DNMT1-mediated demethylation of lncRNA MEG3 promoter suppressed breast cancer progression by repressing Notch1 signaling pathway

Breast carcinoma is one of the common causes of cancer-related mortality in women. Maternally expressed gene 3 (MEG3), a lncRNA located at 14q32, can be involved in carcinogenesis. In this study, we discovered that MEG3 was downregulated by CpG hypermethylation within its gene promoter. Functionally, treatment of breast cancer cells with the DNA methylation inhibitor 5-AzadC as well as silencing of DNA methyltransferase-1 (DNMT1) could decrease the abnormal hypermethylation of the MEG3 promoter, reverse MEG3 expression, inhibit cell proliferation and promote cell apoptosis. In addition, we found that MEG3 expression was negatively correlated with DNMT1. Mechanistically, MEG3 knockdown combined with 5-AzadC or sh-DNMT1 treatment restored the expression of Notch1 receptor, leading to the Notch1 pathway activation, and promoted the progression of epithelial mesenchymal transformation (EMT). Finally, the mice tumor model experiments showed that DNMT1 knockdown can increase MEG3 expression and inhibit tumor growth. Collectively, our findings uncovered that DNMT1-mediated MEG3 demethylation leads to MEG3 upregulation, which in turn inhibits the Notch1 pathway and EMT process in breast cancer.

乳腺癌是导致女性癌症相关死亡的常见病因之一。定位于14q32区域的长链非编码RNA（long non-coding RNA，lncRNA）母系表达基因3（Maternally expressed gene 3，MEG3）可参与肿瘤发生进程。本研究发现，MEG3的表达可通过其基因启动子区内的CpG高甲基化而下调。功能实验结果显示，采用DNA甲基化抑制剂5-AzadC处理乳腺癌细胞，或敲低DNA甲基转移酶1（DNMT1），均可降低MEG3启动子的异常高甲基化水平，恢复MEG3的表达，抑制细胞增殖并促进细胞凋亡。此外，我们观察到MEG3的表达与DNMT1呈负相关。机制研究表明，联合敲低MEG3并辅以5-AzadC或sh-DNMT1处理，可恢复Notch1受体的表达，进而激活Notch通路，并促进上皮间质转化（epithelial mesenchymal transformation，EMT）进程。最后，小鼠肿瘤模型实验证实，敲低DNMT1可上调MEG3的表达并抑制肿瘤生长。综上，本研究结果揭示了DNMT1介导的MEG3去甲基化可导致MEG3表达上调，进而抑制乳腺癌中的Notch通路与上皮间质转化过程。