Abstract
The well-known tumour suppressor p53 is known to prevent the development of tumours. Recent research has highlighted the importance of an N-truncated isoform, Δ133p53, in the progression of metastatic cancers. This particular isoform has been shown be expressed in a number of human cancers and to enhance the metastatic capabilities of cells. As a result, Δ133p53 promotes cancer progression and contributes to the development of metastases. The underlying mechanisms of this are unclear and one such mechanism is that Δ133p53 alters cytoskeletal dynamics to promote a migratory phenotype.
The aim of this thesis was to test this hypothesis and investigate alterations to cytoskeletal dynamics in response to the expression of Δ133p53, with a particular interest in the early stages of migration. SAOS-2 osteosarcoma cells and B16 mouse melanoma cells were retrovirally transduced with either Δ133p53, or the mouse analogue Δ122p53, respectively. To analyse alterations in both cytoskeletal dynamics and protein expression, adherent, near- confluent cells were scratched to elicit a wound closing migratory response. Immunofluorescence results revealed cytoskeletal remodelling, consistent with a migratory phenotype, in both the transduced SAOS-2 and B16 cells. In particular, F-actin and acetylated alpha tubulin reflected striking changes to cellular morphology, with significant alterations in cellular protrusions and protein localisation characteristic of migratory cells. This occurred in cells expressing the p53 isoform, regardless of whether they were scratched or not, suggesting that Δ133p53/Δ122p53 promotes a migratory phenotype even in the absence of a migratory stimulus. Live imaging also revealed an increase in the rate of microtubule dynamics with expression of Δ122p53, confirming the immunofluorescence results of alpha tubulin in cells with Δ133p53.
Results from the p53-null transduced SAOS-2 cell line showed that Δ133p53 can act in a p53 independent manner to promote remodelling of the cytoskeleton, as these cells responded in a similar manner to p53 wild-type B16 Δ122p53 cells. It is possible that Δ133p53 is also interacting with p53 family members and tumour suppressor proteins. Levels of p27 were increased in the B16 cells in response to both Δ122p53 expression and a migratory stimulus. This was not the case for the transduced SAOS-2 cells suggesting that in the presence of full length p53, Δ122p53 (but not Δ133p53) may be modulating p27 expression.
To inform this research of the clinical significance of Δ133p53 expression in cancer, breast cancer cell lines from the Cancer Cell Line Encyclopaedia were analysed with respect to p53 expression. p53 mutation status does not correlate with clinical outcomes, suggesting that the role of Δ133p53 expression, with that of other p53 family members, may provide insight into the way that Δ133p53 influences clinical cancer outcomes.
The results from this thesis provide novel findings that Δ133p53/Δ122p53 directly influences cytoskeletal dynamics to promote a pro-migratory phenotype. The exact mechanisms of these actions are as yet unclear and further research into possible interactions with known regulators of the cytoskeleton and migration, such as RhoGTPases, may elucidate this further. This work contributes to the expanding evidence that Δ133p53/Δ122p53 influences cellular responses to stress and promotes cell migration.