Zerumbone, a sesquiterpene derived from tropical ginger, induces heme oxygenase-1 expression via activation of Nrf2 signaling in mouse epidermal cells and hairless mouse skin in vivo

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Multiple lines of evidence suggest that a wide variety of phytochemicals present in our daily diet possess chemopreventive properties. Zerumbone, a sesquiterpene derived from tropical ginger Zingiber zerumbet Smith (Zingiberaceae), was found to suppress chemically-induced colon and skin carcinogenesis. Chemopreventive effects triggered by zerumbone have been documented recently, and its chemopreventive potential partly depends on the ability to induce phase 2 detoxifying/antioxidant enzymes, which are involved in the elimination of harmful chemical carcinogens from our body. However, the molecular mechanisms underlying phase 2 or antioxidant enzyme induction by zerumbone are poorly understood. In our present study, treatment of mouse epidermal JB6 cells with zerumbone at 10 μM resulted in increased protein and mRNA expression of heme oxygenase-1 (HO-1), a respresentative antioxidant enzyme that degrades heme to produce iron, carbon monoxide and biliverdin. Zerumbone treatment also enhanced nuclear translocation of nuclear factor E2-related factor 2 (Nrf2), a transcription factor regulating expression of phase 2 detoxifying/antioxidant enzymes including HO-1. Moreover, topical application of zerumbone (10 μmol) onto dorsal skin of HR-1 hairless mice also induced HO-1 protein expression in a time manner with maximal induction observed at 6 h. One of the plausible mechanisms responsible for Nrf2 activation is modification of critical cysteine thiol(s) present in Keap1, a repressor of Nrf2. We hypothesize that zerumbone, because of its characteristic α,β-unsaturated functional moiety, can modify cysteine thiols of Keap1 by acting as a Michael reaction acceptor. This will diminish the affinity of Keap1 for Nrf2, releasing this redox-sensitive transcription factor to nucleus. α-humulene and 8-hydroxy-α-humulene, structural derivatives of zerumbone that lack an α,β-unsaturated carbonyl group, failed to induce HO-1 expression as well as Nrf2 nuclear translocation, lending support to a critical role of this functional moiety in zerumbone-mediated induction of HO-1. To further verify the role of Nrf2 on HO-1 expression by zerumbone, we compared HO-1 protein levels in Nrf2 wild-type (+/+) and Nrf2 knock out (-/-) mice. Topical application of zerumbone resulted in much lower expression of HO-1 in Nrf2 (-/-) mice than that of Nrf2 (+/+) mice. Taken together, the above findings suggest that the upregulation of HO-1 by zerumbone is mediated, at least in part, via the Keap1-Nrf2 signaling pathway in mouse skin and epidermal cells in culture, providing a new insight into the skin cancer preventive potential of this phytochemical.