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

Phosphorylation of histone H2AX (termed γ-H2AX) was recently identified as an early event after induction of DNA double strand breaks (DSBs). We have previously shown that coexposure 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 coexposure to BaP and UVA generates γ-H2AX in CHO-K1 cells. Single treatment with BaP (10−9–10−7 M) or UVA (∼2.4 J/cm2) did not result in γ-H2AX, however, coexposure drastically induced foci of γ-H2AX in a dose-dependent manner. γ-H2AX could be detected even at very low concentration of BaP (10−9 M) plus UVA (0.6 J/cm2), which did not change cell survival rates. NaN3 effectively inhibited the formation of γ-H2AX induced by coexposure, indicating the contribution of singlet oxygen. This is the first evidence that coexposure to BaP and UVA induced DSBs, involving γ-H2AX.