In vitro transformation of primary rat tracheal epithelial cells by 5-azacytidine.

The ability of the hypomethylating agent 5-azacytidine to transform normal primary rat tracheal epithelial cells was determined. A single 24-h exposure to 5-azacytidine resulted in the transformation of rat tracheal epithelial cells at concentrations of the cytosine analogue ranging from 1 to 4 microM. Doses of 5-azacytidine that produced these enhanced growth transformants were nonmutagenic as measured by the induction of 6-thioguanine or ouabain resistance. In addition, the hypomethylating agent 5-azadeoxycytidine also transformed primary rat tracheal epithelial cells, whereas 6-azacytidine, a cytosine analogue which does not induce DNA hypomethylation, did not. Enhanced growth transformants resulting from 5-azacytidine exposure continued to progress in the absence of further treatment to give rise to variants with indefinite growth capacity which ultimately became neoplastic. These results indicate that hypomethylating agents can transform normal cells and suggest that changes in DNA methylation may occur during the initial stages of neoplastic transformation.