BCL6 is a context-dependent mediator of the glioblastoma therapy response

Abstract

<p>Glioblastoma is a rapidly fatal brain cancer with no cure. The resistance of glioblastoma tumours to available therapies means that more effective treatments are desperately needed. Previous research showed that the transcriptional repressor protein BCL6 is upregulated by chemo- and radiotherapy in glioblastoma and that inhibition of BCL6 enhances the effectiveness of these therapies. Therefore, BCL6 is a promising target to improve the efficacy of available treatments for glioblastoma. BCL6 is known as a transcriptional repressor in germinal centre B cells and an oncogene in lymphoma, as well as in other cancers. However, previous research indicated that BCL6 induced by therapy in glioblastoma may not act as a transcriptional repressor. This thesis aimed to clarify the role of BCL6 in the therapy response of glioblastoma. The effect of BCL6 inhibition on the whole proteome response of glioblastoma cells to DNA-damaging treatments was investigated. This confirmed that BCL6 was involved in the therapy response of glioblastoma and that acute irradiation appeared to cause BCL6 to switch from a repressor of the DNA damage response to a promoter of stress response signalling. Rapid immunoprecipitation mass spectrometry of endogenous proteins enabled identification of proteins associated with BCL6 in untreated and irradiated glioblastoma cells. BCL6 appeared to be a transcriptional regulator in untreated glioblastoma and its association with the corepressor NCOR2 was validated using proximity ligation assays. However, association with nuclear proteins was lost in response to acute irradiation. This was accompanied by the irradiation-induced association of BCL6 with plasma membrane proteins. Targeted long-read transcript sequencing did not reveal differential alternative splicing of BCL6 in response to acute irradiation. This indicated that the canonical BCL6 protein was expressed in irradiated glioblastoma cells and that the change in BCL6 function must be mediated by mechanisms other than the production of splice variants, such as through post-translational modification. Overall, these results support the hypothesis that BCL6 is involved in the therapy resistance of glioblastoma cells but reveal that its activity is context-dependent and may be mediated by the intensity of cellular stress.</p>