Ionizing radiations sustain glioblastoma cell dedifferentiation to a stem-like phenotype through survivin: possible involvement in radioresistance.

Glioblastomas (GBM) are some bad prognosis brain tumors despite a conventional treatment associating surgical resection and subsequent radio-chemotherapy. Among these heterogeneous tumors, a subpopulation of chemo- and radioresistant GBM stem-like cells appears to be involved in the systematic GBM recurrence. Moreover, recent studies showed that differentiated tumor cells may have the ability to dedifferentiate and acquire a stem-like phenotype, a phenomenon also called plasticity, in response to microenvironment stresses such as hypoxia. We hypothesized that GBM cells could be subjected to a similar dedifferentiation process after ionizing radiations (IRs), then supporting the GBM rapid recurrence after radiotherapy. In the present study we demonstrated that subtoxic IR exposure of differentiated GBM cells isolated from patient resections potentiated the long-term reacquisition of stem-associated properties such as the ability to generate primary and secondary neurospheres, the expression of stemness markers and an increased tumorigenicity. We also identified during this process an upregulation of the anti-apoptotic protein survivin and we showed that its specific downregulation led to the blockade of the IR-induced plasticity. Altogether, these results demonstrated that irradiation could regulate GBM cell dedifferentiation via a survivin-dependent pathway. Targeting the mechanisms associated with IR-induced plasticity will likely contribute to the development of some innovating pharmacological strategies for an improved radiosensitization of these aggressive brain cancers.

Inhibition of Ku heterodimer DNA end binding activity during granulocytic differentiation of human promyelocytic cell lines.

The heterodimeric Ku protein (composed of the Ku 86 and Ku 70 sub-units) is a nuclear protein which binds to DNA termini without sequence specificity. Ku is the DNA-targeting component of the large catalytic sub-unit of the DNA-dependent protein kinase complex that is required for the repair of DNA double-strand breaks in mammalian cells. We studied the expression and function of Ku/DNA-PK during granulocytic differentiation of two human promyelocytic cell lines, HL60 and NB4, a process associated to decreased radiation resistance. After 3 days exposure to differentiating agents (either all-trans-retinoic acid or DMSO), Ku binding to double stranded (ds)-DNA ends declined dramatically whereas Ku protein levels remain unchanged. The nuclear, but not cytoplasmic, fraction of differentiated HL60 cells extracts exhibited a heat-sensitive inhibitory activity towards DNA binding of recombinant Ku heterodimer. We further demonstrate that immunoprecipitation of Ku is impaired in extracts from differentiated cells by using two antibodies that recognize epitopes within the C-terminus DNA binding domains of Ku 70 and Ku 86 proteins. These results favor the hypothesis of a protein interacting with Ku that would prevent DNA binding of heterodimerized Ku protein by steric hindrance.