ROLE OF ESTROGEN-RECEPTOR GENE DEMETHYLATION AND DNA METHYLTRANSFERASE DNA ADDUCT FORMATION IN 5-AZA-2'-DEOXYCYTIDINE-INDUCED CYTOTOXICITY IN HUMAN BREAST-CANCER CELLS

The cytosine analog 5-aza-2'-deoxycytidine is a potent inhibitor of DN A methyltransferase, Its cytotoxicity has been attributed to several p ossible mechanisms including reexpression of growth suppressor genes a nd formation of covalent adducts between DNA methyltransferase and 5-a za-2'-deoxycytidine-substituted DNA which may lead to steric inhibitio n of DNA function, In this study, we use a panel of human breast cance r cell lines as a model system to examine the relative contribution of two mechanisms, gene reactivation and adduct formation. Estrogen rece ptor-negative cells, which have a hypermethylated estrogen receptor ge ne promoter, are more sensitive than estrogen receptor-positive cells and underwent apoptosis in response to 5-aza-2-deoxycytidine. For the first time, we show that reactivation of a gene silenced by methylatio n, estrogen receptor, plays a major role in this toxicity in one estro gen receptor-negative cell line as treatment of the cells with anti-es trogen-blocked cell death, However, drug sensitivity of other tumor ce ll lines correlated best with increased levels of DNA methyltransferas e activity and formation DNA DNA methyltransferase adducts as analyzed in situ. Therefore, both reexpression of genes like estrogen receptor and formation of covalent enzyme DNA adducts can play a role in 5-aza -2'-deoxycytidine toxicity in cancer cells.