The Role of EZH2 in Breast Cancer Progression and Metastasis.

Abstract

Understanding how breast cancer cells disseminate and metastasize is essential to develop better treatments and to improve survival. Enhancer of Zeste Homolog 2 (EZH2) is a Polycomb group protein characterized as a transcriptional repressor through histone trimethylation. We previously found EZH2 overexpression in human invasive breast carcinomas to associate with worse survival. Here, we have focused on elucidating the mechanisms by which EZH2 promotes aggressive breast carcinomas with metastatic potential. We have found that EZH2 regulates two important processes for metastasis: the epithelial-to-mesenchymal transition and migration, and breast cancer stem cell numbers. We discovered that EZH2 downregulation in breast cancer cells promoted a mesenchymal-to-epithelial transition, reduced invasion and motility, and decreased spontaneous pulmonary metastasis. EZH2 was found to induce the p38 signaling pathway, an established regulator of breast cancer invasion and metastasis. EZH2 was demonstrated to bind phosphorylated-p38 (p-p38) in association with other members of the Polycomb Repressive Complex 2 (PRC2). The effect of p-p38 was confirmed in vivo and correlated with decreased spontaneous metastasis. Through analysis of invasive human breast cancers, EZH2 expression was upregulated in all metastatic cases, and EZH2 and p-p38 were co-expressed in 63% of cases, consistent with the functional results. In our studies on the role of EZH2 on breast cancer stem cells, we found that EZH2 expression levels regulate stem cell numbers in nontumorigenic and malignant breast cells. We revealed a role of EZH2 in activating Notch1 signaling by binding the Notch1 promoter independently of PRC2 and methyltransferase activity. Notch1 inhibition was sufficient in preventing the EZH2-induced expansion of stem cell numbers. In a transgenic model with targeted EZH2 overexpression, we found that EZH2 promoted earlier breast cancer initiation and correlated with Notch1 expression. Additionally, EZH2, Notch1 and stem cell markers were found to correlate in human breast cancer. Altogether, these findings reveal important and novel functional links between EZH2, stem cells and breast cancer migration and invasion, and the underlying mechanisms involving EZH2-mediated regulation of the p38 and Notch1 signaling pathways. This work establishes EZH2 as a regulator of breast cancer progression and metastasis, and identifies potential targets for treatment of this malignancy.