Epigenetic modulation by green tea in prevention of photocarcinogenesis

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DESCRIPTION (provided by applicant): Overexposure of the human skin to solar ultraviolet (UV) radiation is the major etiologic factor for the development of melanoma and non-melanoma skin cancers in the United States, with the non-melanoma skin cancer being the most common cutaneous malignancy. Although epigenetic mechanisms have been implicated in UVB-induced skin carcinogenesis, the mechanisms have not been fully characterized. Importantly, epigenetic mechanisms are potentially reversible and represent attractive targets for the prevention of the development of UVB-induced cutaneous malignancies. We have now generated preliminary data that indicate that chronic exposure of the mouse skin to UV radiation induces epigenetic alterations which include: DNA hypermethylation, increases in DNA methyltransferase (Dnmt) activity, and increases in histone de-acetylation and that these alterations may lead to the silencing of tumor suppressor genes in mouse skin. Furthermore, we have made the unique observation that a non-toxic phytochemical may act to prevent photocarcinogenesis through inhibition of epigenetic mechanisms. We had demonstrated previously that administration of green tea polyphenols (GTPs, 0.2%, w/v) in drinking water of mice significantly inhibits UV- induced skin tumor development in terms of tumor incidence (% mice with tumors) and tumor multiplicity. We have now generated preliminary data that suggest that administration of GTPs in drinking water of mice inhibits UVB-induced DNA hypermethylation and Dnmt activity in the skin. Based on our preliminary data, we have formulated the innovative hypothesis that exposure of the skin to UV radiation results in the DNA hypermethylation which leads to epigenetic silencing of key tumor suppressor genes during photocarcinogenesis. We also hypothesize that GTPs prevent UV-induced skin tumor development, at least in part, through the prevention of epigenetic alterations associated with UV radiation. Our long- term goal is to fully test this hypothesis. The objectives of this exploratory R21 application are to verify and extend our preliminary results using the SKH-1 hairless mouse model in order to generate the data necessary for the design of studies that will rigorously test the central hypothesis in mouse and human skin and establish the molecular basis for the observed data. Thus, we propose two complementary Specific Aims: (1) To determine whether inhibition of photocarcinogenesis by GTPs is mediated through inhibition of UVB-induced DNA hypermethylation and Dnmt activity in UVB-exposed skin and tumors, and (2) To determine the molecular mechanisms by which GTPs control epigenetic modulators which regulate key tumor suppressors during photocarcinogenesis. In vitro analyses using human keratinocytes and epigenetic modulating drugs will be used to verify the data generated using the mouse model. The results obtained from this study will generate new knowledge concerning the identity of the epigenetic mechanisms involved in photocarcinogenesis and should indicate whether GTPs exert their skin cancer chemopreventive effects by inhibiting these responses. We address a major public health concern as overexposure of the human skin to solar UV radiation is the major etiologic factor for the development of melanoma and non-melanoma skin cancers in the United States. The proposed studies will provide critical new data concerning UVB-induced carcinogenesis and will identify mechanisms by which green tea may act to prevent/correct UV-induced deficiencies. The development of new strategies using green tea polyphenols as a dietary supplement may help to reduce the risk of skin cancer in humans. There is world-wide interest in green tea as a cancer chemopreventive agent for humans as it is a non-toxic, affordable, popular beverage and is effective in a wide range of organs. PUBLIC HEALTH RELEVANCE: Chronic exposure of the skin to solar ultraviolet (UV) radiation induces multiple adverse effects including the risk of skin cancer development. The epigenetic modifications that occur in DNA play a significant role in the regulation of gene expression and lead to the development of cancers. In the current application, we will determine the molecular mechanism of epigenetic changes which leads to epigenetic silencing of key tumor suppressor genes during photocarcinogenesis. We will also determine whether administration of green tea polyphenols in drinking water of mice prevents photocarcinogenesis in mice through the inhibition of UVB- induced epigenetic alterations and silencing of tumor suppressor genes.