Molecular Mechanisms of GSTP1 Reactivation by Green Tea Polyphenols

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DESCRIPTION (provided by applicant): Aberrant DNA methylation and chromatin remodeling are common epigenetic changes that play critical role in gene silencing; implicated in the development and progression of prostate cancer. One such alteration is silencing of the expression of glutathione S-transferase-pi (GSTP1), a critical enzyme of carcinogen defense, through methylation of deoxycytidine residue in CpG islands in the 5'-regulating region. Loss of GSTP1 function occurs in vast majority (>70%) of prostate tumors regardless of grade or stage and appears to be characteristic of human prostate tumorigenesis. Unlikely mutated genes; epigenetically silenced genes are intact and are attractive targets for agents that could 'wake up' these dormant genes. Reactivation of such genes could be accomplished by DNA methylation and/or histone deacetylase inhibitors however; clinical utility of these inhibitors has been limited due to severe side- effects and toxicity. In recent years, green tea and its major polyphenolic constituent, epigallocatechin-3-gallate (EGCG) has received much attention as a promising chemopreventive agent for prostate cancer. This notion is supported by recent case-control study (Int. J. Cancer 108:130-5, 2004); clinical trial (Cancer Res. 66:1234-40, 2006) and our observation demonstrating that oral consumption of green tea polyphenols (GTP) at human achievable dose results in significant inhibition of prostate carcinogenesis in transgenic adenocarcinoma of the mouse prostate (TRAMP) model that mimics human disease (Proc. Nail. Acad. Sci. USA 98:10350-5, 2001). Extensive laboratory studies in cell culture systems and in limited animal models have further demonstrated that GTP afford protective effects from diverse types of carcinogens and induce phase II enzyme activity that could lead to enhanced detoxification process. More recently GTP has been shown to inhibit the activity of DNA methyltransferase (Cancer Res. 63:7563-70, 2003). Importantly, if drinking green tea can restore or compensate the loss of GSTP1 activity in the prostate and can slow down the process of prostate carcinogenesis that may have relevance for prevention and/or treatment of this disease. In the proposed studies we further our understanding of how GTP and EGCG mediate reactivation of GSTP1 gene, silenced during prostate carcinogenesis. Through a series of exploration, we will specifically investigate whether these molecular mechanism(s) are mediated through epigenetic pathways related to i) demethylation and activation of aberrantly methylated GSTP1 gene, ii) modulation in binding and expression of transcriptional repressers: methyl CpG binding domain (MBD) proteins, DNA methyltransferases (DNMTs), and histone deacetylases (HDAC) to methylated DNA iii) alteration in activity and expression of histone acetyltransferases (HAT), iv) histone modification, and v) global hypomethylation patterns. These studies will be conducted in human prostate carcinoma LNCaP and MDA PCa 2b cells, which possess hypermethylated GSTP1 CpG island alleles, devoid of GSTP1 mRNA and protein expression. This will be further validated in athymic nude mouse orthotopically implanted with prostate carcinoma cells. Completion of this proposal will establish an important role of green tea polyphenols in modulating epigenetic events and will provide a rationale to conduct prevention-based clinical trial. With the 'proof of principle' more structurally-related non- toxic compounds could be developed in future for prevention of prostate cancer in humans.