The anti-neoplastic effect of DSF does not seem to be restricted to prostate cancer. these genes, but did not significantly alter Squalamine lactate prostate-specific antigen (PSA) expression. DSF significantly inhibited growth and clonogenic survival of prostate cancer cell lines in culture and showed a trend for reduced growth of prostate cancer xenografts. CONCLUSIONS Disulfiram is a non-nucleoside DNMT1 inhibitor that can reduce global 5meC content, reactivate epigenetically silenced genes, and significantly inhibit growth in prostate cancer cell lines. <0.05 was considered statistically significant. RESULTS DSF Inhibits DNMT1 Catalytical Activity In Vitro and Results in Reduction of Global 5meC Contentin Prostate Cancer Cells Previous reports have demonstrated that DSF NS1 can inhibit enzyme activity by reacting with thiol groups in the catalytically active site of the protein. Since the catalytical unit of DNMT1 uses a thiol group we hypothesized that DSF could also interfere with the catalytical activity of DNMT1. To investigate this, we tested the ability of DNMT1 to methylate a hemi-methylated DNA oligonucleotide substrate in an in vitro assay as described previously [13]. Recombinant DNMT1 was incubated with hemimethylated oligos, tritium labeled SAM, and increasing concentrations of DSF. DSF decreased the level of incorporated SAM in a dose-dependent manner showing a 95% reduction of activity at a concentration of 200 M (Fig. 1A), Squalamine lactate indicating that DSF indeed inhibits DNMT1 catalytic activity. Open in a separate window Fig. 1 Disulfiram inhibits DNMT1 in vitro and results in reduction of 5meC content in prostate cancer cells. A: DNMT1enzyme activity assays were performed by incubating recombinant His6-DNMT1 with hemimethylated oligonucleotide substrates and <0.05. To assess DNMT1 expression levels in normal human PrEC and prostate cancer cell lines (CWR22Rv1, PC3, C4-2B, DU145) we performed Western blot analysis (Fig. 1B). Whereas PrEC cells showed very low DNMT1 expression, all prostate cancer cell lines expressed high levels of DNMT1. Since inhibition of DNMT1 could result in decreased maintenance methylation and therefore gradual loss of DNA methylation marks, we tested the effect of DSF treatment on the global 5meC content in androgen sensitive (CWR22Rv1) and androgen insensitive (PC3) prostate cancer cell lines. CWR22Rv1 and PC3 cells were treated with DSF or DMSO control for 3 and 10 days and 4, 8, and 21 days, respectively. DNA was extracted and global methylation status (5meC content) was determined as described previously [3]. Both cell lines showed a statistically significant reduction in 5meC content after 10 or 21 days of DSF exposure suggesting that DSF could also inhibit DNMT function in vivo (Fig. 1C). DSF Restores Expression of Hypermethylated Genes in Prostate Cancer Cells Hypermethylation of promoter regions can result in epigenetic silencing of genes [9]. Since DSF treatment affected maintenance methylation in prostate cancer cells, we asked whether DSF treatment could also reverse promoter CpG island methylation of genes known to be methylated in prostate cancer [2,29]. Conversion of DNA using sodium bisulfite results in a change of sequence composition dependent on the methylation status [30]. PCR amplification reactions using primers specific to either the Squalamine lactate methylated or unmethylated locus allow a qualitative assessment of the methylation status. We identified genes that were previously described to be methlyated in prostate cancers and assessed the methylation status using methylation-specific PCR (MSP) to monitor changes in promoter methylation upon DSF treatment [31]..