Ultraviolet (UV) radiation is a major risk factor for skin cancer, particularly squamous cell carcinoma. Skin cancer is an escalating yet potentially preventable disease. Direct DNA damage, oxidative stress generated by reactive oxygen species (ROS), and inflammation all participate in skin tumor development, but the magnitude of their contributions depends on UV wavelength. These detrimental processes are counteracted by a family of intrinsic cytoprotective proteins (e.g., glutathione transferases, NAD(P)H: quinone oxidoreductase 1 [NQO1], heme oxygenase 1) whose gene expression can be upregulated by small molecules (inducers) via the Keap1/Nrf2/ARE pathway. The aim of this study was to test the hypothesis that induction of the intrinsic cytoprotective proteins via the Keap1/Nrf2/ARE pathway provides protection against UVA radiation-induced oxidative stress. Mouse embryo fibroblasts (MEF) isolated from wild-type, keap1-knockout, and nrf2-knockout mice were compared for their levels of NQO1, a marker Nrf2-dependent gene, and their sensitivity to UVA irradiation in terms of generation of ROS and glutathione (GSH) depletion. The levels of NQO1 were highest in keap1-knockout cells, and lowest in nrf2-knockout cells. Nrf2-knockout cells are most sensitive, whereas keap1-knockout cells are most resistant to UVA-dependent generation of ROS and depletion of GSH. This correlates with the enzyme activity levels of NQO1 and with the basal levels of GSH in each cell line. Similarly to mouse embryo fibroblasts, exposure to escalating doses of UVA radiation (from 1 to 20 J/cm2) of primary cultures of keratinocytes and dermal fibroblasts isolated from SKH-1 hairless mice led to generation of ROS and depletion of GSH in a dose-dependent manner. Pre-treatment with 1 \#956;M of the isothiocyanate sulforaphane, a potent inducer of the Keap1/Nrf2/ARE pathway, increased the levels of GSH in keratinocytes by 50% and dermal fibroblasts by 17% and the activity of NQO1 by 3-fold over untreated controls. Furthermore, this resulted in protection against generation of ROS by 30% in keratinocytes and 50% in dermal fibroblasts. Thus, induction of the Keap1/Nrf2/ARE pathway by a dietary phytochemical agent is a promising strategy for protection against oxidative stress, an important consequence and mediator of cellular damage following exposure to UVA radiation.
Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 958.
100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO