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Open Access Highly Accessed Research article

Estrogen receptor α-coupled Bmi1 regulation pathway in breast cancer and its clinical implications

Huali Wang1, Haijing Liu1, Xin Li1, Jing Zhao1, Hong Zhang1, Jingzhuo Mao1, Yongxin Zou1, Hong Zhang2, Shuang Zhang2, Wei Hou1, Lin Hou1, Michael A McNutt1 and Bo Zhang1*

Author Affiliations

1 Department of Pathology, Health Science Center of Peking University, 38 Xueyuan Road, Haidian District, Beijing 100191, China

2 Department of Pathology, Peking University First Hospital, Beijing 100034, China

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BMC Cancer 2014, 14:122  doi:10.1186/1471-2407-14-122

Published: 24 February 2014

Abstract

Background

Bmi1 has been identified as an important regulator in breast cancer, but its relationship with other signaling molecules such as ERα and HER2 is undetermined.

Methods

The expression of Bmi1 and its correlation with ERα, PR, Ki-67, HER2, p16INK4a, cyclin D1 and pRB was evaluated by immunohistochemistry in a collection of 92 cases of breast cancer and statistically analyzed. Stimulation of Bmi1 expression by ERα or 17β-estradiol (E2) was analyzed in cell lines including MCF-7, MDA-MB-231, ERα-restored MDA-MB-231 and ERα-knockdown MCF-7 cells. Luciferase reporter and chromatin immunoprecipitation assays were also performed.

Results

Immunostaining revealed strong correlation of Bmi1 and ERα expression status in breast cancer. Expression of Bmi1 was stimulated by 17β-estradiol in ERα-positive MCF-7 cells but not in ERα-negative MDA-MB-231 cells, while the expression of Bmi1 did not alter expression of ERα. As expected, stimulation of Bmi1 expression could also be achieved in ERα-restored MDA-MB-231 cells, and at the same time depletion of ERα decreased expression of Bmi1. The proximal promoter region of Bmi1 was transcriptionally activated with co-transfection of ERα in luciferase assays, and the interaction of the Bmi1 promoter with ERα was confirmed by chromatin immunoprecipitation. Moreover, in breast cancer tissues activation of the ERα-coupled Bmi1 pathway generally correlated with high levels of cyclin D1, while loss of its activity resulted in aberrant expression of p16INK4a and a high Ki-67 index, which implied a more aggressive phenotype of breast cancer.

Conclusions

Expression of Bmi1 is influenced by ERα, and the activity of the ERα-coupled Bmi1 signature impacts p16INK4a and cyclin D1 status and thus correlates with the tumor molecular subtype and biologic behavior. This demonstrates the important role which is played by ERα-coupled Bmi1 in human breast cancer.

Keywords:
Bmi1; Estrogen receptor α; p16INK4a; Cyclin D1; Breast cancer