Variable DNA methylation changes during differentiation of human melanoma cells

The DNA 5-methylcytosine content has been analyzed in the human melanoma cell line M21 at several time points after induction of differentiation by a variety of inducers. 5-Aza-2'-deoxycytidine reduces DNA methylation to about 50% of the control level and this demethylation occurs prior to the establishment of the differentiated phenotype. The DNA synthesis inhibitors cytosine arabinoside, aphidicolin, and hydroxyurea exert different effects on DNA methylation in these cells. Cytosine arabinoside induces an early DNA hypermethylation, which is however reversible and drops to the original level after 24 h. Hydroxyurea induces DNA hypermethylation after a lag period of more than 48 h and the DNA polymerase alpha inhibitor aphidicolin has no effect on the DNA methylation level. Treatment of cells with phorbol 12-myristate 13-acetate, another potent inducer of melanoma cell differentiation, does not result in a change of total DNA methylation over a period of 96 h. These results indicate that differentiation of human melanoma cells can be accompanied by variable changes of the DNA methylation pattern. These changes can be neither generally related to the differentiation process itself nor related to the effects of DNA synthesis inhibition on DNA methylation, but may more likely reflect a direct or indirect particular effect of the inducer on the DNA methylation process.