5-Azacytidine-induced recovery of O6-alkylguanine--DNA alkyltransferase activity in mouse Ha821 cells.

Mouse Ha821 cells, Harvey murine sarcoma virus-transformed NIH3T3 cells, have extremely low O6-alkylguanine--DNA alkyltransferase (O6AGTase) activity and are hypersensitive to an anti-tumor chloroethylating agent 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU). The treatment of Ha821 cells with a DNA demethylating agent, 5-azacytidine, resulted in an increase in the frequency of ACNU-resistant cell clones. All randomly isolated ACNU-resistant cell clones were found to have O6AGTase activity comparable to the level of the parental NIH3T3 cells. These results suggest that reversible loss of O6AGTase activity in Ha821 cells is caused at least in part by inactivation of the O6AGTase gene due to methylation of cytosine in the gene.