Sulforaphane Potentiates Non-Melanoma Skin Cancer in UVB-Treated Nrf2 Knockout Mice

Abstract

Sulforaphane is a natural product found in cruciferous vegetables which is known to have many chemopreventive properties including the induction of apoptosis and the inhibition of inflammation, cellular proliferation, and reactive oxygen species (ROS) formation. The reduction of ROS activity by sulforaphane is likely linked to the activation of NF-E2 related factor-2 (Nrf2), a transcription factor involved in cytoprotection against ROS and electrophilic stress. The skin is particularly vulnerable to oxidative stress caused by ultraviolet (UV) light, which is an established complete carcinogen. Sulforaphane has been shown to reduce both chemical and UVB-induced skin carcinogenesis in mouse models. Suppression of DMBA/TPA-induced skin tumorigenesis by sulforaphane has been shown to be dependent upon Nrf2 activity. Additional studies have shown that genetic activation of Nrf2 can protect keratinocytes against UVB-induced ROS. Nrf2 has also been implicated in regulating inflammatory responses after UVB exposure in the skin. However, the role of Nrf2 in the antitumorigenic activity of sulforaphane in the context of UVB-induced skin tumors is not well understood. We therefore performed murine experiments in order to clarify whether sulforaphane requires Nrf2 in order to block UVB-induced non-melanoma skin cancer. Consistent with the literature, we observed that wildtype (WT) mice topically treated with sulforaphane were less susceptible to UVB-induced tumor incidence and tumor burden compared to the vehicle control WT group. However, Nrf2 KO mice treated with sulforaphane presented with significantly greater UVB-induced tumor incidence and burden compared to the WT sulforaphane group, suggesting that sulforaphane may potentiate tumorigenesis in the context of UVB exposure if Nrf2 is absent. We therefore performed acute in vivo and in vitro experiments using topical sulforaphane (as per the tumor experiment) to investigate why Nrf2 KO mice developed more tumors than WT mice during UVB and sulforaphane treatment. Topical treatment of SKH-1 mice with sulforaphane did result in slight reduction of UV-induced epidermal hyperplasia in wildtype mice which was not present in Nrf2 KO mice (trends were not significant). Surprisingly, while wildtype mice developed significantly more epidermal inflammation in our acute treatment model than did the Nrf2 KO strain (as measured by skin fold thickness), inflammation was not significantly influenced by topical sulforaphane treatment in either strain of mice. However, cell culture studies using primary mouse keratinocytes indicate that sulforaphane’s ability to block UVB-induced ROS is lost in Nrf2 KO cells. Taken together, our ROS data may strengthen the hypothesis that sulforaphane increases the oxidative stress of cells during UVB treatment in the absence of Nrf2.