Inhibition of ultraviolet light-induced oxidative events in the skin and internal organs of hairless mice by isoflavone genistein

We have previously demonstrated that soybean isoflavone genistein inhibits ultraviolet-B (UVB)-induced skin tumorigenesis in hairless mice. In the present study, we further investigated the possible mechanism(s) of action whereby genistein inhibits photocarcinogenesis with focuses on UVB-induced oxidative events, including hydrogen peroxide (H2O2) production, lipid peroxidation (as represented by malondialdehyde, MDA), and 8-hydroxy-2′-deoxyguanosine (8-OHdG) formation in vivo. We demonstrated that subacute exposure to UVB substantially increased the level of H2O2, lipid peroxides, and 8-OHdG in skin of hairless mice. In addition, chronic exposure to low-dose UVB (0.9–1.2 kJ/m2 for 20 weeks) substantially increased the levels of 8-OHdG not only in the epidermis, but also in the internal organs such as liver, brain, and spleen of mice with exception of kidney. However, genistein did not affect the level of UVB-induced pyrimidine dimmers in the same UVB exposed mouse skin, indicating selective inhibition of oxidative DNA damage by genistein. Induction of H2O2 was independent of UVB fluences whereas the levels of MDA and 8-OHdG were induced in an UVB fluence-dependent manner. The results suggest that H2O2 be generated as an acute cutaneous response to UVB irradiation, while MDA and 8-OHdG are accumulated with increasing UVB exposure and more closely related to chronic effects of UVB radiation. Pre-treatment of animals with 10 μmol of genistein 1 h prior to UVB exposure significantly inhibited UVB-induced H2O2 and MDA in skin and 8-OHdG in epidermis as well as internal organs. Suppression of 8-OHdG formation by genistein has been corroborated in purified DNA irradiated with UVA and B. In summary, our results suggest that UVB irradiation elicit a series of oxidative events, which can be substantially inhibited by isoflavonoid genistein through either direct quenching of reactive oxygen species or indirect antiinflammatory effects. Thus, the antioxidative properties of genistein may explain for the mechanisms of anti-photocarcinogenic action of genistein.