DNA methyltransferase and demethylase in human prostate cancer

Recent studies have shown that cytosine-5 methylation at CpG islands in the regulatory sequence of a gene is one of the key mechanisms of inactivation. The enzymes responsible for CpG methylation are DNA methyltransferase (DNMT) 1, DNMT3a, and DNMT3b, and the enzyme responsible for demethylation is DNA demethylase (MBD2). Studies on methylation-demethylation enzymes are lacking in human prostate cancer. We hypothesize that MBD2 enzyme activity is repressed and that DNMT1 enzyme activity is elevated in human prostate cancer. To test this hypothesis, we analyzed enzyme activities, mRNA, and protein levels of MBD2 and DNMT1, DNMT3a, and DNMT3b in human prostate cancer cell lines and tissues. The enzyme activities of DNMTs and MBD2 were analyzed by biochemical assay. The mRNA expression was analyzed by reverse transcriptase-polymerase chain reaction and by Northern blotting. The protein expression was measured by immunohistochemistry with specific antibodies. The results of these experiments demonstrated that (1) the activity of DNMTs was twofold to threefold higher in cancer cell lines and cancer tissues, as compared with a benign prostate epithelium cell line (BPH-1) and benign prostatic hyperplasia (BPH) tissues; (2) MBD2 activity was lacking in prostate cancer cell lines but present in BPH-1 cells; (3) immunohistochemical analyses exhibited higher expression of DNMT1 in all prostate cancer cell lines and cancer tissues, as compared with BPH-1 cell lines and BPH tissues; (4) MBD2 protein expression was significantly higher in BPH-1 cells and lacking in prostate cancer cell lines and, in BPH tissues, MBD2 protein expression was poorly observed, as compared with no expression in prostate cancer tissues; and (5) mRNA expression for DNMT1 was upregulated in prostate cancer, as compared with BPH-1, and mRNA expression for MBD2 was found to be significantly expressed in all cases. The results of these studies clearly demonstrate that DNMT1 activity is upregulated, whereas MBD2 is repressed at the level of translation in human prostate cancer. These results may demonstrate molecular mechanisms of CpG hypermethylation of various genes in prostate cancer.