title: Estrogen receptor α-coupled Bmi1 regulation pathway in breast cancer and its clinical implications.
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.
authors.forename: Huali
authors.initials: H
authors.lastname: Wang
authors.forename: Haijing
authors.initials: H
authors.lastname: Liu
authors.forename: Xin
authors.initials: X
authors.lastname: Li
authors.forename: Jing
authors.initials: J
authors.lastname: Zhao
authors.forename: Hong
authors.initials: H
authors.lastname: Zhang
authors.forename: Jingzhuo
authors.initials: J
authors.lastname: Mao
authors.forename: Yongxin
authors.initials: Y
authors.lastname: Zou
authors.forename: Hong
authors.initials: H
authors.lastname: Zhang
authors.forename: Shuang
authors.initials: S
authors.lastname: Zhang
authors.forename: Wei
authors.initials: W
authors.lastname: Hou
authors.forename: Lin
authors.initials: L
authors.lastname: Hou
authors.forename: Michael A
authors.initials: MA
authors.lastname: McNutt
authors.affiliation: Department of Pathology, Health Science Center of Peking University, 38 Xueyuan Road, Haidian District, Beijing 100191, China. zhangbo@bjmu.edu.cn.
authors.forename: Bo
authors.initials: B
authors.lastname: Zhang
mesh_terms: D001943:Breast Neoplasms / Q000175:diagnosis* / Q000378:metabolism*; D047628:Estrogen Receptor alpha / Q000378:metabolism*; D005260:Female; D006801:Humans; D061986:MCF-7 Cells; D063150:Polycomb Repressive Complex 1 / Q000096:biosynthesis*; D015398:Signal Transduction / Q000502:physiology*
chemical_list: C069077:BMI1 protein, human; C506487:ESR1 protein, human; D047628:Estrogen Receptor alpha; D063150:Polycomb Repressive Complex 1