Benzo[a]pyrene enhances the formation of 8-hydroxy-2'-deoxyguanosine by ultraviolet a radiation in calf thymus DNA and human epidermoid carcinoma cells

The objective of this study is to investigate if benzo[a]pyrene (BaP) and ultraviolet (UV) radiation synergistically induce oxidative DNA damage. Calf thymus DNA was incubated with BaP and irradiated with UVB (280-320 nm) and UVA (335-400 nm). BaP substantially enhanced the formation of 8-hydroxy- 2'-deoxyguanosine (8-OHdG) by UVA, but only moderately increased the level of 8-OHdG by UVB. Formation of 8-OHdG proportionally correlated with both UV dose and BaP concentration. Human epidermoid carcinoma cells were incubated with 10 μg of BaP/mL for 24 h and then exposed to 10 kJ/m² UVB and 25 kJ/m² UVA. UVB plus BaP did not affect the level of 8-OHdG in cultured cells, whereas UVA plus BaP substantially increased 8-OHdG by over 4-fold compared to BaP and UVA controls. To confirm what reactive oxygen species (ROS) are involved in BaP plus UVA-induced oxidative DNA damage, less or more specific ROS quenchers were added to DNA solution. The results showed that only superoxide dismutase and genistein significantly quenched BaP plus UVA- induced 8-OHdG, whereas catalase, sodium azide, and mannitol exhibited no effect. Our studies suggest that BaP enhances the formation of 8-OHdG in purified DNA and cultured cells by UVA, but not by UVB, and that superoxide anion plays an important role in the synergistic induction of oxidative DNA damage.