Effects of sodium butyrate and 5-azacytidine on DNA methylation in human tumor cell lines: variable response to drug treatment and withdrawal.

The effect of butyrate (Btr) and 5-azacytidine (azaC) on genomic DNA methylation was examined in a variety of human tumor cell lines. Butyrate treatment differentially affected 5-methylcytosine (m5C) content in a cell type-specific manner. For example, incubation of CBT cells (glioblastoma multiforme) for 48 h in medium containing 2 mM Btr resulted in the hypomethylation of their DNA; after removal of the drug, the m5C content was rapidly (24 h) restored to its original level. In contrast, when HeLa cells (cervical carcinoma) were cultured for 48 h in 3 mM Btr, their genomic DNA became more extensively methylated relative to the untreated control, and the hypermethylated state was maintained for long periods in culture. Treatment of HeLa cell lines with azaC resulted in extensive demethylation of their DNA. Following removal of the drug, the DNA was rapidly remethylated to levels equal to or greater than those of the untreated cells. The hypermethylated state has remained stable for over 180 generations. Together, the data suggest that cells respond rapidly to alter drug-induced reductions in the m5C content of their DNA, and this response can subsequently result in overmethylation. In contrast, when drug-induced hypermethylation of genomic DNA occurs, it may persist for many cell generations.