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

Abstract

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.

Fig. 1

Chemical Structure and ring-numbering system for the 16 priority PAHs.

Fig. 2

Viability of HaCaT cells due to the exposure to a PAH with (◆) or without (□) light radiation. The cell viability was measured without further incubation following treatment. The light dose was 1.1 J/cm² UVA + 2.1 J/cm² visible, equivalent to 5 min under the outdoor sunlight. The results were expressed as a percentage of the reading in control wells. Error bars were the mean of six culture wells.

Fig. 3

Excited state reactions of PAHs leading to the formation of reactive intermediates

Fig. 4

Relationship between the exited state energy and cell viabilities after 5 min irradiation in the presence of 5 μM of a PAH without (a) or with (b) and (c) 24 h incubation.