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Identification of therapeutically relevant tumour-associated alterations of the retinoic acid signalling pathway in glioblastoma
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Date
2024
Authors
Flynn, Patricia Margaret
Journal Title
Journal ISSN
Volume Title
Publisher
University College Cork
Published Version
Abstract
Glioblastoma is one of the deadliest cancers in the world, and treatment options are limited for the nearly 150,000 individuals worldwide with this diagnosis annually. The understanding of the stem cell component of glioblastoma tumours is ever evolving. As research advances rapidly, one therapeutic avenue is to develop approaches to control these proliferative cells and suppress their growth. A potential method to control the aggressive and inappropriate tumour growth is to promote the differentiation of these glioblastoma stem cells that make up the highly proliferative and heterogenous tumour population. The bioactive metabolite of vitamin A, retinoic acid (RA), is essential for embryonic development and cellular homeostasis and plays a physiological role throughout life, mainly influencing cellular growth, differentiation, and death, through control of gene expression. Using in vitro and in silico methods, we aim to further our understanding of the different types of glioblastoma stem cell-like cell lines (classical, mesenchymal and proneural) using a panel of representative cell lines to investigate the roles of the different retinoic acid receptor (RAR) isoforms (RAR alpha, beta and gamma). We hypothesise that the different cell lines will respond in a variety of ways to RA treatment and that the different RAR isoforms expressed in these cell lines will be capable of inducing different growth patterns in the cells.
We aim to identify the RAR isoforms that can suppress the growth of the glioblastoma stem cells. Results indicate that some cell lines demonstrate enhanced proliferation following RA treatment, some show reduced growth, while others remain unresponsive to RA. Notably, RAR alpha was identified as the isoform likely responsible for mediating growth in faster-growing cell lines, while RAR gamma may contribute to slower growth in specific contexts.
Another aim was to assess the functionality of the RA metabolic and signalling pathway in different glioblastoma stem cell subtypes and explore the implication of the expression of genes in the RA pathway on patient survival using glioblastoma data from The Cancer Genome Atlas (TCGA). These in silico findings reveal that although the genes of the RA pathway are expressed in the proneural, classical and mesenchymal glioblastoma stem cells, their expression levels vary significantly among different stem cell subtypes. The mesenchymal subtype shows higher expression of key RA pathway components, correlating with poorer patient survival outcomes.
The next objective was to identify changes in gene expression patterns associated with glioblastoma stem cell response to RA treatment. RNA-Seq analysis was performed on three cell lines belonging to different stem cell subtypes that demonstrated differing growth patterns to RA treatment. Results indicate that RA upregulates distinct genes and pathways in each cell line which could explain their respective growth responses.
The final objective was to alter the expression of the RAR isoforms and their variants to further analyse their contribution to the differing growth patterns observed in stem cell lines. The initial plan was to overexpress these RAR isoforms using lentiviral vectors, but technical difficulties required an alternate approach using chemical antagonists to block specific RAR activity. The findings showed that while the pan- RAR agonist retinoic acid produces significant growth modifications in some cell lines, the inhibition of RARs did not alter this response.
The studies presented in this thesis contribute valuable new insights to the complex fields of glioblastoma and RA research by potentially identifying the RAR isoform RAR alpha as contributing to the growth of some glioblastoma stem cells. It is associated with poor survival in patients and is upregulated following RA treatment. This information refines the understanding of why the potential of RA as a therapy for glioblastoma was not realised in past clinical trials, and reopens the possibility of using RA and the RA pathway to promote the suppression of glioblastoma, using a novel approach that differentiates the RAR isoforms being targeted.
Description
Keywords
Cancer , Glioblastoma , Glioma , Retinoic acid , Retinoids
Citation
Flynn, P. M. 2024. Identification of therapeutically relevant tumour-associated alterations of the retinoic acid signalling pathway in glioblastoma. PhD Thesis, University College Cork.