Glioblastoma is the most common and fatal adult primary brain tumour. Despite maximum therapy, median survival time is 14 months after diagnosis. Senescence is a cellular stress response that results in irreversible growth arrest with continued metabolic activity. It has been shown to be a novel mechanism to inhibit tumorigenesis and tumor progression. However, the role of senescence in glioblastoma is poorly understood. Furthermore, resistance to therapy is believed to be in large part due to extensive heterogeneity in glioblastoma at the molecular level. While this has shed light on the biological understanding of glioblastoma, the impact of such heterogeneity in glioblastoma with respect to therapeutic mechanisms such as senescence induction is largely unknown and warrants further investigation.
Primary glioblastoma cells constitute an important model of study as they are a closer representation of the parent disease. In the present study, previously isolated primary glioblastoma cells from human patients were characterized according to their molecular subtype by microarray expression analysis. PriGO7A, PriGO8A and PriGO9A cells were predominantly of the classical subtype whereas PriGO17A were predominantly mesenchymal. I investigated the response of these PriGO cells towards various stress inducing agents to determine their capability to undergo senescence.
PriGO8A and PriGO9A cells underwent senescence in response to serum characterized by increased SAβGal activity, PML bodies, p21 and morphological changes characteristic of senescence. This occurred in the absence of a detectable DNA damage response as seen without an increase in γH2AX foci. There was also a lack of a senescence-associated secretory phenotype. Ionizing radiation, known to induce senescence in fibroblasts by inducing double stranded DNA damage, caused cell death but not senescence in PriGO8A and PriGO9A cells. Similarly, exposure of PriGO8A and PriGO9A cells to Triapine (an agent known to cause single stranded DNA damage) induced cell death without senescence. PriGO17A cells did not show evidence of cell death or senescence upon exposure to any of the above agents.
In subsequent studies, I investigated the molecular mechanism responsible for induction of senescence in PriGO cells. Microarray expression analysis revealed that serum exposure in PriGO8A cells increased the expression of genes associated with the Transforming growth factor-β (TGFβ) pathway. The response of PriGO8A cells to serum was attributed at least in part to TGFβ that was dependent on basal expression of the TGFβ activator protein thrombospondin. PriGO7A cells lacked basal thrombospondin expression and did not undergo senescence in response to serum, but exhibited senescence in response to TGFβ. PriGO17A cells, on the other hand, exhibited senescence in response to TGFβ only when Ras activity was blocked.
In conclusion, primary glioblastoma cells retain a functional senescence program capable of undergoing senescence in response to TGFβ, which suggests senescence can potentially be exploited therapeutically in glioblastoma. In addition, the response to therapeutic agents in glioblastoma is influenced by the molecular heterogeneity present in primary glioblastoma cells not only of different subtypes but also within the same subtype.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/37934 |
| Date | 30 July 2018 |
| Creators | Kumar, Ritesh |
| Contributors | Lorimer, Ian |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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