Abstract 2427: Polycomb group protein BMI1 protects neuroblastoma cells from DNA damage-induced apoptotic death in cooperation with L3MBTL2
Authors: Akita Nobuhiro , Takenobu Hisanori , Chikaraishi Hiroshi , Ohira Miki , Kamijo Takehiko 
Published: July 2016

[Introduction] BMI1 is a polycomb protein and its overexpression has been correlated with cancer development and aggressiveness. We previously reported that v-myc avian myelocytomatosis viral oncogene neuroblastoma-derived homolog (MYCN)-induced BMI1 positively regulated neuroblastoma (NB) cell proliferation via the transcriptional suppression of tumor suppressors in NB cells (ONCOGENE, 2010). The role of BMI1 in DNA damage repair pathways has recently been highlighted. The ubiquitination of H2AK119 is also up-regulated locally at DNA damage sites, namely those of double-strand breaks and UV-damaged lesions. BMI1 and other PcG proteins make foci at DNA damage sites and are involved in DNA damage repair pathways; however, the exact role of PcG proteins in DNA damage repair pathways remains to be elucidated.

[Results and Discussion] In order to evaluate the potential of BMI1 as a new target for NB therapy, we herein examined the effects of BMI1 reductions on NB cell differentiation and apoptotic NB cell death. BMI1 knockdown (KD) up to 7 days in NB cells significantly induced their differentiation. BMI1 depletion up to 14 days significantly induced apoptotic NB cell death along with the activation of p53, increases in p73, and induction of p53 family downstream molecules. BMI1 depletion in vivo markedly suppressed NB xenograft tumor growth. A pathological analysis using the TUNEL assay indicated the induction of apoptotic cell death. BMI1 reductions activated ATM and increased γ-H2AX in NB cells. The recently identified polycomb protein L3MBTL2 was clearly decreased in BMI1 KD NB cells. BMI1 KD-induced L3MBTL2 reductions occurred at the protein level. BMI1 interacted with Ring1b, but not with L3MBTL2. Furthermore, L3MBTL2 KD by siRNA effectively accelerated apoptosis induced by the BMI1 inhibitor, PTC-209, related to apoptotic cell death accompanied by Jun/EGR-1/p53 pathway activation in NB cells. In neuronal cells, NGF depletion induced JUN pathway activation and apoptotic cell death. Furthermore, JUN modulates myeloma cell apoptosis by interacting with EGR-1, which down-regulates Survivin and triggers caspase signaling; JUN up-regulates EGR-1 expression by binding its promoter, which suggests that EGR-1 is the direct downstream target gene of the JUN transcription factor. In solid tumors, EGR-1 regulates radiation-induced cell death via apoptotic pathways including p53-dependent apoptosis.

These results clarified, for the first time, the cooperation between PcG BMI1 and L3MBTL2 in DNA damage responses. Therefore, BMI1 and L3MBTL2 appear to be promising targets in new therapies for NB tumors.

Citation Format: Nobuhiro Akita, Hisanori Takenobu, Hiroshi Chikaraishi, Miki Ohira, Takehiko Kamijo. Polycomb group protein BMI1 protects neuroblastoma cells from DNA damage-induced apoptotic death in cooperation with L3MBTL2. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2427.