Ferguson A T, Lapidus R G, Davidson N E
Oncology Center, Johns Hopkins University School of Medicine, Baltimore, Maryland 21231, USA.
Oncogene. 1998 Aug 6;17(5):577-83. doi: 10.1038/sj.onc.1201961.
Progesterone receptor (PR) is an estrogen-stimulated gene which has a CpG island that is heavily methylated in a significant fraction of estrogen 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 demethylation and expression of ER and PR. However, simultaneous treatment with antiestrogen prevented PR transcription, suggesting that demethylation of PR alone is not sufficient to reactivate the PR gene. To examine the effects of ER on the methylation status of the PR CpG island, we stably transfected MDA-MB-231 cells with an inducible expression vector for ER. Surprisingly, in two cell clones, we found that induction of PR gene expression by ligand-bound ER does not require demethylation of the PR CpG island. In contrast, induction of PR transcription was inhibited by blocking the interaction of ER with SRC-1A, a coactivator of ER function. For the first time, we show that a transcription factor with the potential to remodel heterochromatin can activate gene expression without altering the methylation status of the CpG island. These results raise the possibility that demethylation and histone acetylation are distinct but complementary mechanisms for destabilizing heterochromatin and activating transcription.
孕激素受体（PR）是一种雌激素刺激基因，其具有一个CpG岛，在相当一部分雌激素受体（ER）阴性/PR阴性的人类乳腺癌及细胞系（包括MDA-MB-231细胞）中该CpG岛高度甲基化。用去甲基化剂5-氮杂-2'-脱氧胞苷（脱氧胞苷）处理MDA-MB-231细胞会导致ER和PR的去甲基化及表达。然而，同时用抗雌激素处理会阻止PR转录，这表明仅PR的去甲基化不足以重新激活PR基因。为了研究ER对PR CpG岛甲基化状态的影响，我们用一个可诱导的ER表达载体稳定转染了MDA-MB-231细胞。令人惊讶的是，在两个细胞克隆中，我们发现配体结合的ER诱导PR基因表达并不需要PR CpG岛的去甲基化。相反，通过阻断ER与ER功能的共激活因子SRC-1A的相互作用，PR转录受到抑制。我们首次表明，一个具有重塑异染色质潜力的转录因子可以在不改变CpG岛甲基化状态的情况下激活基因表达。这些结果增加了这样一种可能性，即去甲基化和组蛋白乙酰化是使异染色质不稳定并激活转录的不同但互补的机制。
