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 DNA methyltransferase. Its cytotoxicity has been attributed to several possible mechanisms including reexpression of growth suppressor genes and formation of covalent adducts between DNA methyltransferase and 5-aza-2′-deoxycytidine-substituted DNA which may lead to steric inhibition of DNA function. In this study, we use a panel of human breast cancer cell lines as a model system to examine the relative contribution of two mechanisms, gene reactivation and adduct formation. Estrogen receptor-negative cells, which have a hypermethylated estrogen receptor gene 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 methylation, estrogen receptor, plays a major role in this toxicity in one estrogen receptor-negative cell line as treatment of the cells with anti-estrogen-blocked cell death. However, drug sensitivity of other tumor cell lines correlated best with increased levels of DNA methyltransferase 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.