ERα positively regulated DNMT1 expression by binding to the gene promoter region in human breast cancer MCF-7 cells.

Estrogen receptors (ER) are expressed in approximately 65% of human breast cancer. Clinical trials and retrospective analyses showed that ER-positive (ER+) tumors were more vulnerable to development of chemotherapy resistance than ER-negative (ER-) tumors. The underlying mechanism is still to be elucidated. Aberrant DNA methylation has been recognized to be associated with cancer chemotherapy resistance. Recently, steroid hormone and their receptors have been found to be involved in the regulation of methyltransferases (DNMTs) and thereby contribute to chemotherapy resistance. The purpose of this study is to explore whether ERα could directly regulate the DNMTs expression. We first analyzed the methylation alterations and its correlation with the expression levels of three types of DNMTs in our established paclitaxel-resistant breast cancer lines, MCF-7(ER+)/PTX and MDA-MB-231(ER-)/PTX cell lines, using qMSP, real-time PCR and Western blot. Then we determined the function of ERα in regulation of DNMT1 using luciferase report gene systems. Our data demonstrated for the first time that ERα could upregulate DNMT1 expression by directly binding to the DNMT1 promoter region in MFC-7(ER+)/PTX cells.

ERα directly activated the MDR1 transcription to increase paclitaxel-resistance of ERα-positive breast cancer cells in vitro and in vivo.

Chemotherapy is commonly used to treat early-stage invasive and advanced-stage breast cancer either before or after surgery. Increasing evidence from clinical analysis and in vitro studies has shown that ER-positive breast cancer cells are insensitive to chemotherapy. Complete understanding of how ERα mediates drug resistance is prerequisite to improvement of the chemotherapeutic efficacy. Over-expression of P-glycoprotein (P-gp) encoded by MDR1 gene is one of the major causes of drug resistance. The association between ERα and MDR1 in breast cancer is still unclear and the limited reports are conflict. This study systematically explored intrinsic link between ERα and the P-gp over-expression in paclitaxel-resistant ERα(+) breast cancer cell lines and mouse model in molecular details. Our data showed that ERα activated the MDR1 transcription in MCF-7/PTX breast cancer cells by binding to ERE1/2 and interacting with Sp1 that bridged to the downstream CG-rich element within the MDR1 promoter. Knockdown of MDR1 restrained the effect of ERα in MCF-7 cells and sensitized the cells to paclitaxel. Treatment of ICI 182,780 that selectively suppressed ERα significantly decreased the MDR1 expression and increased the sensitivity of drug resistant breast cancer cells and xenograft tumors to paclitaxel. Our data strongly demonstrated that ERα was able to increase drug resistance of breast cancer cells through activating MDR1 transcription. This novel mechanism provides new insight to how the ERα signaling regulates response of ERα(+) breast tumors to chemotherapy, which may be exploited for developing novel therapeutic strategies for breast cancer in the future.