Abstract 2607: BMI1 and dominant negative p53 cooperate to suppress AKT-mediated oncogene-induced senescence and promote transformation in human neural stem cells.
Authors: Ding Dacheng , Kahlert Ulf , Maciaczyk Jarek , Nikkhah Guido , Eberhart Charles , Raabe Eric 
Published: August 2014

Abstract
Purpose: BMI-1 is required for the maintenance of normal brain stem cells and is associated with aggressive brain tumors such as glioblastoma multiforme (GBM). BMI1 is a polycomb group protein that can modify chromatin to silence the pro-senescence tumor suppressor p16INK4a. The PI3K/AKT pathway is also associated with clinically aggressive brain tumors. Interestingly, persistent activation of AKT can cause oncogene-induced senescence. Dominant negative R248W p53 mutation blocks induction of the tumor suppressor p21CIP1 and is found in aggressive brain tumors such as GBM. Telomere maintenance through telomerase enzyme activity is also found in aggressive brain tumors. We investigated if constitutively active AKT, dominant negative p53, BMI1, and hTERT can cooperate to bring about the transformation of human neural stem and progenitor cells, and in particular if BMI1 can suppress AKT induced senescence.
Experimental Design: We used lentivirus to introduce singly and in combination BMI1, R248W p53, hTERT and constitutively active AKT into human cortex derived stem and progenitor cells. We assayed the effects of one, two, three or four oncogenes on the proliferation and induction of cellular senescence. We measured proliferation using bromodeoxyurine (BrdU) and senescence by western blotting for p16INK4a and p21 CIP1.
Results: Increased expression of BMI1 alone did not have a significant effect on human neural stem cell proliferation, but did lead to the suppression of p16INK4a. AKT alone transduced cells showed no increased proliferation compared to control cells, but did show upregulation of p16INK4a. AKT alone expressing cells stopped growing after several passages in culture, in association with increased p16 INK4a expression. Cells transduced with BMI1 and AKT initially grew at a similar rate to control cells, but after several passages, similar to AKT alone transduced cells, BMI1/AKT cells stopped proliferating and expressed p21CIP1and p16INK4a. Control cells continued to proliferate for multiple additional passages. Cells transduced with R248W p53/hTERT/BMI1/AKT proliferated at an increased rate compared to control cells (17% BrdU positive for pBABE control cells compared to 37% for R248W p53/hTERT/BMI1/AKT p=0.0003, t-test). These cells did not undergo senescence, did not induce p16INK4a or p21CIP1 and continued proliferating more than control or single oncogene transduced cells long after other cells had senesced. Human neural stem cells transduced with R248W p53/hTERT/BMI1/AKT formed aggressive orthotopic xenograft glial tumors, while control cells did not form tumors.
Conclusions: BMI1 alone cannot suppress AKT induced senescence in human cortex derived neural stem cells. But BMI1 combined with R248W p53 and hTERT caused suppression of AKT induced senescence, and resulting cells showed signs of oncogenic transformation
Citation Format: Dacheng Ding, Ulf Kahlert, Jarek Maciaczyk, Guido Nikkhah, Charles Eberhart, Eric Raabe. BMI1 and dominant negative p53 cooperate to suppress AKT-mediated oncogene-induced senescence and promote transformation in human neural stem cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2607. doi:10.1158/1538-7445.AM2013-2607