DEMETHYLATION OF THE ESTROGEN-RECEPTOR GENE IN ESTROGEN RECEPTOR-NEGATIVE BREAST-CANCER CELLS CAN REACTIVE ESTROGEN-RECEPTOR GENE-EXPRESSION

Approximately one third of breast cancers grow independently of estrog en, lack detectable estrogen receptor (ER) protein, and rarely respond to hormonal treatment, Previous studies correlated the lack of ER gen e expression in ER-negative breast tumor cells with hypermethylation o f a CpG island in the 5' region of the ER gene, In order to determine whether demethylation of the ER gene in the PR-negative human breast c ancer cell line MDA MB-231 could affect ER transcription, cells were t reated with two inhibitors of DNA methylation, 5-azacytidine or 5-aza- 2'-deoxycytidine. DNA from cells treated with either drug became parti ally demethylated at several methylation-sensitive restriction enzyme sites, including HhaI, NotI, and SacII, within the ER CpG island, This demethylation correlated with reexpression of the ER gene as detected by reverse transcriptase PCR and production of ER protein as detected by Western blot analysis, ER produced in drug-treated cells was funct ionally active as demonstrated by its ability to activate transcriptio n of estrogen-responsive genes, These results suggest that DNA methyla tion of the ER CpG island may play a role in suppression of ER gene ex pression in PR-negative breast cancer cells.