Estrogen Receptor α (ERα) Increased Drug Resistance Of MCF-7 Cells Through Directly Up-regulating Expression Of DNMT1 And DNMT3b

Breast cancer is one of the most common worldwide diseases in women. Drug resistance is a major obstacle for successful treatment of breast cancer. It is becoming clear that Estrogen receptor a (ERa) plays critical role in chemotherapy resistance of human breast cancer. However, the underlying mechanism still has not been fully understood.Aberrant DNA methylation is the first epigenetic mark to be associated with drug resistance. Accumulating evidence has shown that abnormal expression of DNMTs alters response of cancer cells to chemotherapy agents. Bioinformatic analysis identified EREs within the promoters of DNMT1 and DNMT3b genes, and our previous study proved that ERa could up-regulate the transcriptional activity of DNMT1 reporter gene. The purpose of this work is to test the hypothesis that ERa may contribute to chemotherapy resistance of ERa(+) breast cancer through regulating expression of DNA transmethylases. The effects of ERa/estrgen on expression of DNMT1, DNMT3a and DNMT3b and the impact on drug response of MCF-7 breast cancer cells were systematically investigated with real time PCR, Western blot, ChIP, MSP and reporter system. Our experimental data showed that ERa was able to upregulate transcriptional activity of DNMT1 and DNMT3b genes, but not DNMT3a, through directly binding to promoters of the genes. ERa knockdown significantly reduced expression levels of DNMTl and DNMT3b. The ERa-induced upregulation of both DNMT1 and DNMT3b increased resistance of MCF-7 cells to paclitaxel. Knockdown of either DNMT1 or DNMT3b partially blocked the effects of ERa overexpression and restored the drug sensitivity of the cells. Estrogen treatment enhanced expression of DNMT1 and DNMT3b through activating ERa and increasing ERa binding on the gene promoters. Additionally, it was DNMT1 rather than DNMT3b that was responsible for increased global genomic methylation. These data provided first evidence that ERa/estrogen could increase the drug resistance of ERa (+) breast cancer cells through directly up-regulating expression of DNMT1 and DNMT3b, and thus is of great importance given that estrogen receptor a (ERa) is expressed in approximately 65% of human breast cancer.