Targeted depletion of BMI1 sensitizes tumor cells to P53-mediated apoptosis in response to radiation therapy

Abstract

Overexpression of BMI1 correlates with cancer development, progression, and therapy failure; however, the underlying molecular mechanisms remain to be fully elucidated. Using the C666-1 nasopharyngeal cancer (NPC) model, the role of BMI1 in mediating response of NPC cells to radiation therapy (RT) was investigated. The results showed a novel radioresistance function for BMI1 in NPC, wherein BMI1 depletion sensitized NPC cells to RT. Cell cycle analysis and transmission electron microscopy (TEM) showed apoptosis as the major mode of cell death, and the mitochondria as a primary targeted cellular organelle. Genomewide microarray and pathway analyses revealed that the P53 pathway is a critical mediator of this process. Cotransfection with siP53 rescued C666-1 cells from cytotoxicity upon BMI1 depletion and RT, thereby corroborating the role for P53. Pretreatment with the antioxidant, Trolox, inhibited apoptosis, indicating that production of reactive oxygen species (ROS) is also mediating cytotoxicity. In vivo, BMI1 depletion combined with RT abrogated tumor-forming capacity in SCID mice, showing the relevance of this process in a more complex tumor environment. Hence, we show a novel role for BMI1 in conferring radioresistance in cancer cells through the downregulation of p53-mediated apoptosis. These results suggest a potential strategy of BMI1 depletion combined with RT for tumors wherein BMI1 appears to be driving disease progression.

Direct effects of Bmi1 on p53 protein stability inactivates oncoprotein stress responses in embryonal cancer precursor cells at tumor initiation

“…40,41 Here, for the first time, we found that Bmi1 directly bound, together with Ring1A and Ring1B, to the p53 protein and induced its polyubiquitination and subsequent degradation.…”

Monoubiquitination of H2AX Protein Regulates DNA Damage Response Signaling

“…Collectively our data indicate that RNF2-BMI1 plays a distinct role at an early stage of DDR by initiating monoubiquitination of H2AX at Lys 120 . It is worth noticing that Ismail et al (26), and Alajez et al (27), also reported that the RNF2-BMI1 complex mediates the monoubiquitination of H2AX. However, both of them focused on that BMI1 knockdown led to decreased 53BP1 and BRCA1 foci formation at relative late time points after IR, which occur downstream of ␥-H2AX formation.…”

“…They are core components in the polycomb repressive complex 1 and are required for H2A ubiquitination-associated transcriptional silencing (24). During the course of our study, several groups reported the involvement of BMI in DDR (25)(26)(27). However, the mechanisms by which the RNF2-BMI1 complex regulate DDR is still unclear.…”

BMI1 Confers Radioresistance to Normal and Cancerous Neural Stem Cells through Recruitment of the DNA Damage Response Machinery

“…BMI1 activity in DNA DSB response is independent of P53 BMI1 deficiency can result in P53 activation (Alajez et al, 2009;Chatoo et al, 2009). In nasopharyngeal cancer, BMI1 inactivation was shown to renders cells more sensitive to P53-dependent Figure 6.…”

“…Scale bars in E: 1 mm (left and middle panels); 50 m (right panels). apoptosis after irradiation (Alajez et al, 2009). Because the status of P53 is highly variable among our different GBM cell lines, we evaluated this in normal human NSCs, which are wild type for P53.…”