Department of Pathology, Comprehensive Cancer Center, University of Michigan Medical School, Ann Arbor, Michigan 48109, USA.
Cancer Res. 2011 Mar 15;71(6):2360-70. doi: 10.1158/0008-5472.CAN-10-1933.
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.
EZH2 水平升高，作为细胞记忆的关键调节因子，提示乳腺癌患者存在转移和不良预后。EZH2 水平高与核多形性、雌激素受体表达缺失以及乳腺癌中 BRCA1 肿瘤抑制蛋白核水平降低有关。EZH2 过表达促进低分化浸润性癌生长的机制仍有待确定。在这里，我们表明 EZH2 控制 BRCA1 蛋白的细胞内定位。在良性乳腺上皮细胞中，条件性多西环素诱导 EZH2 上调导致 BRCA1 蛋白核输出、有丝分裂异常伴额外中心体和基因组不稳定性。CAL51 乳腺癌细胞中 EZH2 的抑制诱导 BRCA1 的核定位，并挽救倍性和有丝分裂缺陷。从机制上讲，EZH2 过表达足以通过激活 Akt 同工型 1 特异性激活磷酸肌醇 3-激酶/Akt (PI3K/Akt) 途径。EZH2 诱导的 BRCA1 核输出、非整倍体和有丝分裂缺陷可通过 PI3K 抑制剂 LY294002 或渥曼青霉素治疗来预防。针对 Akt-1、Akt-2 和 Akt-3 同工型的靶向抑制表明，EZH2 诱导的表型需要 Akt-1 的特异性激活。EZH2 蛋白水平高与磷酸化 Akt-1(Ser473)上调和磷酸化 BRCA1(Ser1423)核表达降低相关，在 39%的浸润性乳腺癌中发现这一事实突出了我们研究与人类乳腺癌的相关性。这些结果使我们能够确定 EZH2 通过 PI3K/Akt-1 途径调节 BRCA1 表达和基因组稳定性的一种机制。
