Activated Toxicity of Diesel Particulate Extract by Ultraviolet a Radiation in Mammalian Cells: Role of Singlet Oxygen

Article excerpt

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 ([A.sub.L]) cells and their underlying mechanisms.

METHODS: We exposed exponentially growing [A.sub.L] 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 ([.sup.1][O.sub.2]) trapping probe, 2,2,6,6-tetramethyl-4-piperidone, coupled with electron paramagnetic resonance spectroscopy, we determined the production of [.sup.1][O.sub.2].

RESULTS: Treatment of [A.sub.L] 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 CD[59.sup.-] 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 [.sup.1][O.sub.2] 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 [.sup.1][O.sub.2] played an important role in these processes.

KEY WORDS: [A.sub.L] cell, cytotoxicity, diesel particulate extracts, genotoxicity, singlet oxygen, UVA. Environ Health Perspect 117:436-441 (2009). doi:10.1289/ehp.0800029 available via http://dx.doi.org/ [Online 15 September 2008]

The popularity of diesel engines has been steadily increasing recently because of fuel efficiency, longevity, high torque at highway speeds, safety, and economy concerns (Lloyd and Cackette 2001). Diesel exhaust emitted at ground level is generated during the combustion process and consists of hundreds of organic and inorganic compounds in either gaseous or particulate phases (Kagawa 2002). The dominant pollutant in ambient air, diesel exhaust particles (DEPs) consist of inert carbonaceous cores with large surface areas. This property is ideal for the adsorption of trace transition metals and various organic substances, including polycyclic aromatic hydrocarbons (PAHs), nitroaromatic hydrocarbons, quinine, and acids (McClellan 1987). Several national and international agencies have classified DEPs as a "potential" or "probable" human carcinogen [International Agency for Research on Cancer (IARC) 1989]. DEP exposure causes DNA and chromosomal damage, including bulky DNA adducts, oxidized bases, deletions, and chromosomal aberrations, which may lead to a broad spectrum of mutations (DeMarini et al. 2004; Muller et al. 2004; Tsurudome et al. 1999). Recent evidence has demonstrated that DEP and diesel particle extracts (DPEs) are highly mutagenic to TA98 and TA100 Salmonella strains in the Ames test and induce a dose-dependent increase in mutation yield that is suppressed by the S9 mixture (DeMarini et al. 2004). However, results at the hypoxanthine-guanine phosphoribosyl transferase (Hprt) locus in mammalian cells and the [lambda]/lacl locus in transgenic mice varied in different studies (Dybdahl et al. 2004; Gu et al. 2005; Risom et al. 2003; Sato et al. 2000).

Ultraviolet (UV) radiation from sunlight, including UVA and UVB, is a major factor for causing skin aging, skin cancer, and cytogenetic damage in lung, bone marrow, and peripheral blood erythrocytes (Balansky et al. 2003; Placzek et al. 2004; Yin et al. 2001). UVB (280-320 nm) is directly absorbed by DNA and induces damage, such as cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts (de Gruijl 2000; Ravanat et al. …