The DNA methylation system in proliferating and differentiated cells.

The human melanoma cell line M21 can be induced to differentiate into oligodendrocyte-like cells with concommitant cessation of cell division. Cytosine-arabinoside, 5-aza-2′-deoxycytidine, hydroxyurea, aphidicolin, and phorbol-12-myristate-13-acetate were found to be potent differentiation inducers. We have analyzed the changes of methylation of DNA cytosines that occur after treatment of M21 cells with these compounds. Although DNA methylation levels remain unchanged in the presence of aphidicolin and phorbol ester, 5-aza-2′-deoxycytidine-induced differentiation of these cells results in a 40% DNA demethylation. On the other hand, hydroxyurea and cytosinearabinoside treatment causes DNA hypermethylation, which, in the case of the cytidine analogue is of only transient nature. These results show that the differentiation of human melanoma cells can be accompanied by variable changes of DNA methylation levels.