DEMETHYLATION OF THE PROGESTERONE-RECEPTOR CPG ISLAND IS NOT REQUIREDFOR PROGESTERONE-RECEPTOR GENE-EXPRESSION

Progesterone receptor (PR) is an estrogen-stimulated gene which has a CpG island that is heavily methylated in a significant fraction of est rogen receptor (ER)negative/PR-negative human breast cancers and cell lines, including MDA-MB-231 cells. Treatment of MDA-MB-231 cells with the demethylating agent, 5-aza-2'-deoxycytidine (deoxyC) led to demeth ylation and expression of ER and PR, However, simultaneous treatment w ith antiestrogen prevented PR transcription, suggesting that demethyla tion of PR alone is not sufficient to reactivate the PR gene. To exami ne the effects of ER on the methylation status of the PR CpG island, w e stably transfected MDA-MB-231 cells with an inducible expression vec tor for ER, Surprisingly, in two cell clones, we found that induction of PR gene expression by ligand-bound ER does not require demethylatio n of the PR CpG island. In contrast, induction of PR transcription was inhibited by blocking the interaction of ER with SRC-1A, a coactivato r of ER function. For the first time, we show that a transcription fac tor with the potential to remodel heterochromatin can activate gene ex pression without altering the methylation status of the CpG island. Th ese results raise the possibility that demethylation and histone acety lation are distinct but complementary mechanisms for destabilizing het erochromatin and activating transcription.