Expression level and DNA methylation status of glutathione-S-transferase genes in normal murine prostate and TRAMP tumors.

BACKGROUND: Glutathione-S-transferase (Gst) genes are downregulated in human prostate cancer, and GSTP1 silencing is mediated by promoter DNA hypermethylation in this malignancy. We examined Gst gene expression and Gst promoter DNA methylation in normal murine prostates and Transgenic Adenocarcinoma of Mouse Prostate (TRAMP) tumors. METHODS: Primary and metastatic tumors were obtained from TRAMP mice, and normal prostates were obtained from strain-matched WT mice (n = 15/group). Quantitative real-time RT-PCR was used to measure GstA4, GstK1, GstM1, GstO1, and GstP1 mRNA expression, and Western blotting and immunohistochemical staining was used to measure GstM1 and GstP1 protein expression. MassARRAY Quantitative Methylation Analysis was used to measure DNA methylation of the 5' CpG islands of GstA4, GstK1, GstM1, GstO1, and GstP1. TRAMP-C2 cells were treated with the epigenetic remodeling drugs decitabine and trichostatin A (TSA) alone and in combination, and Gst gene expression was measured. RESULTS: Of the genes analyzed, GstM1 and GstP1 were expressed at highest levels in normal prostate. All five Gst genes showed greatly reduced expression in primary tumors compared to normal prostate, but not in tumor metastases. Gst promoter methylation was unchanged in TRAMP tumors compared to normal prostate. Combined decitabine + TSA treatment significantly enhanced the expression of 4/5 Gst genes in TRAMP-C2 cells. CONCLUSIONS: Gst genes are extensively downregulated in primary but not metastatic TRAMP tumors. Promoter DNA hypermethylation does not appear to drive Gst gene repression in TRAMP primary tumors; however, pharmacological studies using TRAMP cells suggest the involvement of epigenetic mechanisms in Gst gene repression.

Stage-specific alterations of DNA methyltransferase expression, DNA hypermethylation, and DNA hypomethylation during prostate cancer progression in the transgenic adenocarcinoma of mouse prostate model.

We analyzed DNA methyltransferase (Dnmt) protein expression and DNA methylation patterns during four progressive stages of prostate cancer in the transgenic adenocarcinoma of mouse prostate (TRAMP) model, including prostatic intraepithelial neoplasia, well-differentiated tumors, early poorly differentiated tumors, and late poorly differentiated tumors. Dnmt1, Dnmt3a, and Dnmt3b protein expression were increased in all stages; however, after normalization to cyclin A to account for cell cycle regulation, Dnmt proteins remained overexpressed in prostatic intraepithelial neoplasia and well-differentiated tumors, but not in poorly differentiated tumors. Restriction landmark genomic scanning analysis of locus-specific methylation revealed a high incidence of hypermethylation only in poorly differentiated (early and late) tumors. Several genes identified by restriction landmark genomic scanning showed hypermethylation of downstream regions correlating with mRNA overexpression, including p16INK4a, p19ARF, and Cacna1a. Parallel gene expression and DNA methylation analyses suggests that gene overexpression precedes downstream hypermethylation during prostate tumor progression. In contrast to gene hypermethylation, genomic DNA hypomethylation, including hypomethylation of repetitive elements and loss of genomic 5-methyldeoxycytidine, occurred in both early and late stages of prostate cancer. DNA hypermethylation and DNA hypomethylation did not correlate in TRAMP, and Dnmt protein expression did not correlate with either variable, with the exception of a borderline significant association between Dnmt1 expression and DNA hypermethylation. In summary, our data reveal the relative timing of and relationship between key alterations of the DNA methylation pathway occurring during prostate tumor progression in an in vivo model system.

Lack of evidence for green tea polyphenols as DNA methylation inhibitors in murine prostate.

Green tea polyphenols (GTP) have been reported to inhibit DNA methylation in cultured cells. Here, we tested whether oral consumption of GTPs affects normal or cancer-specific DNA methylation in vivo, using mice. Wild-type (WT) and transgenic adenocarcinoma of mouse prostate (TRAMP) mice were given 0.3% GTPs in drinking water beginning at 4 weeks of age. To monitor DNA methylation, we measured 5-methyl-deoxycytidine (5mdC) levels, methylation of the B1 repetitive element, and methylation of the Mage-a8 gene. Each of these parameters were unchanged in prostate, gut, and liver from WT mice at both 12 and 24 weeks of age, with the single exception of a decrease of 5mdC in the liver at 12 weeks. In GTP-treated TRAMP mice, 5mdC levels and the methylation status of four loci hypermethylated during tumor progression were unaltered in TRAMP prostates at 12 or 24 weeks. Quite surprisingly, GTP treatment did not inhibit tumor progression in TRAMP mice, although known pharmacodynamic markers of GTPs were altered in both WT and TRAMP prostates. We also administered 0.1%, 0.3%, or 0.6% GTPs to TRAMP mice for 12 weeks and measured 5mdC levels and methylation of B1 and Mage-a8 in prostate, gut, and liver tissues. No dose-dependent alterations in DNA methylation status were observed. Genome-wide DNA methylation profiling using the HpaII tiny fragment enrichment by ligation-mediated PCR assay also revealed no significant hypomethylating effect of GTP. These data indicate that oral administration of GTPs does not affect normal or cancer-specific DNA methylation in the murine prostate.