Akt-mediated phosphorylation of Bmi1 modulates its oncogenic potential, E3 ligase activity, and DNA damage repair activity in mouse prostate cancer
Karim Nacerddine, … , Shridar Ganesan, Maarten van Lohuizen
Karim Nacerddine, … , Shridar Ganesan, Maarten van Lohuizen
Published April 16, 2012
Citation Information: J Clin Invest. 2012;122(5):1920-1932. https://doi.org/10.1172/JCI57477.
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Research Article Oncology

Akt-mediated phosphorylation of Bmi1 modulates its oncogenic potential, E3 ligase activity, and DNA damage repair activity in mouse prostate cancer

Abstract

Prostate cancer (PCa) is a major lethal malignancy in men, but the molecular events and their interplay underlying prostate carcinogenesis remain poorly understood. Epigenetic events and the upregulation of polycomb group silencing proteins including Bmi1 have been described to occur during PCa progression. Here, we found that conditional overexpression of Bmi1 in mice induced prostatic intraepithelial neoplasia, and elicited invasive adenocarcinoma when combined with PTEN haploinsufficiency. In addition, Bmi1 and the PI3K/Akt pathway were coactivated in a substantial fraction of human high-grade tumors. We found that Akt mediated Bmi1 phosphorylation, enhancing its oncogenic potential in an Ink4a/Arf-independent manner. This process also modulated the DNA damage response and affected genomic stability. Together, our findings demonstrate the etiological role of Bmi1 in PCa, unravel an oncogenic collaboration between Bmi1 and the PI3K/Akt pathway, and provide mechanistic insights into the modulation of Bmi1 function by phosphorylation during prostate carcinogenesis.

Authors

Karim Nacerddine, Jean-Bernard Beaudry, Vasudeva Ginjala, Bart Westerman, Francesca Mattiroli, Ji-Ying Song, Henk van der Poel, Olga Balagué Ponz, Colin Pritchard, Paulien Cornelissen-Steijger, John Zevenhoven, Ellen Tanger, Titia K. Sixma, Shridar Ganesan, Maarten van Lohuizen

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Figure 6

Effect of Bmi1 phosphorylation on H2A ubiquitination and cellular senescence.

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Effect of Bmi1 phosphorylation on H2A ubiquitination and cellular senesc...
(A) Bmi1/Ring1B recombinant complex was first treated with AP or Na3V04-inactivated AP, then assayed for ubiquitin ligase activity on purified nucleosomes. Dephosphorylation of Bmi1 reduced Bmi1/Ring1B E3 ligase activity on H2A (Ub-H2A); Mel18/Ring1B is shown as a positive control. Autopolyubiquitination of Bmi1/Ring1B and Mel18/Ring1B complexes is also shown. (B) Bmi1/Ring1b complex was first incubated with Akt or inactive Akt (inAkt), then assayed for ubiquitin ligase activity as in A. (C) Global Ub-H2A levels were not affected in doxycycline-treated (Dox) LNCaP-tet-shBmi1 cells expressing Bmi1-3A, nor in the presence of Akt inhibitor. (D) Phosphorylation of Bmi1 was not required for repression of cellular senescence. Bmi1–/– MEFs were infected with a control retrovirus (EV; empty vector) or with retroviruses encoding Bmi1-WT or Bmi1-3A. 3T3 assay was performed over a period of 32 days; Western blot against Bmi1, p16INK4A, and tubulin (loading control) is also shown.