Epigenetic mechanisms through aberrant silencing of critical genes are central to the development and progression of prostate cancer. These epigenetic events are often associated with methylation of cytosine residues in CpG dinucleotides, present as clusters to form CpG islands, in the promoter and 1st exon of these genes. Silencing of glutathione S-transferase-pi (GSTP1) gene encoding the pi-class GST, a critical enzyme of carcinogen defense, through methylation of deoxycytidine residue in CpG islands in the 5’-regulatory region has been frequently observed in human prostate carcinomas, prostatic intraepithelial neoplasia and prostate cancer cell lines. GSTP1 plays an important role in the conjugation and detoxification of potential carcinogens and early loss of GSTP1 expression could lead to increased susceptibility to carcinogens, promoting mutation and prostate cancer progression. Unlike mutated genes, epigenetically silenced genes are intact and are attractive targets for agents that could reactivate these dormant genes. Reactivation of such genes could be accomplished by DNA methylation inhibitors however; clinical utility of these inhibitors has been limited due to severe side-effects and toxicity. Green tea polyphenols (GTP) has received much attention over the last few years, as a potential cancer chemopreventive and cancer chemotherapeutic agent, possibly because of its wide range of effects on a number of cellular processes and efficacy in many tumor model systems. Extensive laboratory studies have demonstrated that GTP affords protective effects from diverse types of carcinogens and induce phase II enzyme activity that could lead to enhanced detoxification process. We investigated whether GTP can activate silenced GSTP1 gene in human prostate cancer LNCaP cells, which possess hypermethylated GSTP1 CpG island alleles, devoid of GSTP1 mRNA and protein expression. Treatment of LNCaP cells with GTP (1-10 μg/ml) for 3, 7 and 14 days resulted in a dose- and time- dependent reactivation of GSTP1 activity and expression as observed by ELISA, mRNA and protein expression. These results correlated with decrease in DNMT1 mRNA and protein expression. GTP treatment of LNCaP cells resulted in increased demethylation of the GSTP1 promoter observed after bisulfite modification and methylation-specific PCR. Furthermore, GTP treatment resulted in demethylation of CpG islands in GSTP1 promoter as confirmed by methyl-specific PCR and genomic sequencing. Importantly, treatment of LNCaP cells with 10-μg/ml GTP exhibited minimal toxicity compared to the DNMT inhibitor, 5’-deoxy-azacytidine (10 μM) at the clinical recommended dose. Taken together, these results suggest that green tea polyphenols have the potential to repress DNA promoter hypermethylation in CpG islands through DNMT inhibition thereby reactivating the silenced GSTP1 gene in prostate cancer cells.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]