Histone Methyltransferase EZH2 Induces Akt-Dependent Genomic Instability and BRCA1 Inhibition in Breast Cancer

Abstract

Increased levels of EZH2, a critical regulator of cellular memory, signal the presence of metastasis and poor outcome in breast cancer patients. High levels of EZH2 are associated with nuclear pleomorphism, lack of estrogen receptor expression, and decreased nuclear levels of BRCA1 tumor suppressor protein in invasive breast carcinomas. The mechanism by which EZH2 overexpression promotes the growth of poorly differentiated invasive carcinomas remains to be defined. Here, we show that EZH2 controls the intracellular localization of BRCA1 protein. Conditional doxycycline-induced upregulation of EZH2 in benign mammary epithelial cells results in nuclear export of BRCA1 protein, aberrant mitoses with extra centrosomes, and genomic instability. EZH2 inhibition in CAL51 breast cancer cells induces BRCA1 nuclear localization and rescues defects in ploidy and mitosis. Mechanistically, EZH2 overexpression is sufficient for activation of the phosphoinositide 3-kinase/ Akt (PI3K/Akt) pathway specifically through activation of Akt isoform 1. EZH2-induced BRCA1 nuclear export, aneuploidy, and mitotic defects were prevented by treatment with the PI3K inhibitors LY294002 or wortmannin. Targeted inhibition of Akt-1, Akt-2, and Akt-3 isoforms revealed that the EZH2-induced phenotype requires specific activation of Akt-1. The relevance of our studies to human breast cancer is highlighted by the finding that high EZH2 protein levels are associated with upregulated expression of phospho-Akt-1 (Ser473) and decreased nuclear expression of phospho-BRCA1 (Ser1423) in 39% of invasive breast carcinomas. These results enable us to pinpoint one mechanism by which EZH2 regulates BRCA1 expression and genomic stability mediated by the PI3K/Akt-1 pathway. Cancer Res; 71(6); 2360-70. Ó2011 AACR.

EZH2 expands breast stem cells through activation of NOTCH1 signaling

“…S4A). We used a conditional doxycycline (DOX)-mediated system to overexpress EZH2 in MCF10A cells (18). Whereas DOX treatment of MCF10A-pLVX-EZH2 cells significantly induced EZH2 overexpression in ALDH1 + and ALDH1 − cells, EZH2 mRNA levels were significantly higher in ALDH1 + cells (Fig.…”

Multifaceted role of EZH2 in breast and prostate tumorigenesis

“…81 EZH2 knockdown upregulated BRCA1, a nuclear protein that plays a major role in DNA repair and estrogen receptor modulation. 89 The proposed mechanisms underlying EZH2 overexpression, which ultimately causes uncontrolled cell proliferation leading to tumorigenesis in breast, is discussed in the following section ( Fig. 2A).…”

“…15,79,80 Mechanistically, upregulation of EZH2 in benign mammary epithelial cells inhibits BRCA1 phosphorylation (serine 1423) and causes nuclear export of BRCA1 protein with high levels of Cdc2-CyclinB1 complex (essential for G2/M checkpoint control), ultimately resulting in uncontrolled cell division, aberrant mitoses with extra chromosomes and genomic instability. 89 Conversely, both in vivo and in vitro studies confirm that EZH2 inhibition increases the protein levels of total BRCA1 and of BRCA1 phosphorylation at serine 1423 and induces BRCA1 nuclear localization, which restores defects in mitosis and ploidy level. Gonzalez et al 89 have demonstrated that EZH2 overexpression in breast cancer cells causes activation of the phosphoinositide-3-kinase/Akt (PI3K/Akt) pathway, specifically through activation of the Akt isoform.…”

“…89 Conversely, both in vivo and in vitro studies confirm that EZH2 inhibition increases the protein levels of total BRCA1 and of BRCA1 phosphorylation at serine 1423 and induces BRCA1 nuclear localization, which restores defects in mitosis and ploidy level. Gonzalez et al 89 have demonstrated that EZH2 overexpression in breast cancer cells causes activation of the phosphoinositide-3-kinase/Akt (PI3K/Akt) pathway, specifically through activation of the Akt isoform. In 39% of invasive breast carcinomas, high EZH2 protein levels are associated with increased expression of phospho-Akt-1 (serine 473) and decreased nuclear localization of phospho-BRCA1 (serine EZH2 in prostate carcinogenesis.…”

Differential expression of enhancer of zeste homolog 2 (EZH2) protein in small cell and aggressive B-cell non-Hodgkin lymphomas and differential regulation of EZH2 expression by p-ERK1/2 and MYC in aggressive B-cell lymphomas

“…3 In a subset of invasive breast carcinomas and moderately to poorly differentiated non-small cell lung cancers, AKT(Ser473) phosphorylation was closely associated with EZH2 overexpression. 4,5 In prostate and hepatocellular carcinomas, upregulation of MYC protein results in the overexpression of EZH2 through reduction of miR-26a, miR26b, and miR-101. In addition, MYC can directly bind to and activate the EZH2 promoter.…”