicity test with 10% S9 as the activation system. Benzo[b]fluoranthene, benzo[k]fluoranthene, chrysene, acenaphthylene, and fluorene are weakly mutagenic, while the rest of the PAHs are not. In general, the photomutagenicity of PAHs in TA102 does not correlate with their S9-activated mutagenicity in either TA102 or TA98/TA100 since they involve different activation mechanisms.

Environmental carcinogenic polycyclic aromatic hydrocarbons: photochemistry and phototoxicity

Polycyclic aromatic hydrocarbons (PAHs) are a class of environmental contaminants that has long been of interest in the fields of organic chemistry, theoretical chemistry, physical chemistry, environmental science, toxicology, cancer research, and energy sciences. Concerning environmental science and cancer research, majority of the research has focused on the occurrence, environmental fate, degradation/remediation, chemical transformation, genotoxicity, metabolism and metabolic activation, DNA adduct formation, mutagenesis, and carcinogenesis. Although many books and reviews on these subjects have been published, PAH photochemistry and phototoxicity have received much less attention. Therefore, it is intended for this article to provide an up-to-date source of photochemical reaction, photo-transformation, and phototoxicity of PAHs and their oxygenated, nitrated, halogenated, and amino substituted derivatives on a molecular basis. A perspective for future work is also discussed.

Environmental carcinogenic polycyclic aromatic hydrocarbons: photochemistry and phototoxicity

Polycyclic aromatic hydrocarbons (PAHs) are a class of environmental contaminants that has long been of interest in the fields of organic chemistry, theoretical chemistry, physical chemistry, environmental science, toxicology, cancer research, and energy sciences. Concerning environmental science and cancer research, majority of the research has focused on the occurrence, environmental fate, degradation/remediation, chemical transformation, genotoxicity, metabolism and metabolic activation, DNA adduct formation, mutagenesis, and carcinogenesis. Although many books and reviews on these subjects have been published, PAH photochemistry and phototoxicity have received much less attention. Therefore, it is intended for this article to provide an up-to-date source of photochemical reaction, photo-transformation, and phototoxicity of PAHs and their oxygenated, nitrated, halogenated, and amino substituted derivatives on a molecular basis. A perspective for future work is also discussed.

Phototoxicity of benzo[a]pyrene by ultraviolet A irradiation: induction of apoptosis in Jurkat cells

The toxicology of benzo[a]pyrene (BaP) has been mainly studied with regard to the carcinogenicity of its metabolites, but its phototoxicity is not well understood. Although some studies have indicated the lethal phototoxicity of BaP, there have been no reports regarding the pattern of cell death induced by this agent. In this study, we investigated the pattern and mechanism of cell death induced by coexposure to BaP plus ultraviolet A (UVA) in Jurkat cells. Coexposure to BaP plus UVA showed dose-dependent cytotoxicity. The pattern of cell death was apoptotic as determined by cell shrinkage, chromatin condensation, appearance of subdiploid apoptotic nuclei and translocation of phosphatidylserine to the outer membrane leaflet. Coexposure also strongly increased caspase-3/7 activity and slightly elevated those of caspase-8/6 and -9. The pan caspase inhibitor Z-VAD-CH(2)-DCB partially inhibited the phototoxicity of BaP. Cytochrome c release was observed 6 h after coexposure, but not after 1 h. Furthermore, the phototoxicity was inhibited by NaN(3) (quencher of singlet oxygen), but not by mannitol (quencher of hydroxy radicals). Chromatin condensation and translocation of phosphatidylserine were also inhibited by NaN(3), suggesting that the induction of apoptosis by coexposure to BaP plus UVA was due to singlet oxygen production.